Removal of potentially toxic metals from soil by para-sulphonato-thiacalix[4]arene: competitive extraction and selectivity sequence

Abstract

para-Sulphonato-thiacalix[4]arene (STC[4]A) is an emerging supramolecular receptor for enhancing the extraction of potentially toxic metals (PTMs) from soil. Batch extraction experiments were conducted to evaluate the competitive extraction of cadmium (Cd), copper (Cu), and lead (Pb) by STC[4]A from single- and multi-metal contaminated soils. The extraction capacity and selectivity mechanism were investigated by determination of the conditional stability constants (log K) of the STC[4]A–M complex via spectrophotometric titration and the metal speciation using sequential extraction. Under STC[4]A deficiency, the order of metal extraction efficiencies was Cd > Cu > Pb for single-metal contaminated soils, while the selectivity sequence of metal extraction was Cu > Cd > Pb for multi-metal contaminated soils. Spectrophotometric titration indicated that the log K values of the STC[4]A–M complex were in the order of STC[4]A–Cu > STC[4]A–Cd > STC[4]A–Pb and that the values governed the metal extraction selectivity of STC[4]A in multi-metal contaminated soils. Although the log K values for STC[4]A–Cd were intermediate, STC[4]A showed the largest extraction yield for Cd from single-metal contaminated soil due to its larger proportion of exchangeable fraction in soil. The effectiveness of STC[4]A washing appeared to be highly dependent on the metal–STC[4]A formation constants and the metal distribution in soil.