Accepted Manuscript

Title: Modeling isotherms, kinetics and understanding the mechanism ofphosphate adsorption onto a solid waste: Ground Burnt Patties

Authors: Prangya Ranjan Rout, Puspendu Bhunia, Rajesh Roshan Dash

PII: S2213-3437(14)00090-6

DOI: 10.1016/j.jece.2014.04.017

Reference: JECE 336

To appear in: Journal of Environmental Chemical Engineering

Received date: 10 February 2014Revised date: 23 April 2014Accepted date: 24 April 2014

Please cite this article as: Rout Prangya Ranjan, Bhunia Puspendu, Dash Rajesh Roshan, Modelingisotherms, kinetics and understanding the mechanism of phosphate adsorption onto a solid waste:Ground Burnt Patties, Journal of Environmental Chemical Engineering (2014), doi:10.1016/j.jece.2014.04.017

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Modelling isotherms, kinetics and understanding the mechanism of phosphate

adsorption onto a solid waste: Ground Burnt Patties

Prangya Ranjan Rout, Puspendu Bhunia* and Rajesh Roshan Dash

Department of Civil Engineering, School of Infrastructure, Indian Institute of Technology

Bhubaneswar, Odisha, India 751 013

*Corresponding Author

Email ID: pbhunia@iitbbs.ac.in

Tel: +91-674-2306355; fax: +91-674-2301983

Department of Civil Engineering, School of Infrastructure, Indian Institute of Technology

Bhubaneswar, Odisha, India 751 013

ABSTRACT

The objective of the present study was to investigate the adsorption behaviour of Grounded

Burnt Patties (GBP), a solid waste generated from cooking fuel used in earthen stoves, as an

adsorbent for phosphate removal from aqueous solution. The characterization of adsorbent was

done by Proton Induced X- ray Emission (PIXE), and Proton Induced γ- ray Emission (PIGE)

methods and the adsorption mechanisms by Fourier Transferred Infra- Red spectroscopy (FTIR),

Abbreviations:

GBP: Grounded Burnt Patties

PIXE: Proton Induced X- ray Emission

PIGE: Proton Induced γ- ray Emission

GBPT: Grounded Burnt Patties Treated

XRD: X-Ray Diffraction

FTIR: Fourier Transferred Infra- Red spectroscopy

SEM: Scanning Electron Microscopy

ZPC: Zero Point Charge

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X-Ray Diffraction (XRD) and Scanning Electron Microscopy (SEM) analysis. The effects of

adsorbent dose, contact time, initial solution concentration, agitation, etc. on the uptake of

phosphate by the adsorbent in batch mode were examined. The equilibrium data were fitted to

different types of adsorption isotherms and kinetic models. Freundlich isotherm model and pseudo

second order kinetic model illustrated best fit to the data. The favorability and spontaneity of the

adsorption process are established by the values of experimentally calculated parameters such as

separation factor (RL), 0.03, Freundlich exponent (n), 3.57 and Gibb’s free energy change (ΔG°), -

1.32 kJ/mol. The presence of coexisting anions showed no competing effects on phosphate removal

efficiency. Breakthrough curves obtained from column study revealed that the lower flow rate and

higher bed heights results in longer column saturation time. The results of this study suggested that

GBP can be used as a low cost highly efficient adsorbent for phosphate removal from aqueous

solution.

Keywords: Grounded burnt patties, Phosphate removal, Aqueous solution, Adsorption isotherm,

Adsorption kinetics.

1. Introduction

Phosphorous is an essential element for growth of microorganisms, plants and animals in

most of the ecosystems, thus known as nutrient or biostimulant. Typically in aqueous environment,

phosphorous exists in the form of orthophosphate, polyphosphates, pyrophosphate, organic

phosphate esters and organic phosphonates, and all these forms could be hydrolyzed to

orthophosphate and subsequently could be utilized by bacteria, algae, and plants. On the other hand,

phosphorous is the limiting nutrient and sustained inputs of phosphorous (more than 1 mg/L) to

aquatic environments lead to increased rates of eutrophication a widespread problem throughout

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the world affecting the quality of domestic, industrial, agricultural and recreational water resources

[1, 2]. Some of the ill effects of eutrophication include low dissolved oxygen, fish kills, murky

water, and depletion of desirable flora and fauna [3]. In addition, presence of nutrients in

wastewater stimulate the activity of a harmful microbe known as Pfisteria and accelerate the

production of microsystin, an environmental toxin that poison aquatic animals and can cause

hepatocellular carcinoma in humans [4]. In order to prevent these problems phosphorous removal

from wastewater is highly desirable before discharging. The World Health Organization (WHO)

recommended a maximum discharge limit of phosphorous as 0.5 to 1.0 mg/L [5]. Therefore, in the

current scenario, more and more stringent regulatory limits of phosphate discharge have been set by

many nations and regions worldwide.

The primary input of phosphorous into the water bodies occurs by the discharge of

municipal wastewater and industrial wastewater from detergent manufacturing and metal coating

industries. Usually the concentration of phosphorous in municipal wastewater varies in the range of

3-15 mg/L, out of which approximately 3 mg/L forms by the breakdown of protein wastes and the

remaining comes through the use of detergents [2]. Industrial wastewater may contain phosphorous

in the concentration well in the excess of 10 mg/L [6]. So as to meet the effluent discharge

standards, in many regions both municipal and industrial wastewater is treated before discharged to

the nearest watercourses. But the phosphorous concentration in the secondary effluents of treated

municipal and industrial wastewater still remains more than 2 mg/L, which is well above the

recommended maximum discharge limit of phosphorous by WHO. Therefore, secondary effluents

containing substantial amounts of phosphorous needs to be treated effectively by other

technologies. On-site systems using media filters have emerged as a promising solution for

secondary effluent treatment and are of particular interest for nutrient removal [7]. The main

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mechanism of phosphorous removal in filter bed systems is generally through adsorption and

precipitation within the filter material. Absorption is one of the promising approaches for the

removal of phosphate and could be easily applied to small-scale treatment facilities or wastewater

with relatively low phosphate concentrations without yielding harmful by products [8]. The

competency of the adsorption process depends upon the adsorbent materials, which should have the

property of low cost, easy availability and high uptake capacity [9]. Thus, at the time of designing

on-site systems meant for phosphorous removal, appropriate selection of filter media (adsorbent)

plays a vital role [10]. The most gainful materials are usually found among various waste materials,

by-products and among natural minerals [11]. Till date diverse materials from industrial wastes like

red mud, activated alumina, Fe, Al, Mg, Ca and Si based substrates, fly ash, blast furnace slag etc.,

natural materials like various soils, laterite, dolomite, andensite, granite, etc, other waste materials

like refuse concrete, waste paper, mussel shell, limestone waste, used bricks etc. and from

agricultural wastes like coir pith, date palm fibre etc. have been reported as appropriate filter media

that have been used to reduce phosphorous concentrations in the effluents efficiently. [12-21, 11,

22-24].

