Water Treatment Residuals as a Resource for the Recovery of Soil and Water Polluted with Sb(V): Sorption and Desorption Trials at Different pH Values

In this study, the ability of two different water treatment residuals (Fe- and Al-WTRs) to accumulate antimony(V) from an aqueous solution was investigated at different pH values (pH 4.5 and 6.5). Both WTRs showed a maximum Sb(V) sorption capacity of approx. 0.22 mmol g⁻¹ at pH 4.5 which declined at pH 6.5, particularly for Fe-WTR (i.e., 0.059 and 0.163 mmol g⁻¹ of Sb(V) sorbed by Fe- and Al-WTRs respectively). The greater capacity of WTRs to accumulate antimonate at pH 4.5 seemed to be linked to their chemical properties, such as the pHPZC and the specific surface area. At both pH values, the Sb(V) sorption by Al- and Fe-WTRs followed a pseudo-second-order kinetic model, while the sorption isotherms data fitted the Freundlich model better than the Langmuir one, suggesting the presence of heterogeneous Sb(V) adsorption sites. The sequential extraction of WTR-Sb(V) systems showed that a significant amount of Sb(V) was retained by WTRs through chemical interactions, i.e., through the formation of inner sphere surface complexes [e.g., Fe/Al–O–Sb(V)]). This was particularly relevant at higher pH values (pH 6.5) where more than 60 and 50% of the Sb(V) sorbed by Fe- and Al-WTRs respectively was retained by specific chemical bonding. The residual Sb(V) was higher for the Al-WTR at both pH values, and the highest amount of residual Sb(V) was recorded at pH 4.5 [> 65% of the total Sb(V) sorbed]. SEM-EDX analysis of the WTR-Sb(V) systems showed that antimony was mainly associated with Fe and Al, thus supporting the Sb(V) affinity for Al/Fe oxy-hydroxides. Treatment of WTR-Sb(V) systems with citric and malic acids, at concentrations relevant in the rhizosphere, indicated that Sb(V) could be released by both acids, with 4.5 mM citric acid favoring the highest Sb(V) release in both WTRs. The results from this study suggest that WTRs could be used as alternative amendments for the in situ immobilization of Sb(V) in acidic or circumneutral polluted soils.