Abstract
A study was carried out to evaluate how the adsorption process of arsenate on hematite is influenced by the presence of thiosulfate ion. The competition in sorption between the two anions was examined in relation to potential use of thiosulfate as an additive for phytoextraction in arsenic contaminated soils. Results obtained from sorption isotherms can be described according to the Langmuir equation which allows, by comparing adsorption maxima, a quantitative evaluation of the effect of increasing concentration of thiosulfate on arsenate adsorption. Due to the competition between the two anions for available sites on hematite, arsenic sorption decreased at increasing thiosulfate concentration. The negligible effect of ionic strength on the sorption process, strongly suggests that thiosulfate can substitute arsenate ions in the inner sphere complexes on the surface of hematite. The interpretation of the As adsorption process on hematite in terms of distribution coefficient (K d) shows that thiosulfate ions competing for sorption on the same surface may significantly reduce the arsenate adsorption and enhance the release of arsenic in the liquid phase of soil. Results have a direct influence on the applicability of phytoextraction in arsenic contaminated soils since plants can uptake arsenic only from the liquid phase. The obtained results can explain the increase in As concentration in plants, reported in a recent paper in which the addition of thiosulfate was shown to promote As phytoavailability.
Highlights
Soil is an important sink for arsenic, which is considered a hazardous priority pollutants (Nickson et al, 1998; Berg et al, 2001; Centeno et al, 2002; Nriagu et al, 2009) and its uptake by living organisms in contaminated soils, poses a serious threat to human health and to food security
The remediation of arsenic contaminated sites is an essential priority for the protection of human health
The shapes of the isotherms were similar (L type), but the maximum of sorption greatly decreased. These findings indicate that lesser adsorbing sites were available for arsenate, because thiosulfate ions preferentially adsorbed on the solid phase as a consequence of a competition between the two ions
Summary
Soil is an important sink for arsenic, which is considered a hazardous priority pollutants (Nickson et al, 1998; Berg et al, 2001; Centeno et al, 2002; Nriagu et al, 2009) and its uptake by living organisms in contaminated soils, poses a serious threat to human health and to food security.One of the major environmental emergencies worldwide is due to the high concentrations of As in the aquifers of Bangladesh, which have adversely affected the health of the local populations (Smedley and Kinnburg, 2002). Soil is an important sink for arsenic, which is considered a hazardous priority pollutants (Nickson et al, 1998; Berg et al, 2001; Centeno et al, 2002; Nriagu et al, 2009) and its uptake by living organisms in contaminated soils, poses a serious threat to human health and to food security. The remediation of arsenic contaminated sites is an essential priority for the protection of human health. The most common approach to clean-up As contaminated soil is excavation followed by landfilling, which is a highly disruptive procedure for the environment. Alternative technologies are required in order to protect the environment without destroying the soil, which must be considered as an essential and non-renewable resource. Phytoremedation has great potential as a remediation technology: it is less invasive and is cost effective compared to conventional technologies
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