Use of waste materials as adsorbents for phosphorous removal sounds promising. Moreover,

exploiting the adsorption capacity of abundantly available local waste materials can be undoubtedly

adjudged as the most cost-effective and environmental friendly technology. By doing so either an

increasing toxic threat to the environment can be prevented or waste disposal techniques can be

streamlined making them more affordable. This study is thus relevant as it involves the evaluation

of phosphate adsorption capacity of burnt patties, a solid waste generated after burning patties in

earthen cooking stoves or chullahs in households for cooking. Generally patties are prepared by

mixing coal dust or coal cinder with a definite proportion of soil and cow dung and by drying them

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in the sun before use. Coal cinder is burned or partly burned coal, which is not reduced to ash and

that can burn further but without flame. In city slums and rural areas of developing countries like

India, the majority of the populations are not able to get cooking gas and electricity supply. The

other sources of fuel such as wood and coal are in difficult supply due to depletion of plant biomass.

Petroleum products such as kerosene is in short supply and the price is unaffordable. Therefore the

remaining alternative of fuel source for rural masses is cow dung cakes and for city masses is

patties. More over restricting the use of LPG (Liquefied Petroleum Gas) cylinder by India

government is compelling small food stalls, dhabas and hotels of city areas to switch over to patties

as a source of cooking fuel. Increased use of the patties leads to the generation of a large amount of

solid wastes which are casually dumped triggering environmental and hygiene problems like

mosquito breeding. On the other hand the patties are rich in aluminum, iron, calcium and

magnesium and these compositions add to the possibility of patties to be used as a phosphorous

adsorbent. Exploring phosphorous removal ability of patties can address two major issues like the

phosphorous removal and waste recycling.

The main objective of this study was to inspect the use of Grounded Burnt Patties (GBP) as

an adsorbent for the adsorption of phosphorous. Surface characteristics and physico-chemical

properties of the patties have been investigated. Kinetic and isotherm models have been analyzed

and presented to envisage the phosphorous sorption characteristics of the patties. Although very few

literatures have been observed to report phosphorous adsorptive behavior of coal cinder, so far to

the best of our knowledge, no study has been reported to evaluate the phosphorous adsorption

potential of GBP.

2. Materials and methods

2.1. Adsorbent

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Burnt patties were collected from different waste disposal sites of Bhubaneswar city,

Odisha, India. The collected patties are grounded, sieved, washed several times with distilled water

to remove surface adhered particles, soluble materials and dried in hot-air oven at 100º C for

overnight. The Grounded Burnt Patties (GBP) of particle size less than 0.3 mm were used in the

adsorption study. The properties and average chemical composition of the material were given in

Table 1. For the composition analysis of GBP, highly sensitive multi component analytical methods

like Proton Induced X- ray Emission (PIXE) and Proton Induced γ- ray Emission (PIGE) were used

2.2. Aqueous solution of phosphate

Synthetic phosphate stock solutions of 1000 mg/L were prepared by dissolving defined

amount of analytical grade anhydrous potassium dihydrogen phosphate (KH2PO4) in distilled water.

The stock solution was further diluted with distilled water to get the desired concentrations of

experimental working solution. This synthetic phosphate solution was used for optimizing different

adsorption parameters in both batch and column studies.

2.3. Analytical methods

The chemical compositions of GBP were analyzed with the help of Proton Induced X- ray

Emission (PIXE) and Proton Induced γ- ray Emission (PIGE). Measurements were carried out using

the 2 MeV proton beam obtained from 3 MV Tandem pelletron accelerator. In order to get better

resolution and clarity of results, PIXE was done for analysis of elements with atomic number as low

as 12 (low Z elements) and PIGE was done for analysis of high Z elements following the method as

described by Kennedy et al. (1999) [25].

The adsorbents before adsorption and after adsorption were termed as GBP and GBPT,

respectively, and were characterized by Fourier Transferred Infra- Red spectroscopy (FTIR), X-Ray

Diffraction (XRD) and Scanning Electron Microscopy (SEM). The FTIR spectra were recorded on

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Bruker ALPHA-FTIR Spectrophotometer. Samples were prepared in KBr disks (2 mg sample in

200 mg KBr). The scanning range was 500-4000 cm-1

and the resolution was 2 cm-1

with a scanning

rate of 16. The XRD analysis was performed in a X’pert PW 3040/00 (PANalytical) diffractometer

at room temperature, with Cu Kα radiation at a scan speed range of 3º/min, step size of 1 sec, 30

KV voltage and 20 mAmp current. The XRD patterns were recorded in the 2θ range of 20-80º.

Surface microstructure and the morphological characteristics of the adsorbent before and after

adsorption were evaluated using a scanning electron microscope, (SEM, JOEL JSM-JAPAN) with

an accelerating voltage of 15 KV and a maximum magnification of 1000X. The specific surface

area of BGP was determined by the BET nitrogen gas sorption method using a specific surface area

analyzer (Gemini2360, Micromeritics, USA)

Phosphate was analyzed by the vanado molybdo phosphoric acid method, 4500-P according to

standard methods for the examination of water and wastewater [26]. Vanadate-molybdate reagent of

1 mL and 0.5 mL of distilled water were added to 3.5 mL of filtered sample. The mixed solution

was analyzed after 10 min with a Perkin Elmer Lambd-25 UV/VIS spectrophotometer at the

detection wavelength of 470 nm. pHzpc of the adsorbent was measured following a slightly modified

pH drift method as described by L.A. Rodrigues and M.L.C.P. da Silva (2010) [8]. To a series of

Elnermayer flasks 100 mL of 0.01M NaCl solution was transfered. The pH was adjusted to a value

between 2 and 14 by adding 0.1 M HCl or 0.1 M NaOH solutions. Then, 4 g of GBP was added to

each flask and the final pH was measured after 48 h under agitation at room temperature. The

difference between the initial (pHi) and the final pH (pHf) values (ΔpH= pHi − pHf) was plotted

against the initial pH (pHi). The pHzpc is the point of intersection of the resulting curve at which

pH=0.

