Tannic acid and saponin for removing arsenic from brownfield soils: Mobilization, distribution and speciation

Plant biosurfactants were used for the first time to remove As and co-existing metals from brownfield soils. Tannic acid (TA), a polyphenol, and saponin (SAP), a glycoside were tested. The soil washing experiments were performed in batch conditions at constant biosurfactant concentration (3%). Both biosurfactants differed in natural pH, surface tension, critical micelle concentration and content of functional groups. After a single washing, TA (pH 3.44) more efficiently mobilized As than SAP (pH 5.44). When both biosurfactants were used at the same pH (SAP adjusted to 3.44), arsenic mobilization was improved by triple washing. The process efficiency for TA and SAP was similar, and depending on the soil sample, ranged between 50%–64%. Arsenic mobilization by TA and SAP resulted mainly from decomposition of Fe arsenates, followed by Fe3+ complexation with biosurfactants. Arsenic was efficiently released from reducible and partially from residual fractions. In all soils, As(V) was almost completely removed, whereas content of As(III) was decreased by 37%–73%. SAP and TA might be used potentially to remove As from contaminated soils.