Ernst Bekker final draft

i

VARIATION IN CLAY SURFACE MORPHOLOGY AND MINERALOGY

OF SOILS AFFECTED BY ACID AND SALINE MINE WATER

by

ERNST HENDRIK BEKKER

Submitted in partial fulfilment of the requirements for the degree B.Sc. (Hons.) Environmental Soil Science

In the Department of Plant Production and Soil Science University of Pretoria

Supervisor: Mr. C. de Jager Co-supervisor: Dr. J van der Waals

October 2015

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DECLARATION

I hereby certify that this seminar is my own work, except where duly acknowledged. I

also certify that no plagiarism was committed in writing this thesis.

________________

Ernst Hendrik Bekker

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TABLE OF CONTENTS

LIST OF TABLES ....................................................................................................... v

LIST OF FIGURES ..................................................................................................... vi

ABSTRACT .............................................................................................................. viii

CHAPTER 1: INTRODUCTION .................................................................................. 1

1.1 Literature review ............................................................................................... 1

1.2 Background to study ......................................................................................... 3

CHAPTER 2: METAL BALLANCES ........................................................................... 8

2.1 Background and literature review ...................................................................... 8

2.2 Methodology ..................................................................................................... 9

2.2.1 Sample selection ........................................................................................ 9

2.2.2 Sample and solution preparation and procedure ........................................ 9

2.3 Theoretical soil loadings.................................................................................. 10

2.4 Results ............................................................................................................ 11

2.4.1 Al, Fe and Mn extracts of BC soils ............................................................ 12

2.4.2 Al, Fe and Mn extracts of RS soils ............................................................ 12

2.5 Summary ......................................................................................................... 13

CHAPTER 3: CHANGES IN SOIL MINERALOGY ................................................... 15

3.1 Background and literature review .................................................................... 15

3.2 Methodology ................................................................................................... 17

3.2.1 Sample selection ...................................................................................... 17

3.2.2 Sample preparation .................................................................................. 17

3.2.3 Mineral identification ................................................................................. 17

3.3 XRD results and analysis for BC soils ............................................................. 18

3.3.1 Diffractograms and mineral identification tables of BC Ori and BC MW. .. 18

3.3.2 The influence of ammonium acid oxalate treatment on mine water treated

BC soil. .............................................................................................................. 22

3.3.3 The influence of dithionite citrate extraction on mine water treated BC soil.

........................................................................................................................... 24

3.3.4 Diffractograms of RS original soil .............................................................. 27

3.4 Discussion ....................................................................................................... 30

CHAPTER 4: CHANGES IN SOIL SURFACE MORPHOLOGY ............................... 32

4.1 Background and literature review .................................................................... 32

4.2 Methodology ................................................................................................... 32

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4.2.1 Sample selection ...................................................................................... 32

4.2.2 Sample preparation and analysis .............................................................. 33

4.3 Results of photomicrographs of BC and RS selected samples ....................... 34

4.4 Discussion ....................................................................................................... 37

CHAPTER 5: SUMMARY AND CONCLUSIONS ..................................................... 40

REFERENCES ......................................................................................................... 41

APPENDIX A ............................................................................................................ 44

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LIST OF TABLES

Table 1.1: Pyrite reaction with oxygen and water to various ions and finally ferric

hydroxide and acid…………………………………………………………………………2

Table 1.2: Approximations of the water qualities for the western, central and eastern

basin……………………………………………………………………………………..……3

Table 1.3: Water qualities for the western basin………………………………..….…..6

Table 2.1: Summary of soil metal loading (input), leached (removed) and attenuated

amounts……………………………………………………………………………………..11

Table 2.2: A summary of the AAO and DC soil concentrations of the BC Ori and BC

MW soil…………………………………………………………………………………..….12

Table 2.3: A summary of the AAO and DC soil concentrations of the RS Ori and RS

MW soil………………………………………………………………………………….…..13

Table 3.1 Diffractogram data analysis of figure 3.1…………………………….....….19

Table 3.2 Diffractogram data analysis of figure 3.3 and figure 3.4……….………...22

Table 3.3 Diffractogram data analysis of figure 3.7…………………………………...28

Table 3.4 Diffractogram data analysis of figure 3.9 and figure 3.10………………..30

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LIST OF FIGURES

Figure 1.1: Experimental methodology employed to investigate the effect AMD has

on mineral surfaces and mineralogy. Original soil refers to either the BC or RS

soil………………………………………………………………………………………….…7

Figure 3.1: Diffractograms of the BC MW and BC Ori for 5° - 30° (2θ). Peak 1:

gypsum, peak 2: montmorillonite, peak 3: gypsum, peak 4: quartz, peak 5:

montmorillonite, peak 6: gypsum, peak 7: microline and peak 8:

andesine…………………………………………………………………………………….18

Figure 3.2: Differential diffractogram of the BC MW intensity divided by the BC Ori

intensity for 5° - 30° (2θ). Peak 1: gypsum, peak 2: gypsum, peak 3: quartz and peak

4: gypsum……………………………………………………………………………......…20

Figure 3.3: Diffractograms of the BC MW soil and the BC Ori soil for 35° - 45° (2θ).

Peak 1: gypsum, peak 2: gypsum, peak 3: hematite, peak 4: quartz and peak 5:

quartz……………………………………………………………………………………..…21

Figure 3.4: Differential diffractogram of the BC MW and BW Ori soil for 35° - 45°

(2θ). Peak 1: gypsum, peak 2: gypsum, peak 3: hematite, peak 4: quartz and peak 5:

quartz……………………………………………………………………………………..…21

Figure 3.5: Diffractogram of the BC MW, BC Ori and BC MW AAO for 5° - 30° (2θ).

Peak 1: gypsum, peak 2: gypsum, peak 3: quartz and peak 4:

gypsum……………………………………………………………………………….……..23

Figure 3.6: Diffractogram of the BC MW, BC MW AAO and BC Ori and for 40° - 45°

(2θ) Peak 1: Hematite, peak 2: quartz and peak 3: quartz…………………………….24

Figure 3.7: Diffractogram of the BC MW, BC Ori and BC MW DC for 5° - 30° (2θ).

Peak 1: gypsum, peak 2: gypsum, peak 3: quartz and peak 4:

gypsum……………………………………………………………………………..……….25

Figure 3.8: Diffractogram of the BC MW, BC MW DC and BC Ori and for 40° - 45°

(2θ). Peak 1: Hematite, peak 2: quartz and peak 3: quartz.3.4 XRD analysis for RS

soil…………………………………………………………………………………………...26

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Figure 3.9: Diffractograms of the RS MW and the RS Ori soils for 5° - 30° (2θ). Peak

1: Gypsum, peak 2: montmorillonite, peak 3: gypsum, peak 4: quartz, and peak 5:

gypsum……………………………………………………………………………………...27

Figure 3.10: Diffractograms of the RS MW intensity divided by the RS Ori intensity

for 5° - 30° (2θ). Peak 1: gypsum, peak 2: gypsum, peak 3: quartz and peak 4:

gypsum……………………………………………………………………………………...28

Figure 3.11: Diffractograms of the RS MW and the RS Ori soils for 35° - 45° (2θ).


quartz………………………………………………………………………………………..29

Figure 3.12: Diffractogram of the RS MW intensity divided by the RS Ori intensity for

35° - 45° (2θ). Peak 1: gypsum, peak 2: gypsum, peak 3: hematite, peak 4: quartz

and peak 5: quartz……………………………………………………………………...….29

Figure 4.1: Photomicrographs (a) BC Ori, (b) RS Ori, (c) BC MW, (d) RS MW, (e) BC

MW (CaCO3) and (f) RS MW (CaCO3)……………………...………………...……...…35

Figure 4.2: Rod-like structures found on BC MW soil………………………………..36

Figure 4.3: White spot-like particles found on both (a) BC MW and (b) RS

MW……………………………………………………………………………….......……..37

Figure 4.4: (a) Clay and iron oxide 2:1 composite at a scale of 1 µm, (b) enlargement

of (a) at a 10 µm scale, (c) a sample of pure iron oxide at 1 µm scale…………….37

Figure 4.5: Hematite crystals on a large cristobalite particle 1 µm scale…………..38

Figure 4.6: (a) Sample of bassanite, (b) pure gypsum crystals. (c) and (d) are

photomicrographs taken of the BC MW soil…………………...………………………39

viii

ABSTRACT

Underground mining on the Witwatersrand has produced a great deal of gold and to

a slighter extent other precious metals needed by both the consumer and industry

alike. This has however resulted in great underground voids which are slowly filling

up to environmental critical levels. As the water level rise within the voids more

reactions between the water and the ores take place, generating a great volume of

acidic mine water. One of the proposed methods of remediating acidic mine water is

to directly apply it as is to the soil with neutralising compounds such as limestone. In

a previous study, a black clay (BC) soil and a red sandy loam (RS) soil were used.

