Enhancing Sludge Derived Biochar For Hexavalent Chromium Removal From Aquatic Environment

Chromium is one of the 17 chemicals that pose the greatest threat to humans.Cr(?)is toxic and carcinogenic,and chromium pollution is a serious threat.In this work,municipal sludge was chose as the raw material to produce sludge derived biochar by pyrolysis under oxygen-limited condition.Sludge derived biochar(SDBC)was utilized to mediate the heavy metal pollution in aquatic environment.Effect of pyrolysis temperature on the SDBC property was determined and various modification methods were adopted to improve its adsorption performance.The optimum modification methods as well as loading rates were identified to produce an efficient adsorbate to adsorb hexavalent chromium(Cr(?)).SDBC were characterized using electron microscopy and surface spectroscopy techniques and some environmental variables that affect the sorption behavior along with the proportion of adsorption/reduction were investigated to determine the possible adsorption-reduction mechanisms that involved in the reaction.Fix-bed column were built to estimate the sorption performance of modified SDBC in continuous current condition.The main conclusions are as follows:(1)600? was selected as the optimal temperature for the preparation of biochar by pyrolysis of municipal sludge.Organic material in BC600 is completely carbonized,and a large number of nano-sized pores was distributed in the materia.BC600 contains a large amount of Si,C,and O elements,and the content of highly toxic heavy metal elements is low.The biochar surface contains a certain amount of functional groups such as-OH,C-H and C=C,and has a high degree of aromatization.(2)nZVI modified biochar has the best adsorption capacity,and nZVI is twice the mass of BC600 as Fe is the optimal loading ratio.There is a large amount of uneven floc on the surface of nZVI-BC,forming a stable nZVI-BC composite material,and the loaded nZVI significantly improves the specific surface area of biochar.In the modification process,nZVI reacts with BC600 surface-OH,C=C,aromatic-OH and aromatic CH groups to form functional groups similar to-COO-Feand-CO-Fe-,and part of nZVI is oxidized to Fe(III)binds to a carboxyl group.During preparation and drying,a small portion of nZVI is oxidized to ferrous oxide.(3)Different dosages,initial pH values and competing ions of modified biochar nZVI-BC all have an effect on Cr(?)/ Cr(III)adsorption.Adsorption of Cr(?)and Cr(III)on nZVI-BC is in accordance with the Langmuir equation.The adsorption of Cr(III)conforms to the Elovich kinetic model.The adsorption of Cr(?)conforms to the pseudo second-order reaction model and the three-stage intraparticle diffusion model.The theoretical saturated adsorption capacity is 24.55 mg/g.In the initial use of nZVI-BC,Cr(?)was simultaneously reduced and adsorbed,and the reduction of nZVI was dominant.After the first use,nZVI-BC loses its reducibility and turns into a magnetic(nFe3O4-BC)composite.The adsorption mechanism has electrostatic attraction,complexation reaction with the inner layer coordination.(4)The breakthrough curves of Cr(?)adsorbed on nZVI-BC columns are consistent with the Thomas and Yoon-Nelson models.Flow,initial concentration affect adsorption capacity and dynamic behavior.When the flow rate was 1.0 mL/min and the influent concentration was 100 mg/L,the adsorption capacity was high,and a large adsorption capacity of 34.28 mg/g and a moderate breakthrough time were obtained.Through the modification of sludge-based biochar by nZVI,the adsorption of biochar on Cr(?)is greatly enhanced.nZVI-BC has great potential as a high-efficiency adsorbent for treating Cr(?)-polluted wastewater in a continuous-flow mode of operation.Cr(?)damage to the environment can be minimized by constructing a fixed column.