Proper Mode of Using Rice Straw Biochar To Treat Cd-Contaminated Irrigation Water in Mining Regions Based on a Multiyear in Situ Experiment

Abstract

Biochar has been widely used for immobilizing heavy metals in soils due to its favorable sorption capacity. Previous investigations on heavy metal immobilization in waters by biochar are conducted in laboratory. Knowledge about the long-term effects of in situ biochar application on water bodies contaminated by heavy metals remains scarce. In this research, biochar was derived from cadmium (Cd)-accumulated rice straw. A 35-d laboratory simulation and a three-year field enclosure experiment were carried out to assess the impacts of biochar application on Cd behavior in a water–sediment system of a historically Cd-contaminated irrigation pond. Results indicated that the pH of sediment and overlying water increased with biochar addition in both the lab and field experiments. The rice-straw biochar transformed the Cd in sediments from the exchangeable to residual fraction, because biochar addition increased the pH values and organic matter contents in sediment, leading to the sorption, sequestration, and passivation of Cd by sediment. The Cd concentration in overlying water decreased with the increase in biochar addition, and it continuously decreased with repeated annual application. Overall, for long-term effects of biochar addition on soluble Cd concentrations, the repeated annual addition of 2% biochar exhibited better performance than the repeated annual addition of 1% biochar and one-time addition of 6% biochar. This study provided a proper mode of utilizing rice-straw biochar to treat Cd-contaminated irrigation water in mining regions.