Both soils originate from farmland close to the Brakpan Dam tailings storage facility.

These soils were then treated with acidic mine water from the Western basin of the

Witwatersrand mines and found to have results worth investigating further. A

substantial amount of research has already been done, however, there is still a lot to

be gained on how acidic mine water affects soils at a micro and nano-scale. Acid

ammonium oxalate (AAO) and dithionite citrate (DC) extractions are considered to

be two of the five most widely used methods of extracting different forms of Al, Fe

and Mn. AAO extracted Fe is considered to be “active” Fe, also referred to as the

non-crystalline or amorphous Fe while DC extracted Fe is considered to be “free” Fe

or non-silicate Fe. This provides a basis of what to expect in the XRD and SEM

analyses. Fe, Al, Ca, Mg, Mn and SO4 are the main focus as they can form oxides,

hydroxides or can associate with SO4 in acid mine waters if concentrations permit.

From the AAO and DC extractions it was deduced that the BC soil had more

amorphous Fe whereas the RS soil seemed to have more crystalline Fe. Both soils

showed an increase in Fe content. X-ray diffraction (XRD) was used to differentiate

changes in soil mineralogy caused by acid mine water. All of the diffractograms

generated by the XRD had a distinct background profile which was most probably

due to the high content of poorly crystalline material in the samples. The mine water

treated samples all showed an increase of poorly crystalline material between 5 –

7.5 degrees 2 theta. The hematite peaks formed after treating the soils with acidic

mine water were all short and broad indicating that they are poorly crystalline which

is also an indication of nano-sized particles of Fe oxides. Some of the gypsum

peaks, those found at <30 degrees 2 theta, were found to have formed more

crystalline than those found between 35 – 45 degrees 2 theta. Scanning electron

ix

microscopy (SEM) was employed to assess the variation in clay surface morphology

caused by acid mine water. SEM analyses found that the samples treated with acidic

mine water gained charged nano-sized spheroids on top of the clay. These are

thought to be nano-hematite particles. No typical linear gypsum crystals were found

on any of the samples even though XRD analyses did pick it up. Structures

resembling bassanite, a hemihydrate form of gypsum were however found.

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CHAPTER 1

INTRODUCTION

1.1 Literature review

Underground mining on the Witwatersrand has produced a great deal of gold and to

a lesser extent other precious metals needed by both the consumer and industry

alike. Some of these underground voids have become as vast as 400 Mm3 such as

the Eastern basin. The other two, the Western and Central basins have void volumes

of 43 Mm3 and 280 Mm3 respectively (Akcil and Koldas 2006). These voids were

initially thought not to present a problem at the time partly due to the sheer size of

the voids and the assumption that the water would naturally dissipate into the

environment without a problem. The reality however was that the voids were slowly

filling up to environmental critical levels (ECL). The ECL is defined as “the mine

water level below which, the risk of negative impacts on the shallow economically

exploitable groundwater resources and the surrounding surface water resources is

small” (BKS (Pty) Ltd. 2011). The water was also undergoing various reactions due

to the great surface area within the voids. The reactions between the water and the

ores generated a great volume of acidic mine water which is often referred to as acid

mine drainage (AMD). AMD is defined to be water with a pH of 5.0 or less,

containing sulphates and iron largely as well as various other metals.

Numerous ore minerals have been identified within the conglomerates that contain

contributing sulphide and various heavy metals. The most abundant being pyrite,

uraninite (UO2), brannerite (UO3Ti2O4), arsenopyrite (FeAsS), cobaltite (CoAsS),

galena (PbS), pyrrhotite (FeS), gersdofite (NiAsS) and chromite (FeCr2O4). Pyrite is

by far the greatest contributor to the AMD on the Witerwatersrand and will thus be

the main focus from here on forward (Naicker et al. 2003). The primary ingredients

for acid generation are as follows: (1) sulphide minerals, (2) water or a humid

atmosphere and (3) an oxidant, particularly oxygen from chemical sources or the

atmosphere. Bacteria can also play a major role in accelerating the rate of acid

generation, specifically Acidithiobacillus ferrooxidans which can oxidise pyrite if

conditions are favourable (Akcil and Koldas 2006). The primary factors that

determine the rate of acid generation are: pH; temperature; oxygen content of the

gas phase, if saturation is less than 100%; oxygen concentration in the water phase;

2

degree of saturation with water; chemical activity of Fe3+, chemical activation energy

required to initiate acid generation and bacterial activity.

As table 1.1 shows, the first important reaction is the oxidation of pyrite into

dissolved iron, sulphate and hydrogen ions. The dissolved ions represent an

increase in the total dissolved solids and acidity of the water lowering the pH. If the

surrounding environment is sufficiently oxidising then much of the ferrous iron will

oxidize to ferric iron as per the second equation. At pH values of between 2.3 and

3.5, ferric iron precipitates as ferric hydroxide, leaving little Fe3+ in solution while

simultaneously lowering pH (Coetzee et al. 2007).

Table 1.1: Pyrite reaction with oxygen and water to various ions and finally ferric hydroxide and acid (Coetzee et al. 2007).

1 Pyrite oxidises to form an acidic solution of ferrous iron and sulphate:

4FeS2 (s) + 14O2 (g) + 4H2O (l) → 4Fe2+

(aq) + 8SO4 2-

(aq) + 8H+ (aq)

2 Oxidation of the ferrous ion to ferric ion:

4Fe2+

(aq) + O2 (g) + 4H+ (aq) → 4Fe

3+ (aq) + 2H2O (l)

3 Ferric iron precipitates as ferric hydroxide, producing more acid:

4Fe3+

(aq) + 12H2O (l) → 4Fe(OH)3 (s) + 12H+

(aq)

The Witwatersrand basins each have very high Total Dissolved Solids (TDS) which

consist to a great extent of sulphates followed by calcium carbonates, calcium, iron

manganese and aluminium. The basins do each have quite different amounts of

each dissolved solid. As seen in table 1.2, the western basin has the most TDS, the

lowest pH, highest conductivity, the highest sulphate and iron content of all of the

basins. It is considered the most polluted of the three. The central basin has a similar

pH value to that of the western basin yet has lower TDS, sulphate or iron than any of

the other basins. The eastern basin has an almost neutral pH yet has TDS and other

relevant values between that of the western and central basins.

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Table 1.2: Approximations of the water qualities for the western, central and eastern basin (DWA 2012).

Water Quality Parameters Units Basin within greater Witwatersrand

Western Central Eastern

pH - 3 3.2 7.1

TDS mg/ℓ 5388 3888 4248

Conductivity Ms/m 426 354 367

Acidity/Alkalinity (CaCO3) mg/ℓ 1255 125 541

Aluminium (Al) mg/ℓ - 44 2

Calcium (Ca) mg/ℓ 823 483 421

Chloride (Cl) mg/ℓ - 69 253

Iron (Fe) mg/ℓ 799 177 206

Magnesium (Mg) mg/ℓ - 161 165

Manganese (Mn) mg/ℓ 114 20 6

Sodium (Na) mg/ℓ 243 185 264

Sulphate (SO4) mg/ℓ 3410 2464 2581

Uranium (U) mg/ℓ 0.1 0.2 0.5

For the Western basin in table 1.2, Ca, Fe and SO4 are considerably high relative to

the other elements. Ca come from the chemical weather of calcite [CaCO3], dolomite

[CaMg(CO3)2] and plagioclase [NaAlSi3O8 and CaAl2Si2O8] (Lottermoser 2010). Fe

and SO4 are primarily derived from the oxidation of pyrite as discussed earlier. Other

elements such as Al, Cl, Mg, Mn, Na and U are either leached from rocks or brought

into solution once their minerals are dissolved by the acidity of the water.

1.2 Background to study

Currently there is a lack in the capacity to treat the acid mine water decanted from

the basins as the sheer volume of the problem is just too big to be sustainably

treated with current water treatment methods. Its sustainability is hampered by the

immense cost to run the treatment plants yet it is essential for the future of Gauteng

4

drinkable water supply as well as the surrounding environment. If the water is not

decanted and treated, it will start to surface at lower lying areas and become an

environmental disaster.

Some studies have proposed to first neutralise the acidic mine water and then use it

to irrigate crops. Other proposed methods of remediating acidic mine water is to

directly apply it as is to the soil with neutralising chemicals such as limestone. A lot

of research has already been done on both approaches but there is still a lot to be

gained on how acidic mine water affects soils at a micro and nano-scale.

