Managing Arsenic in the Environment: From Soil to Human Health

656 Pages·2006·0.68 MB·English

by  R. Naidu

MANAGING ARSENIC
IN THE ENVIRONMENT
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86 Managing arsenic in the environment
on birnessite at pH 5 as a result of AsIII oxidation and MnIV reduction. Bothe and Brown (1999)
investigated the conditions required for the formation of stable calcium arsenate phases and
the processes responsible for immobilizing As in the presence of lime. The pH influenced the
type of calcium arsenate precipitate formed and, upon lowering the pH to 7, AsV concentration
in solution increased, suggesting that calcium arsenates were not stable in neutral to acidic pH
(Bothe & Brown 1999).
Little information is available concerning co-sorbing oxyanion and metal contaminants in
the environment, yet in most metal-contaminated areas co-contamination by arsenate (AsO ,
4
AsV) is common. Gräfe et al. (2004) investigated the co-sorption of AsV and Zn on goethite at
pH 4 and 7 as a function of final solution concentration. Complementary extended x-ray
absorption fine structure (EXAFS) spectroscopic data were collected at the As and Zn K-edges
in order to glean information about the coordination environment of As and Zn at the
goethite–water interface. Macroscopic sorption studies revealed that AsV and Zn sorption on
goethite increased in co-sorption experiments beyond site saturation, suggested by the single
sorption isotherms. Arsenate sorption on goethite increased in the presence of Zn by 29% and
more than 500% at pH 4 and 7 respectively. In the presence of AsV, Zn sorption on goethite
increased by 800% and 1300% at pH 4 and 7 respectively. More AsV than Zn sorbed on goethite
below site saturation at pH 7. Above site saturation, the Zn:As site density ratio (SDR) remained
constant at 0.91±0.03. At pH 4, the Zn:As SDR was less than 1 throughout the concentration
range. Below AsV site saturation on goethite, AsV formed bidentate binuclear bridging com-
plexes on Fe and/or Zn octahedra, while Zn mainly formed edge-sharing complexes with Fe at
the goethite surface. Above site saturation, Zn was increasingly complexed by AsO gradually
4
forming adamite-like (Zn (AsO )OH) surface precipitates on goethite.
2 4
In situ speciation of As in contaminated soils
The importance of metal cations in the adsorption mechanisms and solid phase speciation of
As described above is illustrated in findings from As-contaminated soils. A soil contaminated
industrially by CCA was investigated by Gräfe (2004) to elucidate the effects of co-contami-
nating metal cations (Cr, Cu, Zn) on the solid phase speciation of As. Micro-focused synchro-
tron-based x-ray fluorescence (µSXRF) mapping and micro-focused X-ray absorption
spectroscopy (µXAFS) were used to glean detailed in situ information about the spatial distri-
bution of As in relation to other metals and to probe the coordination environment of As at the
molecular level. The upper and lower 20 cm of the soil profile were investigated. The soil pH
ranged between 7.0 and 7.5 due to applications of lime. Micro-SXRF results showed consistent
spatial correlation between As and Zn, Cr and Cu, with Pearson correlation coefficients ranging
between 95% and 66%. Micro-XAFS analysis suggested that 96% of As was in the +5 and c.
4% in the +3 oxidation state, with traces of orpiment (As S ) detectable in some spectra (Figure
2 3
4.5, see p. 251). Abstract factor analysis of 29 sample spectra and target transformation sug-
gested that As occurred mainly as Cu and/or Zn-arsenate surface clusters on Al and Fe oxides
as well as fully precipitated metal-arsenate phases. Linear least-squares combination fit analysis
(LLSF) of individual soil spectra suggested that more than 84% of As occurred as surface
clusters or co-precipitated phases of either mixed Cu–Zn-, Cu- or Zn-arsenates throughout the
upper 40 cm of the soil (Figure 4.6). The remaining 16% of As solid phases unaccounted for by
mixed metal-arsenates were mostly adsorbed AsV surface complexes on Fe and Al-oxides.
Phosphate and NH -oxalate/ascorbic acid solutions were the most effective desorbing agents
4
for As, suggesting that most As was bound as inner-sphere complexes at the mineral–water
interface. In comparison, the shortcomings of indirect solid phase speciation techniques such
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Solid phase speciation of arsenic 87
Figure 4.6 LM-B (lower 20 cm) thin section Map 2 LLSF analysis with µSXRF and fluorescence line
plots. The lack of structure in the residual of the k3*(k) functions indicated that the selected
references and their amounts reproduced the unknown spectrum well. Corresponding Fourier
Transforms, displaying the magnitude of the modulus and the imaginary phase, are shown to the
right. The LLSF results show the dominance of Cu–Cr–As precipitates in this particle
Reproduced with permission from Gräfe et al. (2004a)
as sequential extraction become quite apparent. Nevertheless, the information stemming from
these experiments show which practices should be avoided in a co-contaminated site.
Findings from micro-XAS/XRF studies are very useful in evaluating agriculturally and
industrially contaminated sites, where As and metals coexist in the soil matrix. They also stress
the importance of metal-arsenates as solid phases in As-contaminated environments and
justify further studies on the kinetics and thermodynamics of their formation.
Future directions
While there have been many advances in understanding As reactions and speciation in natural
systems, numerous questions remain. Future research should focus on the following areas:
soluble complex formation of AsIII and AsV with metals, sulfides, DOC and microbes and their
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88 Managing arsenic in the environment
reactivity in soils and sediments; the role that competitive metals and ligands (organic and
inorganic) have on As sorption/redox reactions and speciation; the importance of metal-
arsenate precipitates in As mobility; and increased use of micro-focused spectroscopic tech-
niques to directly speciate As contaminated soils.
Acknowledgements
We appreciate comments of the Environmental Soil Chemistry Group during the preparation
and revision of this chapter. Markus Gräfe appreciates the support of a Competitive Fellowship
from the University of Delaware.
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Part C
Dynamics of arsenic
in groundwater
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