Effects of modified biochar on As-contaminated water and soil: A recent update

Arsenic is deemed to be a highly toxic and non-metallic element, which is widely distributed in nature. Due to natural and anthropogenic factors, more areas across the globe are suffering As contamination, and this has attracted international attention. Biochar, as a low-cost and environmentally friendly adsorbent, has become a popular solution for As remediation. However, the unmodified biochar still has limitations as a sorbent and immobilizer for As in environment. For example, stronger affinity to As, higher removal efficiency of As, and better capacity of adaption to the varied environment on the surface of biochar by varied mechanisms should be improved by different methods. This chapter reviews the latest findings of modified biochar for As remediation and shows that the current trend is mainly for Fe/Mn-containing minerals embedment on biochar. Multiple new modification methods for As removal/immobilization in both water and soil were reviewed, such as Fe-modified biochar, Mn-modified biochar, Zn-modified biochar, Si-modified biochar, rare earth elements-modified biochar, and acid-base-modified biochar. The metal-based modified biochar facilitates the adsorption of As, especially for Fe/Mn-containing minerals, by increasing the active sites of adsorption. Besides, the acid/base-modified biochar increase pore volume and oxygen-containing groups to improve adsorption capacity of As. Nonetheless, further studies are suggested to include engineered biochar for higher stability to overcome alteration in soil environment and promote removals of organic As in both water and soil. A combination of modification methods is proposed for better As removal/immobilization.