The method of applying untreated acidic mine water directly to soil is considered a

viable approach as it exploits the natural integral characteristics of the soil. These

characteristics include buffering capacity, natural alkalinity, cation exchange capacity

(CEC) and sorption capacity to aid in the remediation of acidic mine water (Fey et al.

2014).

There are various elements, compounds and minerals associated with AMD. The

main element of focus for the study will be Fe. Al, Ca Mg, Mn and SO4 will however

also be looked at in certain areas where applicable. These elements can form

oxides, hydroxides or can associate with SO4 in acid mine waters if concentrations

permit.

In these conditions, Fe is most abundant in its Fe3+ form. Fe minerals formed usually

formed in acidic mine water conditions include: ferrihydrite [Fe2OH3 · 2H2O], goethite

[FeO(OH)], hematite [Fe8O8], jarosite [KFe3(SO4)2(OH)6], and schwertmannite

[Fe16O16(OH)12-10(SO4)2-3] (Hudson-Edwards et al. 1999).

In the case of Al it is in its only soluble form as Al3+. Gibbsite [Al(OH)3] and kaolinite

[Al2Si2O5(OH)4] are considered to control its solubility in soil but under acidic

conditions the presence of SO4 can dramatically alter these solubilities. Other, less

soluble minerals then control the aqueous geochemistry of Al. The likely candidates

include: alunite [KAl3(SO4)2(OH)6], alunogen [Al2(SO4)3 · 17H2O], basaluminite

[Al4(SO4)(OH)10 · 5H2O] and jurbanite [Al(SO4)(OH) · 5H2O] according to Nordstrom

(1982).

5

Ca in its only soluble form Ca2+, forms gypsum with SO42-. Gypsum is considered to

be the most common sulphate salt found in acidic mine water environments. Its

solubility is not affected by pH (Lottermoser 2010).

Epsomite [MgSO4 ·7H2O] is formed if concentrations of Mg and SO4 are sufficient

and is commonly precipitated in acidic mine waters (Lottermoser 2010).

Birnessite [(Na, Ca, K) x (Mn4+, Mn3+)2O4 · 1.5H2O] is considered to be one of the

most common Mn minerals in soil environments, while manganite [MnO(OH)] is said

to be the most stable and abundant mineral among those in the MnO(OH) group. It

should however be noted that it is challenging to identify Mn minerals in fresh

precipitates since these materials are typically fine-grained, poorly crystalline, and

contain multiple valence states of Mn (Lee et al. 2002).

The soils used in this study were acquired from a previous study which focussed on

simulating a land treatment method for decontaminating metalliferous mine water. A

black clay soil (BC) and a red sandy loam soil (RS) were used. Both soils originate

from farmland close to the Brakpan Dam tailings storage facility. Both soils were said

to be selected based on their unique properties. The BC soil was classified as

melanic A top soil horizon with a strong structure. The soil is considered to be

dominated by smectite clay with a permanent negative charge. It is also considered

to have a high base status, large CEC and a good ability to retain water. The RS soil

was identified as an orthic A top soil horizon which forms part of a Hutton form. It is

considered to be dominated by kaolinite clay. Consequently it also has a much

smaller CEC and have better drainage relative to soil BC. Both soils were found to

successfully retain and sequestrate the metals and salts to a confident extent. The

BC soil was found to be a better option relative to the RS soil due to it being able to

treat great amounts of mine water more effectively (Storm 2014).

The mine water used came from the Western basin of the Witwatersrand mines as it

has the highest Al, Fe and Mn water qualities of the three basins. The water used in

that study was found to have slightly different concentrations than those of table 1.2

and can be found in table 1.3 of which calculations for this study will also be based

on.

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Table 1.3: Water qualities for the western basin (Fey et al. 2014).

pH EC (dS·m-1

)

mg·ℓ-1

Al

Ca

Cl

Fe

Mg

Mn

Na

SO4

Total

3.38 0.520 54 696 31 342 232 64 92 3494 4942

There are no standard methods in literature on how to approach microscale analysis

of soils probably due to the vast differences in soil properties and the almost infinite

numbers of soil compositions. There are however some generic approaches which

have been proposed and proven over the past few decades with a trial and error

approach. X-ray diffraction (XRD) and scanning electron microscopy (SEM) are

currently considered to provide a manner to assess the variation in clay surface

morphology and mineralogy of soils affected by acid mine water. SEM provides

visual information in surface morphology where as XRD provides information on

mineralogy.

In terms of mineralogical and morphological changes, it was expected that various

metal oxides and hydroxides of Al, Fe and Mn could form. It is also important to

know if formed, how much of these newly formed minerals were crystalline and

amorphous. In order to obtain these quantitative answers acid ammonium oxalate

(AAO) and dithionite citrate (DC) were used to make this separation. AAO extracted

Fe is considered to be “active” Fe, also referred to as the non-crystalline or

amorphous Fe. DC extracted Fe is considered to be “free” Fe or non-silicate Fe

according to the Food and Agriculture Organization of the United Nations (1998).

This method was chosen above the similar citrate bicarbonate dithionite (CBD)

extraction method as the DC method has been found to be more effective by other

research scientist (personal communication with Ms. Leushantha Mudaly).

The overarching aim was to understand what would result when Al, Fe and Mn were

administered. This lead to the following two objectives: 1) To determine if any

minerals had formed, what they look like and what had happened to the clay

surfaces using XRD and SEM; 2) To quantitatively determine the distribution

between amorphous and crystalline forms particularly of Fe administered. It was

hypothesised that some of the clay minerals will dissolve and reconstitute with

elements found in the AMD. It is also hypothesised that some of the elements,

particularly Fe will be adsorbed or precipitated onto the mineral surfaces.

7

A visual outline of the study (Fig 1.1) shows the approach as well as what the train of

thought was throughout as there is no formal approach for such work yet.

Figure 1.1: Experimental methodology employed to investigate the effect AMD has on mineral surfaces and mineralogy. Original soil refers to either the BC or RS soil.

8

CHAPTER 2

METAL BALLANCES

2.1 Background and literature review

The AAO extraction method consists of a solution made of ammonium oxalate and

oxalic acid. The oxalic acid lowers and buffers the pH to about 4, while the oxalate

chelates the metals dissolved by the acidic solution. This extraction is effective at

removing organically complexed and amorphous inorganic compounds of Al, Fe and

to a slighter extent Mn. It should also be carried out in the dark to prevent

photodecomposition of the oxalate solution (Carter and Gregorich 2007).

In the DC extraction, the dithionite creates a reducing environment which dissolves

the metallic oxides. The citrate then chelates the dissolved metals and buffers the pH

to roughly 7 units as to avoid the precipitation of FeS compounds. This extraction is

effective at removing organically complexed and amorphous inorganic compounds of

Al, Fe and Mn. Al extractions should however be interpreted cautiously while the

treatment is considered to be particularly effective at extracting “free” Fe from soils. It

is also often used for removing sesquioxide coatings from soils and clays prior to x-

ray analysis (Carter and Gregorich 2007).

AAO is effective at extracting poorly crystalline and non-crystalline aluminosilicates

while DC is said to be much less effective at extracting these compounds. Both

extractions are said to attack crystalline oxide forms of Mn to some extent but the

differences between the two extracts are not easy to interpret. The DC method is

also effective at extracting finely divided minerals including hematite, goethite and

ferrihydrite while AAO only slightly attacks crystalline Al and Fe oxides. AAO does

however dissolve considerable amounts of magnetite while DC does not (Carter and

Gregorich 2007). AAO treatment is also said to preferentially dissolve ferrihydrite

over goethite (Schwertmann et al. 1982).

It is thus expected that the DC extracts will most probably have higher

concentrations relative to oxalate extracts for Al and Fe while Mn could be higher for

the AAO extracts. These results should also give a better indication of the

crystallinity of the minerals.

9

2.2 Methodology

2.2.1 Sample selection

The samples comprised of the BC original soil sample (BC Ori), BC mine water

treated soil sample (BC MW), RS original soil sample (RS Ori) and RS mine water

treated soil sample (RS MW) of which each were replicated three times for statistical

viability.

2.2.2 Sample and solution preparation and procedure

The soil samples were first homogenously mixed after which a generous amount

was finely ground using a mortar and pestle to get the sample as fine as possible.

This would thus remove some variability among the replicates and samples.

The AAO and DC solution extracts were prepared according to the procedures found

in Soil Sampling and Methods of Analysis by Carter and Gregorich (2007). It is

however recommended that more solution (eg. 3-4 times more) is used for the

extract and that a control extract of different concentrations of the element of interest

is also carried out as the solution may become saturated which would require you to

repeat the experiment.