2.4. Batch study

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To determine maximum adsorption capacity, adsorption kinetics, adsorption isotherms and

to establish the phosphate removal pattern, batch adsorption study is very much important. The

effects of agitation speed, adsorbent dose, settling time and particle size of adsorbent on adsorption

process was carried out prior to main experiments. The effects of contact time, pH, temperature,

initial concentrations of phosphate etc. were also investigated. For the determination of the effect of

various parameters, a known quantity of adsorbent (0.25-10 g) with 100 mL of the phosphate

solution of concentrations varying from1 – 20 mg/L were taken in 250 mL Erlenmeyer flasks and

agitated at (100-200 rpm) at a temperature range of 15 °C - 45 °C for a known period of time i.e., 5

to 60 min. After completion of adsorption process a pre-determined settling period of 1 h was

allowed and filtration of supernatant was done using 0.45 μm filter paper, prior to phosphate

analysis.

2.5. Effect of pH and Mechanism of phosphate removal

The effect of pH on phosphate removal was carried out by adjusting pH of the phosphate

solution from 2 to12 using 1 M HCl or 1 M NaOH prior to the addition of adsorbent. To each of the

Elnermayer flask containing 100 mL of phosphate solution (15 mg/L) with varying pH (2 to12), 4 g

of GBP was added. The flasks were agitated at 150 rpm in orbital shaker maintaining temperature

of 25 ± 2 °C.

2.6. Effect of initial phosphate concentration and Adsorption isotherm

The effect of initial phosphate concentration on adsorption process was determined by

adding 4 g of GBP to 100 mL phosphate solution of various concentrations (1, 5, 10, 15 and 20

mg/L) in 250 mL Erlenmeyer flasks for 60 min. The flasks were agitated at 150 rpm in temperature

controlled orbital shaker maintained at a temperature of 25 ± 2 °C and neutral pH was maintained

for the solution. After equilibrium time the obtained phosphate adsorption data were fitted to the

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Langmuir and Freundlich isotherm models. Though 50 mg/L is comparatively higher concentration

than that of the concentrations of domestic wastewater (3-15 mg/L) [3], higher phosphate

concentrations are required to establish a good adsorption isotherm.

2.7. Effect of contact time and Adsorption kinetics

To determine the effect of contact time on the adsorptive removal of phosphate, the batch

studies were conducted for a series of time intervals until equilibrium was achieved. 4 g of GBP

was dosed into 250 ml Erlenmeyer flasks containing 100 mL of 15 mg/L phosphate solution at

neutral pH. The contents of the Erlenmeyer flasks were then agitated at 150 rpm and 25 ± 2 °C

temperature in an orbital shaker incubator. The samples were withdrawn at the intervals of 5, 10,

15, 20, 30, 40, 50 and 60 min after the start of the reaction and analyzed for residual phosphate

concentration in the solution. The obtained results were analyzed as per pseudo-first order, pseudo-

second order and intra-particle diffusion kinetic equations to find out the best fit kinetic model.

2.8. Effect of coexisting anions

The phosphate adsorption capacity of GBP was explored in the presence of coexisting

anions like SO42-

, NO3- and Cl

-. The individual as well as the combined effect of these anions on

phosphate adsorption onto BGP were studied. Four grams of adsorbent were introduced into

different Elnermayer flasks containing 15 mg/L phosphate solution. A defined amount of each of

the anions has been added individually in the three Elnermayer flasks while in the fourth

Elnermayer flask, all the anions added together. The adsorption experiment in order to observe the

effect of coexisting anions, was carried out fixing all the optimized parameters at constant.

2.9. Desorption study

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Desorption study was carried out by taking spent adsorbents from adsorption experiments.

In this study, approximately 4 g of phosphate loaded GBP was subjected to desorption in 100 mL of

0.1 M KCl solution agitated at 200 rpm for 24 h (other parameters maintained at the same value as

that of adsorption experiment) following slightly modified procedure as described by Ye et al.

(2006) [21].

2.10. Column study

A polyacrylic transparent column having inner diameter 4.5 cm and height 50 cm was used

for this study. The column was operated in down flow mode with adsorbents filled to different bed

heights (10 and 15 cm) and different inflow velocities (1.5 and 3 mL/min) in a continuous mode.

The inflow velocities were maintained with the help of peristaltic pumps (miclins VSP-200-2C).

The phosphate concentration in the inflow was 15 mg/L, pH 7 and temperature 25 ± 2 °C. Periodic

monitoring and data collection was carried out at regular interval of 1 h to obtain the breakthrough

and exhaustion pattern for the adsorbent in the column. Column tests are normally performed for

providing more realistic laboratory results, since it has a greater resemblance to the flux conditions

in full scale constructed filters compared to short-term stirred batch experiments, which can result

in overestimation of sorption capacities [27].

3. Results and discussions

3.1. Characterization of adsorbent

PIXE and PIGE are the highly sophisticated experimental techniques used in the precise

determination of elemental composition of GBP and the major constituents were given in Table 1.

The presence of Al, Fe, Mg and Ca-oxides are known to play an important role in phosphate

removal [28]. In spite of being the major component of GBP, the Si - oxide has a very insignificant

role in phosphate removal [29]. Phosphate ions react with Fe and Al-oxides by ligand exchange

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forming inner-sphere complexes whereas the presence of Mg and Ca ion facilitates phosphate

removal via precipitation [30-32].

