Simultaneous immobilization of As and Cd in a mining site soil using HDTMA-modified zeolite

Arsenic (As) and cadmium (Cd) co-contamination has been a typical problem in Chinese agricultural land adjacent to historical metal mining and smelting activities. Remediation of As and Cd in soil has encountered many difficulties owing to the distinct nature of the two metal(loid)s. In this study, we developed a remediation scheme by adding a hexadecyltrimethylammonium (HDTMA)-modified zeolite to a mining site soil and evaluated the immobilization effect. The result of the increased surface zeta potential indicates that the HDTMA modification conferred the zeolite with adsorbability towards As through the cationic surfactant head. The addition of the highest dosage of HDTMA-modified zeolite (10%) to the contaminated soil greatly improved soil organic matter by 1.4 times, partly due to the elevated C loading on the zeolite from HDTMA. Sequential extraction results show that the addition of HDTMA-modified zeolite not only increased the residual fraction of As (by 2.7-5.9%) but also reduced the toxicity-related fraction (by 2.3-2.7%) when compared to the unmodified zeolite and blank treatments. The oxidizable fractions of Cd in the modified zeolite treatment were significantly higher than that in the blank soil. Besides, the exchangeable fractions of Cd were all significantly reduced in the zeolite treatments. Enzyme activity assays show that the HDTMA-modified zeolite treatment could greatly improve soil microbial environment. The physiologically based extraction test (PBET) also proved that the bioavailability of As and Cd was reduced after the modified zeolite treatment.