Acid ammonium oxalate extraction:

The AAO solution was prepared completely in the dark. It comprised of a 0.2 M

solution of ammonium oxalate [(NH4)2C2O4·H2O] and a 0.2 M solution of oxalic acid

[H2C2O4·2H2O] and which were mixed in a 1.3:1 ratio respectively. The pH must then

be measured and be around 3, if not then either one of the solutions should be used

to achieve this before carrying on. A sample size of 1.0 g was used instead of the

suggested 0.5 g and 150 mL of solution per extraction instead of the suggested 20

mL per 0.5 g of sample. The samples were then shaken at 150 rpm for 4 hours.

Dithionite citrate extraction:

The dithionite citrate solution was prepared by making a 0.68 M solution of tri-sodium

citrate [Na3C6H5O7·2H2O] and adding 1.3 g of dithionite [Na2S2O4] to each sample

tube before the soil sample was added. A sample size of 0.5 g was used and 80 mL

of solution per extraction instead of the suggested 40 mL per 0.5 g of sample. The

samples were then shaken at 150 rpm for 8 hours (until the soil particles were

bleached white).

10

All of the samples were then centrifuged for 30 min and immediately suction filtered

through 0.45 μm PES membrane filters. Their concentrations, including the blanks,

were then determined a few hours later using the ICP-OES of the Department of

Plant Production and Soil Science of the University of Pretoria by Mr. Charl Hertzog.

2.3 Theoretical soil loadings

Al, Fe and Mn are part of the fabric of soil and naturally quite abundant. The aim, in

part, of the theoretical calculations was to determine if the accumulation of Al, Fe

and Mn were appreciably enough to be analytically separated from the natural (or

background) levels of the soil. Table 2.1 is a summary of how much Al, Fe and Mn

was loaded to the soils based on the respective volume of mine water added, the

amount that leached, and concentration of mine water added. These value were then

divided by the soil mass and a mass concentration loading was calculated. The

leached amounts were based on a cumulative leached volume for that respective

treatments. This was reported by Mr. Ignus Storm during the previous study.

Subtracting the elemental quantities of the soil treated with mine water from the

natural soil should give an indication of what has been theoretically attenuated by the

soil. The BC soil should thus had sorbed 748 mg·kg-1, 4711 mg·kg-1 and 739 mg·kg-1

of Al, Fe and Mn respectively. The RS soil on the other hand should theoretically

have sorbed 422 mg·kg-1, 2832mg·kg-1 and 23.4 mg·kg-1 of Al, Fe and Mn

respectively. The calculated enrichment of Al Fe and Mn was expected to be

analytically detectable.

11

Table 2.1: Summary of soil metal loading (input), leached (removed) and attenuated amounts.

Input Leached

Soil BC RS BC RS

Mine water concentration (mg·ℓ-1

)

Al

54 2 74

Fe

342 50 314

Mn

64 207 544

Al, Fe and Mn loading per treatment (mg · 250g-1

soil)

Al

187 125 0.36 19.3

Fe

1186 790 9.00 82.0

Mn

222 148 37.3 142

Al, Fe and Mn loading per treatment (mg · kg-1

)

Al

750 499 1.44 77.3

Fe

4747 3160 36.0 327

Mn

888 591 149 568

Al, Fe and Mn attenuated by soil (mg · kg-1

)

Soil BC RS

Al 748 422

Fe 4711 2832

Mn 739 23

2.4 Results

Please note that It is expected that there will be some error regarding the values of

the extracts as this part of the study was only carried out after a small fraction

(roughly 2-3 g) of the soil fraction <63 μm had been removed and lost which

removed a significant effect on the amount of <63 μm fraction of the soil. The original

soil sample size was 250 g. The soil samples have thus been slightly compromised

overall but should still be able give some indication of the crystallinity of Fe. A full

summary of the soil data and statistics can be found in appendix A.

12

2.4.1 Al, Fe and Mn extracts of BC soils

In table 2.2, Al concentrations for both the AAO and DC extracts are quite similar for

the BC Ori and BC MW soils. The AAO Al concentrations are however more than

twice the concentration of that of the Al DC concentrations which suggests there is a

fair amount of poorly crystalline and non-crystalline aluminosilicates present. This is

however not for certain as literature suggests Al extract concentrations should be

considered with caution.

The BC Fe concentrations for both extraction methods were found to be high. The

AAO Fe concentrations are lower than that of the DC Fe concentrations which is

expected. There is almost no difference between the AAO and DC extractions for the

BC MW soils which suggests the crystalline Fe attenuated in the soil was largely

poorly crystalline or of small crystal size with fairly large surface to volume ratios

making them more susceptible to extraction. The difference between BC Ori and BC

MW soils also suggested this. The Δ BC (4062 mg·kg-1) and theoretical Fe

enrichment (4711 mg·kg-1) was reasonably close.

AAO Mn extracts were found to be almost double that of the DC and suggests that

roughly half of the Mn extracted from both soil treatments could be in the form of

manganite.

Table 2.2: A summary of the AAO and DC soil concentrations of the BC Ori and BC MW soil (in mg kg-1).

Soil BC Ori BC MW Δ BC

Treatment AAO DC AAO DC AAO DC

Al

1768 851 1631 974 -137 123

Fe

7590 9616 11652 11830 4061 2214

Mn

1983 1052 1851 1126 -132 73

2.4.2 Al, Fe and Mn extracts of RS soils

The AAO and DC RS Ori and RS MW Al concentrations have fairly similar values

which suggests there is not much poorly crystalline and non-crystalline

aluminosilicates present in either treatment.

13

Similar to the BC soil, the DC extracts of the different RS treatments have higher

concentrations than that of the AAO. The DC difference is however greater than that

of the AAO difference unlike the BC soils. It should also be noted that there is a lot

more crystalline than amorphous Fe in both the original soil and the mine water

treated soil. The original soil is naturally red indicating the presence of Fe-oxides.

There is a noticeable decrease in Mn content for RS MW relative to RS Ori to. As

with the BC samples, the RS Ori AAO extract is high and the DC low but these

values are fairly similar for the RS MW extracts.

Table 2.3: A summary of the AAO and DC soil concentrations of the RS Ori and RS

MW soil (in mg·kg-1).

Soil RS Ori RS MW Δ RS

Treatment AAO DC AAO DC AAO DC

Al

1595 1506 1187 1167 -408 -339

Fe

4739 18510 3960 19391 -779 871

Mn

1184 657 326 262 -858 -396

2.5 Summary

There are noticeable error regarding the values of the extracts as some

concentrations after being treated with mine water decreased instead of increasing.

This effect was also more prominent in the RS soils than in the BC soils which could

be due to the fact that the BC soil has more clay than the RS soil. Given that the RS

MW values for AAO and DC had both decreased, it would suggest that a

considerable amount of the Al, Fe and Mn was situated in the <63 μm fraction of the

soil.

The BC soil seems to have a lot more amorphous Fe whereas the RS soil seems to

have a lot more crystalline Fe. Both soils do however show an increase in Fe

content.

Given the circumstances of the data, only minor conclusions can be drawn. For all of

the samples, the Fe content was <1.5 % (kg·kg-1) which is below the detection limit

of an XRD, these soil samples are however representative of the whole soil, or at

least intended to be, given a small fraction of the finer material had already been

14

removed. Fortunately, the samples analysed using the XRD were of the <63 μm

fraction. There was thus a chance that crystalline Fe oxides may have formed

sufficiently to be detected.

15

CHAPTER 3

CHANGES IN SOIL MINERALOGY


XRD is considered to be a crucial method in understanding soil mineralogy and is

the technique most heavily relied on presently. It is however important to understand

the nature of XRD data before any interpretations can be made. A powder

diffractometer is said to be the most applicable to analyse soil mineralogy. Powder

diffractometer results are generally plotted as 2θ degrees on the x axis from 0° to 90°

and X-ray intensity per second on the y axis (Harris and White 2007). Clay minerals

occur between 2° to 30° (2θ) whereas oxides occur at higher degrees 2θ.

Relative peak intensities and d-spacing are then used to identify minerals. Peaks

should then be marked and their corresponding 2θ angles converted to d-spacing

values. To convert to d-spacing values, Bragg’s Law is used as in equation (3.1):

2d Sin θ = n λ (3.1)

Where,

d = d-spacing value in Angstroms (Å)

θ = Radians (2 θ°)

n = 1

λ = Element wavelength (Co-Kα) = 1.789 Å

Given the nature of the experiments performed for this study, there will be various

mixtures of minerals which are said to produce complex XRD patterns. Quartz is

often used to correct data shifts between samples however this should be done with

caution as samples high in and containing different mixtures of clays can shift strong

pronounced peaks of well-defined crystal structures of quartz (Harris and White

2007). These present a great challenge in identifying the different minerals.