The FTIR spectra of the adsorbent before and after adsorption are shown in the Fig. 1a,

indicating the changes in the functional groups and surface properties of the adsorbent. The FTIR

spectrum reveals the complex nature of the adsorbent as evidenced by the presence of a large

number of peaks. The infrared absorption between 1250 and 850 cm-1

corresponds to the stretching

frequency region of phosphate species [33]. Different peak positions in the above mentioned range

of frequencies again vary depending upon the species of the phosphate ion and probable mineral

phase of the adsorbent participating in the adsorption process.

The e-FTIR software determined absorption peak around 1127, 1173.5 and 1199.36 cm-1

in

case of GBPT indicates the participation of P=O entities in adsorption process, which is in

agreement with the findings reported earlier [34]. In consequence the above mentioned peaks were

not present before the phosphorous exposure indicating absence of these functionalities in native

adsorbent (GBP). Well accepted experimental results by earlier researchers revealed that frequency

at 1126 cm-1

is mainly due to adsorption of H2PO4- resulting in P=O stretch [34-35]. Thus, the

frequency appearing at 1127 cm-1

in this study is possibly due to the P=O stretch as a result of

adsorbed H2PO4- . The slight difference in our experimental value and other additional peaks in the

frequency ranges may arise due to the presence of different mineral phases within the native

adsorbents.

The adsorbents before and after adsorption were subjected to XRD analysis and the spectra

of the GBP and GBPT are shown in Fig. 1b. Visible difference in the diffraction pattern of

adsorbent before and after adsorption was observed with respect to shifting of peaks, decrease in

intensity of peaks and disappearance of peaks. The difference in diffraction pattern of both the

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adsorbents may indicate phosphate adsorption. While the peak shift represents the contraction of

unit cell, both peak intensity decrease and peak disappearance contributes towards loss of

crystalinity of the material under investigation [36]. The number of peaks and their respective

positions for GBP and GBPT were determined using XPertHighScore software. From Fig. 1b, it

was found that, the peaks of the spectrum before adsorption remained in the same position after the

adsorption. However disappearance of some peaks in 2θ range 59-77 in the XRD spectrum of

GBPT was observed, demonstrating minor destruction of crystalinity due to adsorption of

phosphates [23].

The SEM image of the adsorbent before adsorption (GBP) is shown in Fig. 1c. It is observed

from the figure that the presence of irregular grooves and ridges results in a rough and porous

surface which is considered suitable for the attachment of phosphate to the adsorbent surface. On

the other hand, Fig. 1d shows SEM of GBPT, indicating the presence of fine particles over the

surfaces that are basically absent from the adsorbent before adsorption (Fig. 1c). This deposition of

fine particles is mainly due to the adsorption of phosphates over the surface of GBP. SEM enabled

the direct visualization of the deposition of fine particles of phosphate on the adsorbent surface.

Therefore, adsorption of phosphates by the use of GBP as an adsorbent was confirmed by XRD,

FTIR and SEM analysis.

3.2. Batch Study

3.2.1. Effect of adsorbent dose and agitation speed

The effect of adsorbent doses and agitation time on phosphate adsorption was investigated

by varying the agitation speed from 100 - 200 rpm and GBP concentration from 0.25 - 10 g per 100

mL of synthetic wastewater containing 15 mg/L phosphate. Other experimental parameters were

maintained at optimum level, e.g., 40 min of contact time, 60 min of settling time, pH 7, particle

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size less than 0.3 mm and temperature 25 ± 2 °C. The results of the study are given in Fig. 2. It can

be observed from Fig. 2a that the phosphate removal efficiency, increased from 20.92 % to 97.93 %

by increasing the adsorbent dose from 0.25 g to 10 g per 100 mL. Due to the availability of more

surface area and more adsorption functional sites, the phosphate adsorption rate increases rapidly

with the increasing adsorbent concentration [37]. Beyond a certain extent, i.e., 4 g per 100 mL in

this case, there is no significant uptake of phosphate with the increase in adsorbent dose (Fig. 2a).

This can be attributed to the attainment of saturation level beyond a certain adsorbent dosage during

adsorption process. Therefore the optimum adsorbent dose of 4 g of GBP per 100 mL of phosphate

solution was fixed for the subsequent experiments. GBP is a waste material, produced in large

quantity and is used as an adsorbent as such without undergoing any process of surface

modification or chemical modification. Therefore, comparatively higher dose can be compromised

rather than going for modification of the same which may incur processing costs.

From Fig. 2b, it can be observed that the percentage removal of phosphate increases with increase

in agitational force, from 92.43% at 100 rpm to 97.58% at 150 rpm. Increase in rotational speed

increases the movement of adsorbent particles in the solution, leading to the reduction of mass

transfer boundary. This ultimately improves the surface contact between the adsorbent and the

aqueous solution, thereby increasing the rate of phosphate adsorption [38]. But the rate of

adsorption of phosphate by GBP does not change much beyond 150 rpm, indicating optimum

agitational speed as 150 rpm.

The effects of settling time and particle size on phosphate adsorption were also determined

by varying the time from 15 – 120 min and adsorbent size from 2.36 mm to less than 0.3 mm and

maintaining all other experimental parameters constant at optimum level. Settling time

determination is important to examine the effect of particle sedimentation after shaking. It was

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observed that for GBP, 60 min of settling time and particle size of less than 0.3 mm is the optimum

conditions for maximum (≈ 97.5 %) phosphate removal. Smallest particle always has the largest

specific surface area, which means more available sites for adsorption, thus more phosphate

removal.

3.2.2. Effect of pH and Mechanism of phosphate removal

The effect of pH on phosphate removal and the mechanism of removal were evaluated by

carrying out the experiments at variable pH of 2, 5, 7, 10 and 12, maintaining all other parameters

constant at their optimum values. It was observed that the maximum phosphate removal efficiency

of 97.43% occurred at pH 5 followed by 96.09 % at pH 7 (Fig. 2c). Phosphate speciation in

solution, pHzpc of the adsorbent, the affinity of phosphate ions towards the adsorbent etc. are the

governing factors controlling the effect of pH on phosphate adsorption [23]. There exist three

different forms of phosphate, e.g., H2PO4-, HPO4

2- and PO4

3- in the solution as described in the

equations 1, 2, and 3. Generally, HPO42-

and H2PO4- are the predominant species in the pH region

between 4 and 10, with HPO42-

being more widespread in slightly alkaline conditions and H2PO4- in

slightly acidic conditions. Between pH 10 and 12, HPO42-

dominates over PO43-

species, whereas, at

pH higher than 12.5, PO43-

species are the predominant [35]. pHzpc is the pH at which the net

surface charge on the adsorbent is zero. At pH less than pHzpc value, the surface charge of the

adsorbent is positive, so, a higher Columbic attraction between the binding sites and phosphate ions

leads to a higher phosphate uptake. Whereas, at pH greater than pHzpc, the surface has a net negative

charge that enhances Columbic repulsion between the sites and the phosphate ions resulting in a

decrease in phosphate absorption [39]. The pHzpc of GBP was found to be 8.62 from the Fig. 2d.

