Synergistic effects of phosphoric acid modified hydrochar and coal gangue-based zeolite on bioavailability and accumulation of cadmium and lead in contaminated soil

Abstract

In this paper, a novel compound was developed by mixing H 3 PO 4 -modified cauliflower leaves hydrochar (CLH) and coal gangue-based Na-X zeolite (ZL). An alkaline soil contaminated with cadmium (Cd) and lead (Pb) was amended through the individual and synergistic application of CLH and ZL (1%CLH, 2%CLH, 1%ZL, 2%ZL and 1%CLH + 1%ZL), and Chinese cabbage was grown on it. Individual application of CLH was superior to ZL on decreasing the pH of alkaline soil and increasing soil available phosphorus (Olsen-P) and soil organic matter (SOM). In contrast, their combined application significantly improved the soil cation exchange capacity (CEC). Besides, the 1%CLH + 1%ZL was the most efficient treatment in decreasing diethylenetriamine pentaacetate (DTPA)-extractable Cd/Pb and concentrations of these two metals in cabbage root and shoot. Their synergistic application could better increase Cd and Pb immobilization and cabbage yield than their alone application. Furthermore, the immobilization of Pb for all treatments was higher than that of Cd. The synergistic immobilization mechanism of CLH and ZL reflected that the CLH precipitated and complexed with these two metals, which may block the pores of hydrochar or wrap on the surface of hydrochar. So the continuous adsorption and complexation were prevented. Nevertheless, ZL could probably alleviate this obstacle. This finding provides helpful information about using CLH combined with ZL as a soil stabilizer to immobilize heavy metals in contaminated alkaline soil.