Intensity on the y-axis is usually expressed in counts per second and is considered a

relative measurement at most which is affected by various conditions which include

current and voltage at which the X-ray tube is operating as well as the counter

16

efficiency. The relative intensity of a diffraction peak produced by a given set of

atomic planes in a crystal, assuming all atomic planes are equally represented

(random orientation), is dictated by the composition and arrangement of atoms in the

unit cell. Thus, relative intensity does not provide a simple 1:1 index of the mass

fraction of minerals in a mixture (Harris and White 2007). Minerals have multiple d-

spacing at different temperature and chemical treatments and must thus also be

considered if resources permit it.

It is already known that the BC Ori soil is a dark, clay rich soil with swelling

properties. It is thus expected that there will be some clays detected, specifically

swelling clays such as smectites. The RS Ori is however a red well drained soil and

contains far less clay than the BC Ori soil and could already contain Fe-oxide peaks.

It is expected that the BC MW soil might gain enough crystalline Fe-oxides to be

detected but if not it could indicate that the Fe is in fact mostly amorphous. Similarly

for the RS MW soil, there is an expectation to find Fe-oxides. These expectations are

based on conclusions drawn from chapter 2.

XRD is better suited to crystalline materials but poor crystalline materials are also

identified to some extent. These are given as low but broad peak intensities and a

background profile (Stunda et al. 2011).

If there are in fact Fe-oxide peaks detected then these peaks may have different

shapes and sizes. These may either be short and broad or thin and tall. If they are

short and broad then it is an indication of nano-sized particles of Fe oxides according

to Cheng et al. (2010). If the peaks are however thin and tall then the Fe-oxides are

larger more micro-sized.

17

3.2 Methodology


Samples were selected on their relevance toward understanding the changes in

mineralogy and each of which were replicated three times. Sample selection was as

follows: BC Ori, BC MW, BC MW AAO, BC MW DC, RS Ori and RS MW.

The rationale for only subjecting the BC MW AAO and BC MW DC was to see

whether the changes obtained between the XRD of the BC original and the BC mine

water treated soils was due to a metal adsorbed on the surface, particularly Fe-

oxides.

3.2.2 Sample preparation

All the samples used were first placed in a sieve shaker with multiple sieve

diameters with the smallest being a <63 µm sieve. This approach was followed as

opposed to a chemical separation as any contact with a solution risked to influence

the mineral concentrations. The <63 µm fraction consist of both silt and clay. For the

BC soil this fraction consists of course silt, fine silt and clay as 19.8%, 30.8% and

29.2% respectively. While for the RS soil, the values are 24.5%, 10.3% and 9.5% in

the same respective order. These fractions thus represent 78.8% and 44.3% of the

BC and RS soil particle sizes correspondingly (Storm 2014).

Once the BC MW fraction samples were treated with AAO and DC, they were dried

for 72 hours in a 30 °C oven after which they were then powered using a mortar and

pestle.

All of the selected samples powders were then set into XRD specimen holders. This

was done by compressing the samples into the holders until the top of the

specimens were completely flat. This is done to keep all specimens as consistent as

possible and to minimise variables as rough surfaces can have an effect on the

results.

3.2.3 Mineral identification

The samples were prepared according to the standardized Panalytical backloading

system, which provides nearly random distribution of the particles. The samples

were then analysed using a PANalytical X’Pert Pro powder diffractometer in θ–θ

configuration with an X’Celerator detector and variable divergence- and fixed

18

receiving slits with Fe filtered Co-Kα radiation (λ=1.789Å). The data was then

analysed for inconsistencies using Microsoft Excel which originated when the data

was transformed from its original .asc file format to .xls (MS Excel) format. These

inconsistencies are in the form of majorly patterned peak increases which are easily

removed by deducting an established error value. This ‘fix’ is then confirmed by

assessing the smoothness of the data where there are known to not be peaks. The

peaks were identified using the American Mineralogist Crystal Structure Database

with assistance from Ms. Wiebke Grote from the Department of Geology at the

University of Pretoria X-Ray Diffraction unit.

3.3 XRD results and analysis for BC soils

3.3.1 Diffractograms and mineral identification tables of BC Ori and BC MW.

The main minerals identified in the <63 μm fraction of the BC Ori soil were andesine,

microcline, montmorillonite and quartz. The same minerals were again found in the

soil treated with acidic mine water but also gained hematite and gypsum. Andesine

was identified at peak 8, microline at peak 7, montmorillonite at peaks 2 and 5 and

quartz at peak 4 (Fig 3.1). Gypsum has three peaks, these are peak 1, 3 and 6.

Gypsum and quartz both have two lower order peaks between 35 – 45 degrees 2

theta (Fig 3.4). The identification data for figure 3.1 and 3.4 can be found in table 3.1

and 3.4 respectively.

Figure 3.1: Diffractograms of the BC MW and BC Ori for 5° - 30° (2θ). Peak 1:

gypsum, peak 2: montmorillonite, peak 3: gypsum, peak 4: quartz, peak 5:

montmorillonite, peak 6: gypsum, peak 7: microline and peak 8: andesine.

0

3000

6000

9000

12000

15000

5 10 15 20 25 30

Counts

(s

-1)

Position (2θ)

BC MW

BC Ori

2

3

4

5 6 7 8

1

19

The diffractogram data analysis of figure 3.1 (Table 3.1) shows that the d-spacing’s

for the peaks and their respective minerals are not exactly the same, which occurs

when there are other elements in the crystal structure (Harris and White 2007). This

trend does however change as the relative intensity values decrease.

Table 3.1: Diffractogram data analysis of figure 3.1

Peak 2θ (°) Peak d-spacing (Å) Mineral Mineral d (Å)a

1 13.5001 7.61 Gypsum 7.60

2 23.1321 4.46 Montmorillonite 4.45

3 24.2441 4.26 Gypsum 4.28

4 24.2601 4.25 Quartz 4.25


6 27.1801 3.80 Gypsum 3.80

7 27.6201 3.75 Microcline 3.75

8 28.4041 3.64 Andesine 3.64

(a) Mineral d-spacings as reported by American Mineralogist Crystal Structure Database

To isolate the differences between the BC Ori and the BC MW XRD spectrums, the

intensity of BC MW was divided by the intensity of the BC Ori soil (Fig 3.2). This

differential intensity (IBC MW / IBC Ori) shows that only gypsum formed (peak 1, 2 and 4)

for “spectrum” < 30 degrees 2 theta. All the other minerals remained the same.

There is no definite explanation for the decrease in quartz (peak 3), however, it is

thought to be due to the mix of clay present in the sample. There is also a noticeable

increase in background noise (between 5 - 7.5 degrees 2 theta) which is due to an

increase in amorphous material.

20

Figure 3.2: Differential diffractogram of the BC MW intensity divided by the BC Ori

intensity for 5° - 30° (2θ). Peak 1: gypsum, peak 2: gypsum, peak 3: quartz and peak

4: gypsum.

The higher degrees 2 theta (Fig 3.3) also showed prominent peaks appearing after

the mine water treatment. Gypsum (peak 1 and 2) is present again in the BC MW

sample. Peak 3 has a low yet broad profile at where hematite is usually found. There

is a noticeable shift again between quartz (peak 4 and 5) of the treatment differences

(Fig 3.3). These low yet broad peak observations are indicative of poorly crystalline

minerals. The peaks and their relevant information are summarised in table 3.2.

23 24 25 26 27 28

2

3

4

0

1

2

3

4

5

6

7

8

5 10 15 20 25 30

I BC

MW

/ I B

C O

ri

Position (2θ)

1

21

Figure 3.3: Diffractograms of the BC MW soil and the BC Ori soil for 35° - 45° (2θ).


quartz.

The differential diffractogram (Fig 3.4) isolates the gypsum (peak 1 and 2) and

hematite (peak 3) that was formed after the soil was treated with acidic mine water.

Quartz has again shifted slightly.

Figure 3.4: Differential diffractogram of the BC MW and BW Ori soil for 35° - 45°

(2θ). Peak 1: gypsum, peak 2: gypsum, peak 3: hematite, peak 4: quartz and peak 5:

quartz.

0

500

1000

1500

2000

2500

35 37 39 41 43 45

Counts

(s

-1)

Position (2θ)

BC Ori

BC MW

1

2

3

4

5

0.7

0.8

0.9

1

1.1

1.2

1.3

1.4

1.5

1.6

35 36 37 38 39 40 41 42 43 44 45

I MW

/ I

BC

Ori

Position (2θ)

1

2 3

4

5

22

It should be noted that the peak d-spacing and the mineral d-spacing are the same

where they are slightly different at lower degrees 2 theta.

Table 3.2: Diffractogram data analysis of figure 3.3 and figure 3.4.


