The Adsorption Capacity And Mechanism Of Residual-straw Biochar And Enhanced Biochar For Heavy Metal Removal

Heavy metals caused by industry "three waste",coal release,motor vehicle exhaust,wastewater irrigation,agricultural supplies have been causing the pollution of soil and water more and more serious.Existing engineering and chemical management methods due to the large volume,high cost,repair for a long time,may also carries the risk of secondary pollution is not suitable for large areas of pollution remediation.Adsorption has been proven to be an efficient technology while its large-scale application is limited by the high cost of the adsorbent.So find an adsorbent which is inexpensive,efficient,easy to obtain is necessary.Biochar,produced from incomplete pyrolysis of biomass at a certain temperature?300?700 ??,with the high pH,rich in mineral components,a large amount of exchangeable cations and surface adsorption sites,has shown strong sorption capacity for heavy metals.Through thermal pyrolysis of crop residues to make biochar,not only solved the problem of resources waste,can also play an important role in heavy metal remediation,so that "what is taken from farmers,giving back to farming".In this study,we choose wheat straw,rice straw,maize straw as research subjects,converted to biochars at pyrolysis temperature of 450 ?.Biochar properties were analyzed and batch adsorption experiments were conducted.Then focus on the main factor,for quantitative analysis,to clarify its contribution to the biochar adsorption capacity,provide a reference for understand the mechanism and scientific applications of biochar.Modified the wheat straw biochar for further enhance its adsorption ability.The main results are as follows:?1?Sorption kinetics of Pb²⁺ and Cd²⁺ on biochar particles followed pseudo second order kinetic model.The adsorption rate of Pb²⁺by wheat straw biochar?WSB?,rice straw biochar?RSB?and maize straw biochar?MSB?were 0.044,0.019 and 0.012 mg·g-1·h-1 and for Cd²⁺ were 0.195,0.164 and 0.070 mg·g-1·h-1 respectively.Isothermal adsorption of Pb²⁺and Cd²⁺ by WSB,RSB and MSB fitted well with Langmuir equation,the maximum sorption capacity of Pb²⁺ were 99.65,110.31 and 88.82 mg·g-1;and were 30.64,29.39 and 21.47 mg·g-1 respectively for Cd²⁺.The removal rate increased dramatically when the initial pH was from 2.5 to 3.5.In addition,the removal of Pb²⁺ and Cd²⁺ from aqueous solution might be through chemical precipitation with CO32-,PO43-,SiO2 in WSB and RSB,while through adsorption on the surface of MSB particle with functional groups.?2?Wheat straw biochar was soaked and washed using ultrapure water.Results indicated that washing of wheat straw biochar almost removed the soluble salts include carbonate,sulfate,silicate and phosphate.Organic functional groups were preserved,specific surface area of washed biochar was about 1.5 times of unwashed,pore size almost no change.The sorption kinetics of Pb²⁺ followed pseudo second order kinetic model,the adsorption rate of Pb²⁺ onto washed wheat straw biochar?0.1012 mg·g-1·h-1?was twice times of unwashed biochar?0.0441 mg·g-1·h-1?.Adsorption isotherm fitted well with Langmuir equation,the maximum sorption capacity of Pb²⁺ onto unwashed and washed biochar were 99.66 and 20.27 mg·g-1 respectively.Carbonate and silicate were released little when the pH at 2.5,phosphate was easy to release,but the concentration was low,so the removal rate of Pb²⁺ was low,pH from 3.5 to 6.5,a large amount of carbonate and silicate were released and precipitated with Pb²⁺,the removal rate of Pb²⁺ was high,the release of sulfate was not affected by pH.?3?Pre-treating the wheat straw with iron and manganese oxides then pyrolyse to make the modified biochar.After modification,the content of C5 H,O,N were decreased from 59.84%,3.32%,7.91%,0.62%to 44.94%,2.60%,7.63%,0.38%.Little change in the specific surface area,the bound of C N were reduced,while O=C-O was increased.Iron and manganese oxides in the form of small particles uniformly distributed in the pores and surface of the biochar and SO42-and K+ were introduced.The predicted maximum Pb²⁺adsorption capacities for wheat straw biochar and modified wheat straw biochar were found to be 121.52 and 276.38 mg·g-1,respectively.Meanwhile,the modified wheat straw biochar had a high removal rate even under acidic conditions.Enhanced adsorption capacity due to the specific adsorption of Pb²⁺ on iron and manganese oxides and precipitation with SO₄^2-.