H3PO4 ↔ H+ + H2PO4

- pKa= 2.2 (1)

H2PO4- ↔ H

+ + HPO4

2- pKa= 7.2 (2)

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HPO42-

↔ H+ + PO4

3- pKa=12.3 (3)

When the solution pH becomes less than pKa, the solution starts donating more protons than

hydroxide groups, making the adsorbent surface positively charged that attracts negatively charged

species [40]. In this study, the higher removal of phosphates at pH 5 and 7 indicates that the species

involved in adsorption are those which are related to the pKa value of 7.2. In this case as both the

pHs, at which maximum phosphate sorption took place were less than that of the i) pHzpc (8.62) of

GBP, and ii) pKa value (7.2) of H2PO4-, the conditions that facilitate GBP surface to be positively

charged. Thus, based upon this observation, it can be said that H2PO4- was the major species

involved in this adsorption process [23, 41]. Fig. 2c shows that phosphate removal efficiency at pH

7 (96.09 %) is marginally less than that of removal efficiency at pH 5 (97.43 %). Therefore, for

better convenience, neutral pH was set for subsequent tests.

Regarding mechanism, as already discussed in section 3.1, phosphate removal occurs either

via ligand exchange forming inner-sphere complexes or via precipitation. Hydrolysis of metal

oxides generate metal cation and hydroxyl anion (OH-) as per the following equation.

MaOb + bH2O → aM(2b/a)+

+ 2bOH- (4)

M (2b/a) +

....... H2PO4

- (5)

The cationic species participates in phosphate up taking through electrostatic interaction, whereas

the anionic species is used in inner-sphere ligand exchange mechanism. Similarly phosphate metal

precipitation occurs as per the equation given below:

3Ma+

+ aPO43-

= M3 (PO4) a (6)

The mechanism for maximum phosphate removal at pH 5 and 7 that has been observed in this study

can be explained on the basis of protonation of surface metal hydroxides of the adsorbent. Both the

pH values (5 and 7) discussed here are less than that of the pKa value of H2PO4- (7.2) and pHzpc of

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GBP (8.62). This condition favors the donation of more protons than hydroxide groups by the

solution phase, thereby making the adsorbent surface positively charged and attracting negatively

charged phosphate species. The equation could be presented as:

M-OH + H+ → M-OH2

+ (7)

M-OH2+ + H2PO4

- → M-H2PO4 + H2O (8)

where, M is the metal constituent such as Fe, Al, Ca, Mg, Si etc. present in the adsorbent.

3.2.3. Effect of initial phosphate concentration and Adsorption isotherm

To assess the effect of initial phosphate concentration on the adsorption capacity of the GBP

and adsorption isotherm, the experiments were conducted by varying initial phosphate

concentration from 1 to 20 mg/L while maintaining all the other optimized parameters constant. It is

evident from Fig. 3a that under the lower initial phosphate concentration (1 mg/L), adsorption

saturation could not be reached due to greater availability of free adsorption sites on the GBP as

compared to the number of phosphate molecules to be adsorbed, thus resulting in higher phosphate

removal efficiency (99.97 %). In case of higher initial phosphate concentration (20 mg/L), the

available free active adsorption sites on the GBP decreases [2], resulting in lower phosphate

removal efficiency (94 %). The initial phosphate concentration of 15 mg/L was chosen as the

optimum value for the subsequent experiments. Generally secondary effluents from treatment plants

contain 1-2 mg/L phosphate. But occasional system failures in treatment plants receiving high

strength domestic wastewater may end up in contributing a very high phosphate input (up to 15

mg/L) to the water bodies leading to catastrophic effect on the aquatic ecosystem. Therefore, in

order to accommodate maximum possible levels of phosphate in domestic wastewater which are

usually in the range of 3-15 mg/L [2], the initial phosphate concentration of 15 mg/L was opted in

this study.

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Adsorption isotherms state about the equilibrium relationship between the amount of

adsorbate adsorbed qe (mg/g), and the amount of adsorbate in solution Ce (mg/L) at constant

temperature. The equilibrium uptake capacity qe can be calculated by using the following equation

[42].

qe = (C0-Ce) V/m (9)

where C0 and Ce are the initial and equilibrium phosphate concentrations (mg/L), V is the volume of

phosphate solution (mL), and m is the mass of adsorbent (g). In this study Langmuir and Freundlich

isotherm models were adopted to fit the equilibrium data obtained from adsorption experiments.

3.2.3.1. Langmuir model

Monolayer adsorptions on a homogeneous surface with a finite number of adsorption sites

and without any interaction between the adsorbed molecules are the characteristic features of

Langmuir isotherm. The linearized form of the Langmuir isotherm model is given as:

Ce/qe = 1/(bqm)+ (1/qm) Ce (10)

where b is adsorption constant (L/g), that measures the affinity of the adsorbent for the solute and

qm is the maximum adsorption capacity (mg/g). The value of b signifies the level of adsorption. The

values of qm and b (Table 2) can be obtained considering the slope and intercept of the plot of Ce

versus Ce/qe (Fig. 3b). GBP is found out to have a maximum adsorption capacity (qm) of 0.41 mg/g,

which is at par with the finding reported earlier by Yang et al. (2009) for coal cinder [28].

Comparatively less uptake capacity of GBP can be compromised by the fact that, GBP is an

abundantly available waste material and phosphate loaded exhausted GBP can be used in the

agricultural sector as soil conditioner and phosphate fertilizer.