1 36.2601 2.87 Gypsum 2.87

2 38.9401 2.68 Gypsum 2.68

3 41.5081 2.52 Hematite 2.52

4 42.6841 2.46 Quartz 2.45

5 42.7721 2.45 Quartz 2.45


3.3.2 The influence of ammonium acid oxalate treatment on mine water treated BC

soil.

The aim of treating the sample with AAO was to extract the gypsum and more

importantly the Fe oxides. Poorly crystalline Fe oxides should be soluble in AAO and

it was expected that especially peaks related to ferric iron minerals will either

disappear or become less pronounced. The AAO almost completely dissolved the

gypsum formed), as shown by its peaks all but disappearing (Fig 3.5). All the other

minerals remain unaffected. There also seems to be a shift in the data as peak one

and two seem slightly shifted. The quartz (peak 3) for the BC MW is also broadened

to the left.

23

Figure 3.5: Diffractogram of the BC MW, BC Ori and BC MW AAO for 5° - 30° (2θ).

Peak 1: gypsum, peak 2: gypsum, peak 3: quartz and peak 4: gypsum.

There seems to be a slight decrease of hematite (peak 1) once the BC MW soil was

treated with AAO (Fig 3.6). It therefore seems that the hematite is fairly crystalline.

The hematite peak looks better defined after the oxalate treatment. Given that AAO

is considered not to effectively dissolve crystalline Fe oxides this result is expected

(the y-axis is in counts per second but on different axis and thus not included).

23.5 24.5

3 2

26 27 28 29

4

-3000

-1000

1000

3000

5000

7000

9000

11000

13000

15000

5 10 15 20 25 30

Counts

(s

-1)

Position (2θ)

1

Legend:

BC MW

BC MW AAO

BC Ori

24

Figure 3.6: Diffractogram of the BC MW, BC MW AAO and BC Ori and for 40° - 45° (2θ) Peak 1: Hematite, peak 2: quartz and peak 3: quartz.

3.3.3 The influence of dithionite citrate extraction on mine water treated BC soil.

The aim of treating the sample with DC was to completely extract the Fe oxides and

see if it had had an effect on the soils. Treating the BC MW sample with DC (Fig 3.7)

completely dissolves the formed gypsum (peak 1, 2 and 4). All the other minerals

remain unaffected. Quartz (peak 3) for the BC MW is also again broadened to the

left.

40 41 42 43 44 45

Position (2θ)

BC MW BC MW AAO BC Ori

1

2 3

25

Figure 3.7: Diffractogram of the BC MW, BC Ori and BC MW DC for 5° - 30° (2θ).

Peak 1: gypsum, peak 2: gypsum, peak 3: quartz and peak 4: gypsum.

Similar to the AAO treatment, there is again a slight decrease of hematite (peak 1)

once the BC MW soil is treated with DC (Fig 3.8). The DC treatment effectively

dissolve Fe oxides yet in this result this is not the case. One possible reason for this

could be that the solution was supersaturated and no more Fe oxides could be

further extracted.

23.5 24.5

2 3

26 27 28 29

4

-3000

-1000

1000

3000

5000

7000

9000

11000

13000

15000

5 10 15 20 25 30

Counts

(s

-1)

Position (2θ)

1

Legend:

BC MW

BC MW DC

BC Ori

26

Figure 3.8: Diffractogram of the BC MW, BC MW DC and BC Ori and for 40° - 45°

(2θ). Peak 1: Hematite, peak 2: quartz and peak 3: quartz.3.4 XRD analysis for RS

soil.

40 41 42 43 44 45

Position (2θ)

BC MW BC MW DC BC Ori

1 2

3

27

3.3.4 Diffractograms of RS original soil

The main minerals identified in the RS Ori soils (Fig 3.9) were only montmorillonite

and quartz. The same minerals were again found for the soil treated with acidic mine

water with the addition of gypsum. Montmorillonite (peak 2) and quartz (peak 4) and

gypsum (peak 1, 3 and 5). Gypsum and quartz both have two more peaks on figure

3.10 (35° - 45°). Gypsum and quartz both have two lower order peaks between 35 –

45 degrees 2 theta (Fig 3.9). The identification data for figure 3.9 and 3.10 can be

found in table 3.7 and 3.9 respectively.

Figure 3.9: Diffractograms of the RS MW and the RS Ori soils for 5° - 30° (2θ). Peak

1: Gypsum, peak 2: montmorillonite, peak 3: gypsum, peak 4: quartz, and peak 5:

gypsum.

The diffractogram data analysis of figure 3.7 (Table 3.3) shows that the d-spacing’s

for the peaks and their respective minerals are exactly the same except for

montmorillonite meaning their crystal structures are well formed.

0

3000

6000

9000

12000

15000

5 10 15 20 25 30

Counts

(s

-1)

Position (2θ)

RS Ori

RS MW1

2

3

4

5

28

Table 3.3 Diffractogram data analysis of figure 3.7


1 13.5081 7.60 Gypsum 7.60


3 24.1161 4.28 Gypsum 4.28

4 24.2921 4.25 Quartz 4.25

5 27.2201 3.80 Gypsum 3.80


A similar approach used to isolate the effect the mine water had on the BC soil was

used on the RS soil, by dividing the RS MW data with the RS Ori. Similarly than for

the BC soil, gypsum formed at <30 degrees 2 theta (Fig 3.8) while quartz peaks also

showed. Again there is a decrease in quartz peak which is in fact a shift.

Figure 3.10: Diffractograms of the RS MW intensity divided by the RS Ori intensity

for 5° - 30° (2θ). Peak 1: gypsum, peak 2: gypsum, peak 3: quartz and peak 4:

gypsum.

23 23.5 24 24.5 25 25.5 26 26.5 27 27.5 28

2

3

4

0

1

2

3

4

5

6

7

8

9

5 10 15 20 25 30

Co

un

ts (

s)

Position (2θ)

1

29

Gypsum is present again in the RS MW sample and there is a noticeable shift again

between the quartz peaks in figure 3.11, similar to what happened in the BC soils.

Gypsum peaks, peak one and two, are also less pronounced compared to the same

peaks for the BC MW soil.

Figure 3.11: Diffractograms of the RS MW and the RS Ori soils for 35° - 45° (2θ).


quartz.

The differential diffractogram (Fig 3.10) again emphasise the formation of gypsum

and hematite.

2000

3000

4000

5000

35 37 39 41 43 45

Counts

(s

-1)

Position (2θ)

RS MW

Ori

1 2 3

4

5

0.8

0.9

1

1.1

1.2

1.3

1.4

35 36 37 38 39 40 41 42 43 44 45

I RS

MW

/ I R

S O

ri

Position (2θ)

1 2 3

4

5

30

Figure 3.12: Diffractogram of the RS MW intensity divided by the RS Ori intensity for

35° - 45° (2θ). Peak 1: gypsum, peak 2: gypsum, peak 3: hematite, peak 4: quartz

and peak 5: quartz.

The d-spacing values of the peaks and minerals (Table 3.4) are similar to at least

two decimal places where as some of the BC soil d-spacings were not. An

explanation for this could be that the crystal structures are better formed and also

that the RS soils contain less clay.

Table 3.4 Diffractogram data analysis of figure 3.9 and figure 3.10


1 36.2441 2.87 Gypsum 2.87

2 38.9161 2.68 Gypsum 2.68

3 41.6181 2.52 Hematite 2.52

4 42.6601 2.45 Quartz 2.45

5 42.7401 2.45 Quartz 2.45


3.4 Discussion

The BC Ori soils were found to contain andesine, microcline, montmorillonite and

quartz while the RS Ori soils were found to contain montmorillonite and quartz.

These are however not considered to be the only clays in these soils.

The diffractograms all have a distinct background profile which are most probably

due to the high content of poorly crystalline material in the samples (Stunda et al.

2011).

The hematite peaks formed are also short and broad indicating they are poorly

crystalline (Carlson and Schwermann 1980). This is also an indication of nano-sized

particles of Fe oxides according to Cheng et al. (2010). There was no evidence

found of other Fe-oxides such as ferrihydrite, goethite, jarosite, and schwertmannite

although they may just be under detection limit.

Some of the gypsum peaks, those found at <30 degrees 2 theta, were found to have

formed more crystalline than those found between 35 – 45 degrees 2 theta. The

formation of gypsum is due to the mine water having high concentrations of both

Ca2+ and SO42- which have precipitated (Lottermoser 2010).

31

The two highest intensity gypsum peaks of BC MW’s d-spacing’s were slightly off

and are thought to be a transition interference caused by bassanite before it

becomes gypsum. This is however very speculative but might be due to a

mechanism hypothesised by Wang et al. (2012) who reported a multistep process

which precipitated gypsum via hemihydrate using Ca2+ and SO42- as starting solution.

