Management of a contaminated mine soil: Effect of soil water content on antimony and arsenic immobilisation by iron-based amendments and biochar composites

Alteration of wet and dry periods affects soil geochemical properties that drive contaminant availability and mobility. Consequently, the efficiency of soil remediation may vary, and thus soil management needs to consider these variations. This study reveals the effect of soil moisture manipulation on the behaviour of the toxic metalloids antimony (Sb) and arsenic (As) and the immobilisation efficiency of nanoscale zero-valent iron (nZVI), biochar (BC) and BC modified by iron or iron/ferrous sulphide (nZVI-BC and Fe/FeS-BC). Soil samples from an abandoned Sb mining site were tested under laboratory conditions. Two distinct (organic and mineral) soil layers were incubated with each of the amendments (2% w/w application rate) at 50% and 100% water holding capacity (WHC) of the soils for 10 weeks, and subsequently extracted with demineralised water and rainwater. The immobilisation efficiency of the tested amendments was significantly influenced by the different soil moisture regimes and type of contaminant. Sorption onto secondary Fe minerals was the dominant Sb/As immobilizing mechanism. The iron-based amendments showed up to 97% efficiency for Sb and up to 96% for As. However, Sb/As release was generally higher at 100% WHC and in the rainwater extracts compared to the other conditions, indicating that heavy rainfall and flooding of soils in-situ may increase contaminant availability and mobility. In the organic layer, enhanced formation and leaching of metalloid-DOC complexes and sorption competition of the negatively charged Sb/As species and DOC can be expected. Also, reductive dissolution of Sb or As-bearing metal oxides could lead to the observed higher Sb/As release under flooded conditions, resulting in exacerbated environmental and health risks. Therefore, it is crucial to (1) improve the sorption characteristics of the tested amendments and/or develop other effective amendments for Sb/As immobilisation in different water regimes in soil; (2) test the amendments under real field conditions to evaluate the long term wet/dry exposure; and (3) develop proper risk assessment and soil remediation strategy for the study area and similar sites.