The separation factor (RL), a dimensionless equilibrium parameter can be calculated from

Langmuir equation [38] and can be expressed as:

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RL = 1/(1 +bC0) (11)

where, C0 is the initial concentration of phosphate (mg/L). RL= 1 represents linear adsorption while

RL = 0 stands for the irreversible adsorption process. RL < 1 is for favorable adsorption, while RL > 1

represents unfavorable adsorption. In this case the value of RL was found to be 0.03, suggesting that

the adsorption process was favorable.

Calculation of Gibbs free energy changes (ΔG°) assists in analyzing the impulsiveness of the

adsorption process. Langmuir constant, b is also helpful in finding ΔG° for the adsorption process

as per the following equation [43]

ln (1/b) = ΔG°/(RT) (12)

where, R is the universal gas constant (8.314 J/mol.K) and T is the absolute temperature. Based

upon Eq. (12), the Gibbs free energy ΔG° observed to be negative (-1.32 kJ/mol) which necessarily

represents the spontaneity of the phosphate adsorption process by GBP as an adsorbent.

3.2.3.2. Freundlich model

The adsorption process wherein a heterogeneous adsorbent surface is involved in the

multilayer distribution of the adsorbate with interaction amongst adsorbed molecules is explained

by the Freundlich isotherm model. The linear form of Freundlich isotherm model is mentioned

below as

ln qe = ln kf + (1/n) ln Ce (13)

where, kf (mg/g) is the Freundlich constant which represents the adsorption capacity and 'n' is the

Freundlich exponent that represents the adsorption intensity. Freundlich constant (kf) is related to

temperature and the chemical or physical characteristics of adsorbents whereas Freundlich exponent

(n) is an indicator of the change of intensity of adsorption process and also a measure of the

deviation from linearity of the adsorption. A higher value of the Freundlich exponent (n > 1)

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indicates favorable adsorption where as n < 1 represents poor adsorption characteristics [37]. In

this study the values of kf and n (Table 2) were calculated from the intercept and slope of the

Freundlich isotherm, which are drawn by plotting ln qe versus ln Ce (Fig. 3c). The value of "n =

3.57" suggests favorable adsorption. In order to ascertain the adsorption characteristics, surface

area, pore volume and pore radius of the adsorbent was found out to be 19.07 m2/g, 0.016 cm

3/g and

27.358 nm respectively. Nitrogen adsorption-desorption isotherm of GBP is shown in Fig. 3d,

which indicates the presence of mesoporocity with affinity for adsorption.

The higher values of the correlation coefficient (R2) for both the models suggests that the

experimental data exhibit a very good mathematical fit to both the models and this can be

interpreted in terms of surface nature of the adsorbent and affinities of different mineralogical forms

present in adsorbent towards phosphate. While both the models fit well, the R2 value for Langmuir

model (0.99) is marginally higher than that of the Freundlich model (0.97), which may indicate the

predominance of monolayer adsorption process over intra-molecular interactions amongst the

adsorbed phosphates.

3.2.4. Effect of contact time and Adsorption kinetics

The effects of contact time and phosphate adsorption kinetics were evaluated by varying

contact time from 5-60 min while keeping other experimental parameters constant at optimized

condition. The results are shown in Fig. 4a which clearly indicates that contact time of 40 min can

be considered to be the optimum contact time to reach the equilibrium, resulting in 97.77 % of

phosphate removal. The adsorption process includes diffusion through the fluid film around the

adsorbent particle and diffusion through the pores to the internal adsorption sites. It is observed

from the Fig. 4a, that during the initial phase of adsorption, i.e., up to first 15 min, the slope of the

curve is very steep, indicating a very fast rate of adsorption (0 to 87.19 % removal took place). This

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can be explained on the ground that, at the beginning of the experiment the maximum surface area

of the GBP was uncovered, thus exposing numerous pores and free surface area for adsorption,

thereby results in a large concentration gradient between the film and the available pore sites. With

the passage of time, more and more surface area and pore sites of the GBP get saturated with

phosphates. Thus, the available exposed surface of GBP decreases, rate of pore diffusion of the

phosphates into the bulk of the adsorbent decreases so does the rate of adsorption [44]. This is

evident from the figure that only a small amount of phosphate removal takes place during the last

25 min of equilibrium. Therefore the optimum contact time was fixed at 40 min for the rest of the

experiments.

An insight into the rate of adsorption process, evaluation of reaction coefficients and finding

out of contact time required for an adsorption process can be obtained through the kinetic studies.

To investigate the mechanism of adsorption kinetics of phosphate onto GBP, three kinetic models,

namely, pseudo-first order, pseudo-second order and intra-particle diffusion models were

analyzed. The linear forms of pseudo-first order and pseudo-second order kinetic models are given

below as Eq. (14) and Eq. (15), respectively [2].

log (qe–qt) = log qe – (k1/2.303) t (14)

t/qt = 1/ (k2. qe2) + (1/ qe) t (15)

where, k1 is the first order reaction rate constant (1/min) and k2 represents the second order reaction

rate constant (g/mg.min), qt and qe (mg/g) are respectively, the adsorption capacity at any time t and

at equilibrium. The values of k1 and qe were determined from the graph of pseudo-first order model

(Fig. 4b) that was obtained by plotting log (qe-qt) versus t. For pseudo-second order model, the

values of qe and k2 are determined from the slope and intercept of the plot of t/qt versus t as shown

in Fig. 4c. The calculated kinetic parameters along with R2 values for both the kinetic models are

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given in Table 2. The R2 value for the pseudo-second order model (0.99) was much higher than that

of the pseudo-first order model (R2 = 0.73). Moreover, negative reaction constant (k1 = -0.078) was

observed for the pseudo-first order model, indicating the inadequacy of the kinetic model. The

results indicated that the adsorption kinetics of phosphate on GBP follows the pseudo-second order

kinetics. Pseudo-second order kinetics explains that the rate of adsorption is proportional to the

square of the number of unoccupied sites on the adsorbent surface and to the concentration of

adsorbate in the solution as well. Thus suggesting that the adsorbates can be bound to different

binding sites on the adsorbent. Similar types of findings were also reported by earlier researchers [2,

23, 38].

