Purified Cadmium And Arsenic Contamination By Alkali/Fe-modified Biochar

Cadmium（Cd）and arsenic（As）contamination has seriously damaged ecological stability and human health.Still,their different chemical properties（Cd in the cationic form and arsenic in the anionic form in the aqueous solution）challenge the simultaneous removal of Cd-As contamination.Since adsorption technology is economical,efficient,and operational,controlling target pollutants by adsorbing heavy metals from water and soil has become a trend.Biochar has been widely used to treat heavy metal pollution because of its comprehensive source,strong adsorption capacity,and recyclability.It has also been shown that alkali-modified and iron-modified biochar promote the adsorption of cationic and anionic pollutants,respectively.To this end,this paper tried to prepare alkali/iron composite modified cotton straw biochar（FACSB）,focusing on its performance and mechanism for the adsorption and passivation of Cd（II）and As（V）（abbreviated as Cd,As）pollution in water and soil.Meanwhile,the longitudinal migration of FACSB fine particles（colloids,dynamics diameter<1μm）adsorbed cadmium and arsenic contaminants in the soil（quartz sand）was further explored to provide a reference for the risk assessment of colloidal migration of heavy metal contamination generated by passivation remediation.The following study results were obtained:（1）Experiments investigated the effects of initial p H,kinetics,isotherms,and coexisting ions on the adsorption efficiency of Cd and As in the single Cd、As and binary Cd-As water pollution system.Compared with the pristine biochar,the adsorption of Cd increased 3-fold,and the adsorption of As increased significantly from 0 to 30.69 mg/g.In the binary Cd-As system,the maximum adsorption of Cd and As were 41.27 and 34.51 mg/g,respectively,at p H=6.The presence of Cd in the solution contributed to the adsorption of As.In the coexisting ion effect experiments,the presence of Mg²⁺and Ca²⁺competed with Cd,whereas PO₄^3-inhibited As adsorption,and the high ionic strength negatively impacted Cd adsorption.SEM,BET,FT-IR,and XPS analyses before and after adsorption showed a significant increase in the specific surface area of the modified adsorbent and an increase in the number of functional groups,revealing a possible mechanism of electrostatic attraction,ion exchange,precipitation,and multiple complexations（Fe OCd,Fe OAs,Fe OCd As）for the adsorption of Cd and As in the single or binary system.（2）Passivation experiments on Cd-As composite contaminated soil were conducted at0.5%,2.5%,and 5%（w/w,passivator/soil）for six months of continuous incubation.The results showed that the soil p H and SOM increased significantly after adding FACSB,and the bioavailability of Cd and As decreased considerably.The bioavailability of Cd and As could be reduced by 79.99%and 34.72%under 5%treatment,respectively.The increase of soil p H caused by adding a passivation agent enhanced the electrostatic gravitational force and precipitation effect on Cd.In addition,the complexation of Cd with soil organic matter was also an essential mechanism for reducing its bioavailability.The strong adsorption of iron oxides in FACSB produced complexes on arsenic and limited the migration of arsenic in the soil.This study’s alkali iron-modified cotton straw biochar showed sound remediation effects in water and soil Cd and As pollution.（3）Stability of FACSB colloids was observed by the hydrodynamic diameter HD and zeta potential,with the increase of ionic strength（1→800 m M）,HD becomes more significant to739 nm zeta potential tends to 0.Finally,the critical poly-sink CCC is 207.35 m M obtained by fitting the adhesion coefficient.（4）Colloid adsorption Cd-As breakthrough experiments in porous media show that regardless of the flow rate,the colloid concentration,and ionic strength,the addition of colloid will cause a hysteresis effect of Cd and As migration in solution;the peak penetration of Cd-As is more significant（above 0.9/0.8）in high flow rate condition,and there is a differentiation of Cd-As migration in low flow rate condition,and under high concentration colloid condition Cd has a more prominent peak and As has a smaller height;In contrast,under low concentration colloidal conditions,the result is opposite.It means that soil remediation using FACSB should concern the FACSB colloid migration caused by high permeability soil and its effect on the migration of Cd and As pollution in pore water and the resulting risk of groundwater pollution.