Functionalized Biochar Supported Manganese Sulfide Composite Interaction With Aqueous Cadmium

As one of the most toxic heavy metals recognized by the World Health Organization(WHO),cadmium(Cd)is the first type of pollutant strictly controlled in wastewater discharge standards.Adsorption is considered to be an optimum method for Cd removal from aqueous solutions.The searching for efficient and inexpensive adsorbents has become the focus of research.Manganese sulfide can effectively remove Cd from wastewater,and the use of biochar-supported manganese sulfide can enhance its removal efficiency.In this study,manganese sulfide loaded functionalized biochar(MnS-fBC)was synthesized and the optimal loading ratio was determined.The loading of MnS was evaluated by characterization.Secondly,some influencing factors on the adsorption of Cd by MnS-fBC were investigated,and the adsorption mechanism was explored with the help of characterization results.Subsequently,the effects of different oxidation conditions on the adsorbent and adsorption in solutions were investigated.The dynamic process of the reaction was also determined.Finally,the risk assessment of post adsorption biochar was conducted.Result shows that the Cd in biochar was stabilized by re-pyrolysis,and the potential risk of post adsorption biochar under different re-pyrolysis temperatures was analyzed.The specific conclusions are:1.The MnS-fBC biochar was synthesized.Different loading ratios had wonderful Cd removal capacity,and the optimal load ratio was 1:1.2.A higher solution pH promotes Cd(II)adsorption.The removal rate of Cd(II)gradually decreased with an increasing initial concentration.With a higher reaction temperature,both the adsorption capacity and the adsorption rate would increase.Additional dosage of biochar could enhance the removal efficiency,while reducing the amount of Cd(II)adsorbed per unit volume.Anions can be promoted Cd(II)adsorption while cations do the opposite.Biochar has desirable recyclability in cycle experiments.Possible Cd adsorption mechanisms by MnS-fBC include:(1)aqueous Cd(II)directly binds(or adsorbs)to MnS on MnS-fBC and form complexes;(2)manganese oxides are formed by MnS oxidized by dissolved oxygen(or already existed manganese oxide),which tend to combined(or adsorb)aqueous Cd(II);(3)With manganese coupled with sulfur in the MnS-fBC was released in to solution,the vacated S forms CdS with Cd(II)and solidified in biochar.3.After oxidation,sulfur content disappeared from MnS-fBC and the valence of Mn increases,which reduces the adsorption capacity.The un-oxidized MnS-fBC has a better adsorption capacity on Cd(II)in the anaerobic system than in the aerobic system.Under different oxidation conditions,the adsorption reaction can be divided into three stages,and the reaction mechanism is different in different reaction stages.4.Re-pyrolysis of post adsorption biochar facilitates further stabilization of adsorbed Cd in the carbon matrix,reduces the amount of residual biochar and the risk of releasing bioavailable Cd components into the environment.Re-pyrolysis is an efficient method for safe disposal of post adsorption biochar.And a higher the re-pyrolysis temperature result in a safer solid residue.The as-synthesized MnS-fBC in this study can effectively remove Cd(II)from aqueous solutions.The removal behavior is affected by several factors,and the oxidation conditions have a great influence on the MnS-fBC itself as well as the Cd adsorption dynamics.MnS-fBC has excellent recyclability.Re-pyrolysis can effectively reduce the ecological risk of post-adsorption biochar.The newly reported adsorbent in this study can be used to effectively remove aqueous Cd(II).