The rate limiting steps of the adsorption i.e., either external mass transfer (film diffusion) or

intra-particle diffusion (pore diffusion) or both principally control the process and the same can be

predicted by diffusion coefficients calculated from a diffusion model. The linear form of the

Morris-Weber equation as mentioned in Eq.16 [45] can confirm the presence or absence of intra-

particle diffusion.

qt = kp t1/2

+ C (16)

where qt (mg/g) is the phosphate uptake amount at time t (min), kp is the intra-particle diffusion rate

constant (mg/g.min1/2

) and C is the constant indicating the thickness of the boundary layer. The

coefficient, kp can be determined from the slope and C from the intercept of the plot of qt versus t1/2

as shown in Fig. 4d. The figure clearly shows that the plot is not a straight line, indicating that intra-

particle diffusion is not only the rate limiting step. Along with intra-particle diffusion, other

mechanisms may also be involved in the adsorption process. The presence of 1st step, 2

nd step and

3rd

step on the curve is possibly due to the presence of boundary layer diffusion, intra-particle

diffusion and saturation step, respectively. Therefore, both pore diffusion and film diffusion are

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likely to affect the adsorption process. This supports the discussion about the diffusional nature of

the adsorption process.

3.2.5. Effect of coexisting anions

Wastewater generally contains common anions such as SO42-

, NO3- and Cl

- and

concentrations of SO42-

, NO3- and Cl

- in high strength domestic wastewater were found to be 50

mg/L, 70 mg/L and 90 mg/L respectively [42]. These anions may interfere in phosphate adsorption

by GBP. Assuming that there will be no removal of these anions during primary treatment, the

intervention of coexisting anions on phosphate adsorption was evaluated by considering the original

concentrations, e.g., 50 mg/L of SO4

2-, 70 mg/L

of NO3

- and 90 mg/L

of Cl

- ions as model

competing anions. The results given in Table 3 show no significant change in phosphate adsorption

onto GBP after adding coexisting anions with concentrations more than that of phosphate ions. This

may be credited to the specific binding of phosphate onto the specific active site of the adsorbent,

which is not generally influenced by the presence of other ions [46]. The experiment demonstrates

that GBP has high adsorption selectivity towards phosphate, thereby suggesting its potential

application as an adsorbent for phosphate removal from wastewater.

3.2.6. Desorption study

Desorption study not only helps in recovering the adsorbed phosphate but also in

regenerating the adsorbent. A high quality and competent adsorbent should respond well to

desorption process in order to demonstrate the very basic property of reusability. In this study, the

phosphate loaded GBP was subjected to desorption for 24 h following a slightly modified procedure

of Ye et al. (2006) [21]. The maximum desorption of phosphate was found to be in the range of 89-

94 %, indicating that the adsorption of phosphate onto GBP is reversible. The finding also suggests

that the bonding between GBP and phosphate is not strong consequently the labile phosphate can be

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available for the growth and development of plant species, if the spent adsorbent will be applied as

a soil conditioner. In spite of the high rate of desorption, desorption process is not encouraged in

this study since i) GBP is a copiously available waste material, ii) desorption process incurred any

cost, and iii) applicability of spent GBP in agricultural fields for greater conductivity of water as

well as a source of phosphate fertilizer. To explore the reusability of the adsorbent, the desorbed

adsorbent was washed several times with distilled water to neutral pH and dried in hot-air oven at

100º C for overnight before reusing it. In the second time use (maintaining all optimum parameters)

the phosphate removal efficiency of the adsorbent was observed in the range of 90-93% as

compared to approximately 97.5% in the case of first use. The observed lesser efficiency in second

use is possibly due to the occupancy of some of the active sites of the adsorbent by the non-

desorbed phosphates. There is also probability that the number of active sites for adsorption

decreases after every use of the adsorbent. The result explores and recommends the reusability of

the adsorbent e.g., GBP.

3.2.7. Column study

Column study that simulates the flow conditions of an actual wastewater treatment process,

provides more reasonable laboratory results. So the column study was performed to investigate

phosphorous removal efficiency of adsorbent by implementing all the optimized adsorption process

parameters obtained from batch study. The breakthrough time (tb) represents the time corresponding

to 93.5% removal of phosphates, i.e., effluent phosphate concentration less than 1 mg/L whereas,

exhaust time (te) signifies the time matching to 5% removal of phosphate. The effect of bed height

and flow rate on the breakthrough curve of column study were discussed in the following sections.

3.2.7.1 Effect of bed height

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To investigate the effect of bed height on the breakthrough curve of the column adsorption

process, 3 mL/min flow rate was maintained keeping initial phosphate concentration 15 mg/L, pH 7

and varying bed heights of the column as 10 and 15 cm. Fig. 5a shows the breakthrough curves of

different bed heights at a constant flow rate of 3 mL/min. It is clear from the figure that break

through and exhaust timing increases with increase in column height. For 10 cm bed height tb and te

values were 10 and 64 h, respectively, while the corresponding values for 15 cm bed height were 14

and 76 h. The volumes of water treated corresponding to breakthrough time and exhaust time were

1.8 L and 11.52 L for 10 cm bed height, whereas the corresponding values for 15 cm bed heights

were 2.52 and 13.68 L, respectively. For 10 cm bed height, it took 3.41 h for the formation of

exchange zone, which moves at a rate of 0.28 cm/h. In case of 15 cm bed height, it took 4.03 h for

the formation of exchange zone that moves at a rate of 0.20 cm/h. The results depict that increasing

the adsorbent bed height results in higher efficacy of the column. These findings can be attributed

to the existence of more available surface area and higher contact time for adsorption process

contributing to higher efficiency.

3.2.7.2 Effect of flow rate

The effect of flow rate on the breakthrough curve of the column adsorption process was

assessed by the varying flow rate as 1.5 and 3 mL/min while keeping the bed height 10 cm, initial

phosphate concentration 15 mg/L and pH 7. Fig. 5b shows that higher flow rate results in shorter

column exhaustion time. For lesser flow rate (1.5 mL/min), tb and te values were 18 and 92 h,

respectively, while the corresponding values for 3 mL/min flow rate were 10 and 64 h. For lower

flow rate, it takes 4.81 h for the formation of the exchange zone that moves at a rate of 0.11 cm/h.

