Tannic acid as a cost-effective substitute for saponin in soil remediation

The efficiency of soil washing with tannic acid (TA) and saponin (SAP) was compared. In the contaminated soil, Cu and Zn were more mobile than Pb. At 3% concentration, both biosurfactants removed similar amounts of Zn (48 and 54%, respectively), SAP removed somewhat more Cu (81%), and TA removed considerably more Pb (63%). SAP removed more Cu due to its higher number of carboxylic groups. TA dissolved the more stable forms of metals because of its higher acidity resulting from the content of phenolic groups. Based on the potential ecological Risk Index (RI), both biosurfactants reduced the risk posed by the contaminated soil from "considerable" (RI=364 ) to "low" (RI=118, on average). These results, and the fact that TA is more than three times less expensive than SAP, make TA an attractive substitute for SAP in soil washing.