Alkali/Fe-modified biochar for Cd-As contamination in water and soil: Performance and mechanism

Fabricating low-cost and efficient adsorbents to purify water and soil with Cd-As contaminants remains challenging. Previous studies have demonstrated that alkali-modified and iron-modified biochar promote the adsorption of cationic and anionic pollutants. Herein, alkali/Fe-modified cotton straw-based biochar (FACSB) was fabricated, and its performance and the mechanism for remediation of Cd(II) and As(V) in water and soil were investigated. The initial pH, kinetics, isotherms, and coexisting ions on single and binary-adsorption water systems, whilst the temporal effects of passivation in co-contaminated soils, were systematically examined. The results showed that the maximum adsorption capacity of Cd and As were 42.57 and 30.69 mg/g for single-adsorption system, 41.27 and 34.51 mg/g for binary-adsorption adsorption system, respectively, enhancing adsorption for As when Cd presence. The presence of Mg2+ and Ca2+ competed with Cd, whereas PO43- inhibited As adsorption, and the high ionic strength exerted a detrimental impact on Cd adsorption. The mechanisms for removing Cd and As could mainly involve physisorption and chemisorption, respectively. Continuous soil incubation for half-year revealed a substantial reduction in the bioavailability of Cd and As, decreasing by 79.99% and 34.72%, respectively, at 5% treatment (amendment/soil ratio). The amendment, rising soil pH, and organic matter content (SOM) facilitated to immobilize contaminants. The above results indicate that FACSB is promising for Cd and As pollution remediation applications.