If this is indeed true, the bassanite is so little that it cannot be picked up by the XRD

instrument as a minimum of 1.5 % of it is needed mass/mass. There is also no trace

of any bassanite in any of the other samples using XRD.

All of the RS MW gypsum d-spacings were found to be exact to those in literature

which suggests their crystalline structures are well formed. In contrast to the BC soil

this could suggest that gypsum could have formed via a mass precipitation.

32

CHAPTER 4

CHANGES IN SOIL SURFACE MORPHOLOGY


A fundamental part of the study was to establish whether morphological changes

occurred on the clay surfaces due to the treatment with acidic mine water. SEM is

considered to be uniquely suited for this challenge as it magnifies the surface of the

clays, giving a three-dimensional view of the surface with great depth focus.

Fundamentally, the process works by scanning the sample with a focused beam of

electrons. The electrons interact with atoms in the sample, producing numerous

signals which are then again received by the instrument and used to produce an

image. The signals are unique to different elements and can give compositional

information about specific points or areas of interest if the machine has the additional

hardware and the points/areas of interest are of great enough size (Bohor and

Hughes 1970).

It is however necessary for clays to either be coated with a thin metallic or carbon

coating. This is applied in a vacuum evaporator. The coating is said to prevent a

build-up of electrons on the surfaces by conducting away static electricity and are

usually between 2-3 nm thick (Frost et al. 2002). There are currently no known

universal standard methods of preparing and analysing soil clays but there are

however various successful attempts by scientists in the past with specific clay

materials.

It is expected that there will be some gypsum crystals formed and perhaps

derivatives thereof (BC MW samples). There is also a good chance of hematite

crystals (in both BC MW and RS MW) having formed, more specifically poorly

crystalline forms thereof as found by the XRD.

4.2 Methodology


Both the BC Ori and the RS Ori soils were chosen to serve as the controls. Their

acidic mine water treated counterparts, BC MW and RS MW respectively, to see how

the acidic mine water had affected the clay surfaces. Finally, the same soils treated

with acidic mine water and CaCO3 were then also added to see what affect the

33

CaCO3 had had on the surface of the clays as the water was neutral. All six of the

samples were replicated three times for consistency.

4.2.2 Sample preparation and analysis

All the samples used were firstly placed in a sieve shaker with multiple sieve

diameters in, the smallest being a <63 µm sieve. The samples were purposefully

extracted to 63 µm and smaller to see what it would look like under the SEM. The

<63 µm extracts were then kept separately to be used further for further analysis and

comprised of fine quartz particles, silt and clay. To get the samples to almost pure

clay (<2 µm) without using chemical treatments is nearly impossible. Given that

sample morphological and amorphous changes want to be detected a

water/chemical treatment of any sort would compromise the integrity of the samples

and was thus not considered. A degree of trial-and-error is said to be required as

SEM clay analysis is still a new field and thus does not have defined guidelines on

how to approach it as mentioned.

To analyse the samples in the SEM, a powder mount was used. The powder mount

was done by pressing the sample onto conducting carbon adhesive tape and then

coating it with carbon to insure that the low conducting clay does not build up charge

as this could blur the image of the SEM. An epoxy coating was considered to smooth

the surface of the clay topography but was inevitably decided against as it would not

make a significant difference due to the particle sizes analysed being small enough.

An epoxy coating would also take weeks to dry which given circumstances was not

an option.

The instruments used were a Joel JSM 5800LV SEM for low resolution imaging and

a Zeiss ULTRA Plus FESEM for high resolution imaging. Field emission scanning

electron microscope (FESEM) enables high resolution electron imaging with low

acceleration voltages which makes it possible to analyse also delicate biological

samples and nanostructures.

34

4.3 Results of photomicrographs of BC and RS selected samples

All samples were replicated three times and all were found to have the same visual

appearance. Both the original BC and RS soils were found to have no “white spots”

on them as seen in figure 4.1 (a) and (b) respectively. As soon as the samples were

treated with acidic mine water, both BC and RS soils gained a white spot-like

appearances on top of the clay as seen in figure 4.1 (c) and (d). Figure 4.1 (e) and (f)

are the BC and RS soils treated with the acidic mine water and CaCO3 respectively.

They both have a similar spot-like appearance as in figure 4.1 (c) and (d). These

white spots are caused by a charge difference relative to the surrounding particles

due to the nature of the machine.

These spot-like appearances are considered far too small to do an elemental

analysis using Energy-dispersive X-ray spectroscopy (EDS) on them (verbally

communicated by SEM technician) and would give an inaccurate representation of

the spot composition.

35

Figure 4.1: Photomicrographs (a) BC Ori, (b) RS Ori, (c) BC MW, (d) RS MW, (e) BC

MW (CaCO3) and (f) RS MW (CaCO3).

a b

c d

e f

200nm

200nm

200nm 200nm

200nm

200nm

36

The structures within the red rectangles in figure 4.2 were found to have formed on

the BC MW soil but not on any of the other soils.

Figure 4.2: Rod-like structures found on BC MW soil.

1µm

37

4.4 Discussion

Figure 4.3: White spot-like particles found on both (a) BC MW and (b) RS MW.

Due to the high iron content in the acidic mine water used to treat the soils it was

hypothesised that the iron had sorbed/precipitated onto the surface of the clay. In

study conducted by Oliveira et al. (2003) on clay-iron oxide composites (Fig 4.4 a)

the clay surface resulted. Figure 4.4 (b) is magnification of the white square in figure

4.4 (a). Figure 4.4 (c) is a sample of pure iron oxide. A much greater size than that of

the results obtained which is due to the higher temperature used to synthesise the

iron oxides which was at 70 °C. It is hypothesised that the white spot-like particles

(Fig 4.3) are likely to be small iron oxides, specifically nano hematite particles. The

smaller size could be due to a slow formation as the temperature of the samples

never rose above 30 °C during the experiment which produced the samples nor did

they during this study.

Figure 4.4: (a) Clay and iron oxide 2:1 composite at a scale of 1 µm, (b) enlargement

of (a) at a 10 µm scale, (c) a sample of pure iron oxide at 1 µm scale (Oliveira et al.

2003).

a b

200nm 200nm

38

A sample of hematite crystals (Fig 4.5) attached to a large cristobalite (a polymorph

of quartz) particles, according to a study conducted by Scheidegger et al. (1993).

These particles have similar morphologies to that of the BC MW and RS MW soils

but are at a greater size scale.

Figure 4.5: Hematite crystals on a large cristobalite particle 1 µm scale (Scheidegger

et al. 1993).

The precipitation of gypsum from solution is considered to be a single phase direct

precipitation, however Wang et al. (2012) has reported a multistep process and Van-

Driessche et al. (2012) has synthesised gypsum from a solution of 150 mM solution

of CaSO4 at room temperature and pressure. Wang and Meldrum (2012) observed

an aggregation-based mechanism, where the hemihydrate (bassanite) nanorods

aggregate to form rod-like structures which subsequently recrystallize to gypsum.

The rod-like structures found on only the BC MW soils (Fig 4.5 c and d) have a

similar appearance to that of bassanite (Fig 4.5 a) while Fig 4.5 b is a sample of

gypsum crystals (Wang and Meldrum 2012).

39

Figure 4.6: (a) Sample of bassanite, (b) pure gypsum crystals (Wang and Meldrum

2012). (c) and (d) are photomicrographs taken of the BC MW soil.

a b c

d

500nm

1 µm

40

CHAPTER 5

SUMMARY AND CONCLUSIONS

As expected, there are noticeable errors regarding the values of the extracts as

some concentrations after being treated with mine water decreased instead of

increasing. This effect was also more prominent in the RS soils than in the BC soils

which could be due to the fact that the BC soil had more clay than the RS soil. Given

that the RS MW values for AAO and DC had both decreased, it would suggest that a

considerable amount of the Al, Fe and Mn was situated in the <63 μm fraction of the

soil. The BC soil seems to have a lot more amorphous Fe whereas the RS soil

seems to have more crystalline Fe , which could be due the removal of colloidal

particles. Both soils showed an increase in Fe content.

All of the samples analysed using the XRD were of t<63 μm fraction of the soil.

There was thus a good chance that crystalline Fe-oxides formed would be of

sufficient concentration to be detected. Hematite was the only Fe-oxide found in the

soil samples treated with mine water. All of these had low broad peaks indicating that

they were poorly crystalline and of nanoparticle size. Gypsum was also identified in

the soil samples treated with mine water. Gypsum peaks <30 degrees 2 theta were

found to be narrower and of higher intensity than peaks than peaks found at >30

degrees 2 theta. The two highest intensity gypsum peaks of BC MW’s d-spacing’s

were slightly off and are thought to be a transition interference caused by bassanite

before it becomes gypsum or due to impurities within the gypsum crystal structure.