But in case of higher flow rate, it takes 3.41 h for the formation of exchange zone, which moves at a

rate of 0.28 cm/h. When the flow rate decreases the residence time in the bed increases resulting in

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a longer service time for the column. On the other hand, increasing flow rate decreases the

residence time that results in lower bed utilization. Therefore, bed capacity decreases with increased

flow rate.

4. CONCLUSIONS

The present study estimates the effectiveness of GBP as an adsorbent for the removal of

phosphates from aqueous solution. The adsorption data were observed to fit well to both the

Langmuir and Freundlich isotherms. Data fitting to the pseudo-second order reaction kinetics

explains that the concentration of adsorbate as well as adsorbent are the two controlling parameters

that govern the phosphate adsorption process. The diffusional nature of the adsorption process is

suggested by intra-particle diffusion model, which explains that both film diffusion and pore

diffusion control the rate of adsorption of phosphate onto GBP. It can be concluded from the results

of this study that the application of GBP for adsorptive removal of phosphate is highly favorable

and a spontaneous process. Therefore, it can be anticipated that Grounded Burnt Patties (GBP) can

be used as a low cost and highly efficient adsorbent for removal of phosphates from wastewater.

Acknowledgements

The authors are thankful to School of Infrastructure, Indian Institute of Technology

Bhubaneswar, India, for providing facilities to carry out the research work in the concerned area.

The authors would also like to thank Dr. R. Acharya, and Mr. K. B. Dasari of BARC, Mumbai,

Mr. D. K. Ray of IOP, Bhubaneswar and Dr. A. Basu of NIT, Rourkela for their kind help in doing

PIGE, PIXE and XRD experiments and analysis.

Conflict of interest

The authors have declared that they have no conflict of interest.

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Fig. 1 Characterization of adsorption process:

(a) FTIR spectrum of adsorbent before adsorption (GBP) and after adsorption (GBPT). (b) XRD

spectra of adsorbent before and after adsorption. (c) SEM images of adsorbent before adsorption.

(d) SEM images of adsorbent after adsorption.

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Fig. 2 Effects of various parameters on phosphate adsorption in batch mode:

(a) Effect of adsorbent dose (Conditions: Contact time 40 min, Agitation 150 rpm, Adsorbate 15

mg/L, pH 7 and Temperature 25 ± 2 °C). (b) Effect of agitation (Conditions: Contact time 40 min,

Adsorbent 4 g per 100 mL, Adsorbate 15 mg/L, pH 7 and Temperature 25 ± 2 °C). (c) Effect of pH

(Conditions: Contact time 40 min, Agitation 150 rpm, Adsorbent 4 g per 100 mL, Adsorbate 15

mg/L and Temperature 25 ± 2 °C). (d) pHzpc of GBP

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Fig. 3 Effect of initial phosphate concentration on adsorption and adsorption isotherms:

(a) Effect of initial phosphate concentration (Conditions: Contact time 40 min, Agitation 150 rpm,

Adsorbent 4 g per 100 mL, pH 7 and Temperature 25 ± 2 °C). (b) Langmuir isotherm model. (c)

Freundlich model (Conditions: Contact time 40 min, Agitation 150 rpm, Adsorbent 4 g per 100

mL, Adsorbate 1-20 mg/L, pH 7 and Temperature 25 ± 2 °C) (d) Nitrogen adsorption-desorption

isotherm of GBP

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Fig. 4 Effect of contact time on phosphate adsorption and kinetic models:

(a) Effect of contact time (Conditions: Agitation 150 rpm, Adsorbent 4 g per 100 mL, Adsorbate

15 mg/L, pH 7 and Temperature 25 ± 2 °C) (b) Pseudo-first order kinetic model. (c) Pseudo-second

order kinetic model. (d) Intraparticle diffusion model (Conditions: Contact time 5-60 min, Agitation

150 rpm, Adsorbent 4 g per 100 mL, Adsorbate 15 mg/L, pH 7 and Temperature 25 ± 2 °C)

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Fig. 5 Breakthrough curves of column study:

(a) Effect of bed height on column saturation (Conditions: Flow rate 3 mL/min, pH 7, Bed height

10 and 15 cm and initial phosphate concentration, C0 15 mg/L). (b) Effect of flow rate on column

saturation (Conditions: Flow rate 3 and 1.5 mL/min, pH 7, Bed height 10 cm and initial phosphate

concentration, C0 15 mg/L)

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Table 1 Properties and compositions of Grounded Burnt Patties (GBP)

Properties and compositions Grounded Burnt Patties (GBP)

Particle size (mm) < 0.3

pH zpc 8.62

BET Surface area (m2/g) 19.07

Bulk density (g/cm3) 2.0

Porosity (%) 74.48

Specific gravity 2.41

Specific Yield (%) 62.19

Specific Retention (%) 12.29

SiO2 (%) 52.71-54.49

Fe2O3 (%) 17.95-18.72

Al2O3 (%) 20.74-21.42

MgO (%) 4.85-5.01

Na2O (%) 0.37-0.40

CaO (%) 3.8-4.11

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Table 2 Parameters of isotherm and kinetics models

Langmuir model

qm (mg/g) b (L/mg) R2

0.41 1.87 0.99

Freundlich model

Kf (mg/g) n R2

0.21 3.57 0.97

Pseudo first order model

qe (mg/g) k1 (1/min) R2

0.43 -0.078 0.54

Pseudo-second order model

qe (mg/g) k2 (g/mg. min) R2

0.35 0.26 0.99

Table 3 Effect of coexisting anions

Concentration of anions (mg/L) % Phosphate removal

PO43-

SO42-

NO3- Cl

-

20 0 0 0 95.3

20 50 0 0 95.5

20 0 70 0 95.9

20 0 0 90 96.2

20 50 70 90 94

Highlights

1. Grounded Burnt Patties (GBP), a solid waste is used as an adsorbent for phosphate

removal for the first time.

2. Approximately 98% phosphate removal efficiency of GBP from phosphate rich aqueous

solution.

3. Characterization of adsorption behaviour of phosphate onto GBP by PIXE, PIGE, FTIR,

XRD and SEM.

4. No influence of co-existing anions on phosphate adsorption onto GBP.

5. Suggestive use of spent adsorbent as a source of phosphorous in agriculture.

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Graphical Abstract