Preparation Of Green Ca/Mg-Modified Biochar And Its Adsorption Study Of Phosphate In Wastewater

At present,the phosphorus shortage and the deterioration of aquatic environment have become two major problems that need to be solved urgently all over the world.Among many phosphorus removals processes,chemical adsorption process,is considered a promising and feasible technology,owing to its simple operation,low cost,high efficiency,low sludge yield and high selectivity.In this study,marble waste and calcium-rich sepiolite were employed as Ca and Mg sources,and the agricultural waste was used as carbon source to synthesize two modified-biochar composites（Mar-BC800 and Sep-BC800）for removing phosphate from waste streams.Firstly,the physical and chemical properties of Mar-BC800 and Sep-BC800were analyzed by using characterization methods such as SEM-EDS,Elemental Analyzer,ICP-OES,XRD,FTIR,BET and Zeta potentiometer.Secondly,the effects of pyrolysis temperature,dosage and solution p H on phosphate adsorption by Mar-BC800 and Sep-BC800 were systematically investigated,and the adsorption kinetics,adsorption isotherm,adsorption thermodynamics and adsorption mechanism were explored.Finally,the effects of competing anions,antibiotics and actual wastewater on phosphate adsorption by Mar-BC800 and Sep-BC800 were explored,and the dynamic adsorption behavior of Mar-BC800 and Sep-BC800 on phosphates were studied.The available phosphorus leaching experiment and economic benefit analysis were carried out to comprehensively evaluate the application prospect of Mar-BC800and Sep-BC800 in practical engineering.The main results are as follows:（1）Mar-BC800 and Sep-BC800 exhibited disordered and fragmented surface with porous structure.Meanwhile,Ca and Mg have been successfully incorporated into the surface of biochar,and existed in the form of Ca O、Ca（OH）₂、Mg O and Mg（OH）₂.Furthermore,compared with unmodified biochar,Mar-BC800 and Sep-BC800 have more abundant pore structures and surface functional groups,and the S_BET,pore volume and pore size were significantly improved.It has been proved that biochar modified by Ca/Mg can form more favorable physical and chemical properties for phosphate adsorption.（2）The Ca/Mg-modified biochar showed the highest adsorption capacity for phosphate obtained by pyrolysis at 800°C,and the optimal dosage of Mar-BC800 and Sep-BC800 were therefore determined at 0.3 g/L and 0.6 g/L,respectively.Among,Mar-BC800 exhibits the best adsorption performance for phosphate.Meanwhile,Mar-BC800 and Sep-BC800 exhibited excellent performance for phosphate adsorption from aqueous solution in a wide p H range of3–11,and the maximum adsorption capacity of phosphate by Mar-BC800 and Sep-BC800 was263.17 and 128.21 mg/g,respectively.Furthermore,the adsorption process was best described by the pseudo second-order model and Langmuir model,indicating that the adsorption processes were mainly dominated by the chemisorption process.Meanwhile,the adsorption process was endothermic reaction,and the reaction was spontaneous.（3）The mechanism of Mar-BC800 and Sep-BC800 to adsorb phosphate includes surface precipitation,electrostatic attraction,surface complexation and physical adsorption.The dominating sorption mechanism of Mar-BC800 and Sep-BC800 for phosphate is that Ca²⁺and Mg²⁺with phosphate to form Ca₅（PO₄）₃OH、Mg₃（PO₄）₂ and Mg₃（PO₄）₂·4H₂O precipitate.（4）All the selected anions,antibiotics and actual wastewater on Mar-BC800 and Sep-BC800 adsorption of phosphate were limited,and Mar-BC800 and Sep-BC800 still have excellent adsorption performance for phosphate under dynamic adsorption conditions.In addition,the net costs for Mar-BC800 and Sep-BC800 production is determined to be 0.355and 0.680$/kg,respectively,and the phosphate adsorbed modified biochar can well leach the available phosphorus.The results show that Mar-BC800 and Sep-BC800 had excellent adsorption properties,stability,and adsorption selectivity,indicating that Mar-BC800 and Sep-BC800 have great potential for remediation phosphate contamination from wastewater in practical application.At the same time,the phosphate adsorbed modified biochar can applied as fertilizer to farmland soil,which have the functions of increasing soil fertility and improving soil physical and chemical properties.