This is however this is very speculative. In contrast, all of the RS MW gypsum d-

spacings were found to be exact to those in literature which suggests their crystalline

structures are well formed.

For SEM, both the original BC and RS soils were found to have no white spots on

them. The samples treated with acidic mine water gained white nano-sized particles

on top of the clay which are thought to be nano-hematite particles. No gypsum

structures were found on any of the samples but structures resembling deviations

thereof (BC MW only), such as bassanite, a hemihydrate mineral of gypsum were.

It is thus suggested that various amorphous Fe-oxides may have formed in smaller

than detectible concentrations, however enough nano-sized poorly crystalline

hematite has formed to concentrations detectable by both XRD and SEM.

41

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42

Food and Agriculture Organization of the United Nations. 1998. World Reference

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alluvium contaminated by metal mining in the Rio Tinto area, southwest

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581

Lottermoser BG. 2010. Mine Wastes: Characterization, Treatment and

Environmental Impacts. Springer Science & Business Media.

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43

Oliveira LCA, Rios RVRA, Fabris JD, Sapag K, Garg VK, Lago RM. 2003. Clay–iron

oxide magnetic composites for the adsorption of contaminants in water.

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44

APPENDIX A

BC Ori soil

Raw data

Average

Al Fe Mn

Al Fe Mn

mg/l

mg/l

BC DC 1 52.3 320 36.7

BC DC 5.3 60.2 6.60

BC DC 2 50.5 306 37.4

BC AAO 11.8 50.7 13.2

BC DC 3 49.7 306 35.3 BC AAO 1 178 500 94.4

Standard deviation

BC AAO 2 148 430 88.8

Al Fe Mn

BC AAO 3 106 328 61.1

mg/l

BC DC 0.10 0.53 0.11

(Raw data) - (Blanks)

BC AAO 0.53 1.91 0.39

Al Fe Mn mg/l

Coefficient of variance

BC DC 1 5.35 60.7 6.52

Al Fe Mn

BC DC 2 5.23 60.3 6.72

%

BC DC 3 5.42 59.7 6.55

BC DC 1.79 0.88 1.63

BC AAO 1 12.3 52.5 13.6

BC AAO 4.47 3.77 2.95

BC AAO 2 11.9 50.8 13.4 BC AAO 3 11.3 48.7 12.8

Average

Al Fe Mn

(Raw data) - (blanks)

mg/kg

Al Fe Mn

BC DC 851 9616 1053

mg/kg

BC AAO 1769 7591 1984

BC DC 1 854 9695 1040 BC DC 2 835 9629 1073

Standard deviation

BC DC 3 864 9524 1046

Al Fe Mn

BC AAO 1 1842 7861 2030

mg/kg

BC AAO 2 1777 7614 2002

BC DC 14.9 86.4 17.5

BC AAO 3 1686 7297 1919

BC AAO 78.2 283 57.6

Blank average


Al Fe Mn

Al Fe Mn

mg/l

%

AAO B Ave 0.44 0.48 0.10

BC DC 1.75 0.90 1.66

DC B Ave 0.48 1.08 0.10

BC AAO 4.42 3.73 2.91

45

BC MW soil

Raw data

Average

Al Fe Mn

Al Fe Mn

mg/l

mg/l

BC DC 1 6.39 73.7 6.91

BC DC 6.10 74.1 7.05

BC DC 2 6.74 76.3 7.32

BC AAO 10.9 77.8 12.4

BC DC 3 6.61 75.6 7.23

BC AAO 1 12.1 80.3 12.9

Standard deviation

BC AAO 2 9.79 73.3 11.6

Al Fe Mn

BC AAO 3 12.1 81.2 12.9

mg/l

BC DC 0.18 1.35 0.21


BC AAO 1.33 4.34 0.74

Al Fe Mn

mg/l


BC DC 1 5.91 72.6 6.81

Al Fe Mn

BC DC 2 6.26 75.2 7.22

%

BC DC 3 6.14 74.5 7.13

BC DC 2.93 1.82 3.04

BC AAO 1 11.6 79.8 12.8

BC AAO 12.2 5.58 6.00

BC AAO 2 9.35 72.8 11.5

BC AAO 3 11.7 80.7 12.8

Average

Al Fe Mn


mg/kg

Al Fe Mn

BC DC 974 11830 1126

mg/kg

BC AAO 1631 11652 1851

BC DC 1 944 11603 1088

BC DC 2 997 11990 1150

Standard deviation

BC DC 3 980 11896 1138

Al Fe Mn

BC AAO 1 1740 11970 1916

mg/kg

BC AAO 2 1401 10900 1723

BC DC 27.3 202 33.0

BC AAO 3 1751 12079 1914

BC AAO 198 648 111

Blank average


Al Fe Mn

Al Fe Mn

mg/l

%

AAO B Ave 0.44 0.48 0.10

BC DC 2.80 1.70 2.93

DC B Ave 0.48 1.08 0.10

BC AAO 12.2 5.56 5.99

46

RS Ori soil

Raw data

Average

Al Fe Mn

Al Fe Mn

mg/l

mg/l

RS DC 1 9.05 111 3.84

RS DC 9.43 116 4.12

RS DC 2 10.2 119 3.91

RS AAO 10.6 31.6 7.90

RS DC 3 10.5 121 4.89

RS AAO 1 12.1 33.5 8.41

Standard deviation

RS AAO 2 11.1 32.3 8.09

Al Fe Mn

RS AAO 3 10.1 30.5 7.48

mg/l

RS DC 0.77 5.36 0.59


RS AAO 0.98 1.48 0.47

Al Fe Mn

mg/l


RS DC 1 8.57 110 3.74

Al Fe Mn

RS DC 2 9.67 118 3.81

%

RS DC 3 10.1 120 4.79

RS DC 8.13 4.62 14.3

RS AAO 1 11.6 33.0 8.31

RS AAO 9.21 4.68 5.97

RS AAO 2 10.6 31.8 7.99

RS AAO 3 9.66 30.1 7.38

Average

Al Fe Mn


mg/kg

Al Fe Mn

RS DC 1506 18510 657

mg/kg

RS AAO 1594 4738 1183

RS DC 1 1366 17517 596

RS DC 2 1543 18766 608

Standard deviation

RS DC 3 1609 19246 767

Al Fe Mn

RS AAO 1 1740 4941 1244

mg/kg

RS AAO 2 1595 4767 1198

RS DC 125 893 95.4

RS AAO 3 1449 4508 1107

RS AAO 146 218 69.9

Blank average


Al Fe Mn

Al Fe Mn

mg/l

%

AAO B Ave 0.44 0.48 0.10

RS DC 8.33 4.82 14.5

DC B Ave 0.48 1.08 0.10

RS AAO 9.14 4.60 5.90

47

RS MW soil

Raw data

Average

Al Fe Mn

Al Fe Mn

mg/l

mg/l

RS DC 1 7.76 124 1.76

RS DC 7.30 121 1.64

RS DC 2 8.00 124 1.65

RS AAO 7.95 26.5 2.18

RS DC 3 7.59 119 1.78

RS AAO 1 8.50 26.1 2.38

Standard deviation

RS AAO 2 8.91 28.2 2.29

Al Fe Mn

RS AAO 3 7.76 26.7 2.18

mg/l

RS DC 0.20 2.89 0.08


RS AAO 0.59 1.07 0.10

Al Fe Mn

mg/l


RS DC 1 7.28 123 1.66

Al Fe Mn

RS DC 2 7.51 123 1.55

%

RS DC 3 7.11 118 1.70

RS DC 2.78 2.39 4.65

RS AAO 1 8.06 25.7 2.28

RS AAO 7.39 4.02 4.40

RS AAO 2 8.47 27.7 2.19 RS AAO 3 7.31 26.2 2.08

Average

Al Fe Mn


mg/kg

Al Fe Mn

RS DC 1166 19381 262

mg/Kg

RS AAO 1187 3960 326

RS DC 1 1163 19615 265

RS DC 2 1201 19687 248

Standard deviation

RS DC 3 1136 18840 272

Al Fe Mn

RS AAO 1 1206 3836 341

mg/kg

RS AAO 2 1266 4139 327

RS DC 32.9 469 12.1

RS AAO 3 1090 3904 311

RS AAO 89.4 159 15.0

Blank average


Al Fe Mn

Al Fe Mn

mg/l

%

AAO B Ave 0.44 0.48 0.10

RS DC 2.82 2.42 4.61

DC B Ave 0.48 1.08 0.10

RS AAO 7.53 4.00 4.60

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