ASV for Lability of CD, Pb, Cu

Wat Res Vol. 20, No. I1, pp. 1397-1406, 1986 0043-1354/86 3.0 0+0.00 Printed in Grea t Britain. All rights reserved Copyrigh t © 1986 Pergamon Journ als Ltd ANODIC STRIPPING VOLTAMMETRIC STUDY OF THE LABILITY OF Cd Pb Cu IONS SORBED ON HUMIC ACID PARTICLES T. U. AUALIITIA and W. F. PICKERING Chemistry Department, University of Newcastle, N.S.W. 2308, Australia Received July 1985) Abstract--Th e sorption of Cd, Pb and C u by humic acid particles has been studied at #g 1 t levels using A.S.V. on a Hg film electrode as the measuring technique. The variables examined included amount of solid present (0.01--0.2% w/v), initial metal ion concn (10--100 #g l-l), systems pH (5.3, 6.35, 8.15) and base electrolyte composition. The calculated capacity for specific adsorption of the m etal ions was a few mm ol M 2+ kg -~, or less. The apparent lability of p art of the sorbed material was examined by analysing the base solution before and after filtering, and by adding Chelex 100 particles to the suspension. Some sorbed Cd 2+ was A.S.V. labile, another fraction transferred to the resin. The effect of solution reactions on uptake was studied by making the 1 M CH3COONa base solution 0.2 M in a range o f Na + salts (8 different anions), or in carboxylic acid content (5 acids) or in compoun ds having S-type bonding groups. Formation of complex ions in solution altered the extent of metal ion uptake, and in the case ofC u A.S.V. peak size, shape and position were varied. It is suggested that natural waters containing suspended matter should be analysed by A.S.V. as received , as well as after filtration since response differences provide guidance in respect to the lability of sorb ed ions. Key words--voltametry, anodic stripping, A.S.V., colloid effects, heavy metal-colloid sorption, humic INTRODU TION Colloidal particles contribute greatly to the move- ment and distribution of metal ions within a water- way system with up to 95% of the heavy metals transported by fresh waters from land to sea possibly being associated with particulate matter (Martin and Meybeck, 1979). During migration, sorbed species may be continuously re-distributed between the aqueous and solid phases, until finally incorporated in bed sediments following flocculation (e.g. in estuaries) and particle settling (Hart, 1981). The complexity of natural systems and limited understanding of many of the chemical processes involved has hindered rigorous evaluation of the particulate role. Distribution patterns are usually discerned on the basis of an analysis of metal content of >45/~m particles and aqueous phases separately, but this app roach ignores the possibility that some of the sorbed ion may be quite labile (i.e. loosely bound and m obile under the influence of an applied chemical or electrical potential), and thus available for transfer (e.g. to biological systems). Several more complex schemes have been devised, and these allo- cate the different species present into more specific categories. The results demonstrate, inter alia that a significant fraction of the total metal content can be associated with colloidal organic matter (Gardiner, 1974; Batley and Florence, 1976; Turekian, 1977; Batley and Gardiner, 1978; Florence and Batley, 1977, 1980). When using anodic stripping voltammetry (A.S.V.) as the measuring technique, adding humic acid to dilute metal ion solutions and synthetic seawater has been found to alter the magnitude of the peak currents (ip) and occasionally the peak potential (Ep) positions (thereby signifying formation of a new chemical species), with the magnitude of the effect varying with pH (Ernst et al., 1975; Sagberg and Lund, 1982; Ugapo and Pickering, 1985). Bonding between metal ions and humic acids itself varies with pH, relative concentrations and type of functional groups involved (Zurino et al., 1975; Stevenson, 1977). These parameters also influence the amount of metal ion that can be sorbed by the solids (Farrah and Picketing, 1978). Extraction studies using solids loaded with > 100 mmol M 2+ per kg humic acid, indicate that a high proportion of the M 2+ is ion-exchangeable, and may thus be A.S.V. labile (Slavek et al., 1982). This prediction was partly substantiated when suspensions of humic acid containing /agl -I amounts of Me 2+ were analysed before and after filtration (Ugapo and Pickering, 1985). A voltammetric study of Pb(II) solutions containing hydrous oxide suspensions, however, supports the theoretical viewpoint that metal ions adsorbed on particle surfaces are essen- tially nonlabile (Goncalves et al., 1985). In view of the important role that suspended organic matter may play in metal ion transport, a study has been made of metal sorption, and factors influencing uptake and lability, using initial M 2+ 1397

ASV for Lability of CD, Pb, Cu

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