Study On Removal Efficiency And Mechanism Of Hexavalent Chromium From Electroplating Wastewater By Biochar

Chromium?Cr?is a common pollutant mainly derived from electroplating,mining,wood preservation,and leather tanning.Cr mainly exits in two oxidation states,namely,Cr???and Cr?III?.Cr???is more soluble,less stable,and more toxic than Cr?III?.If it is improperly disposed before discharging into surface water,Cr???will seriously threaten human health and ecosystem.Accordingly,it is an imperative goal to explore an inexpensive and efficient method for Cr treatment.Biochar shows great potential for Cr???removal owing to its large specific surface area,porous structure,and abundant surface functional groups.Lots of biochar has been proved to be effective for Cr???removal from aqueous solution.However,most of the studies simply evaluated the performance of biochar,few of them indicated Cr???removal by biochar mainly caused by electrostatic attraction,reduction and complexation.The important role of functional groups,the existing form and micro spatial distribution of Cr on biochar after the reaction were still unclear.Understanding the deep mechanism of Cr???removal by biochar is the key to evaluate the feasibility of using biochar to treat Cr???polluted wastewater.Therefore,it needs to be further explored with the combination of kinetic study and various characterization methods.In addition,the capacity of unmodified biochar is easily influenced by feedstocks,production method and production environment,the selective adsorption/reduction capacity and rate of unmodified biochar for high concentration of Cr???are limited,the removal capacity is usually lower than 10 mg g^-1,which limits its practical application in industrial wastewater and emergency risk control.Thus,some modified methods should be developed to modify biochar,such as increasing specific surface area,porosity,functional groups,or active sites to some modified methods should be developed to optimize the performance of biochar for environmental remediation.In recent years,the doping of Fe and N on biochar has become a research hotspot to promote its removal capacity.Nano-zero valent iron?nZVI?has large specific surface area,high activity and strong Cr???removal capacity,but nZVI is easy to agglomerate,which greatly reduced its reaction activity.Moreover,there are some disadvantages in the traditional method for nZVI synthesis using hydroboron.Therefore,the development of other low-cost and environmental friendly methods for nZVI synthesis to modify biochar is of great value to improve the dispersion of nZVI and the Cr???removal capacity of biochar.The doping of N on biochar is also an effective method to improve Cr???removal.Three biochar derived from oak wood,pine needle,and apple wood were used for Cr???removal aqueous solution.The removal performance of biochar was investigated and Cr???removal mechanisms were determined using various spectroscopic techniques.Two efficient and novel methods were developed for the optimal oak wood biochar modification to improve the removal capacity of Cr???.One method is utilizing environmental friendly tea polyphenols instead of traditional sodium borohydride to synthesize a novel nZVI modified oak wood biochar?TP-nZVI-OB?;the other method is employing carbothermal reduction method combined with NH₃ treatment to prepare nZVI/N modified oak wood biochar?Fe/N-OB?.The performances of TP-nZVI-OB and Fe/N-OB,and the influencing factors on the removal of Cr???were investigated.The removal mechanism was deeply identified by various techniques,and the performance of these two composites for the actual electroplating wastewater containing Cr???was verified.This paper provides a theoretical basis for the remediation of electroplating wastewater.The following results are obtained:?1?The removal of Cr???using oak wood biochar followed the pseudo second-order kinetic model best.The maximum Cr???removal percentage achieved was 99.9%at a low pH value of 2.0.The initial concentration of Cr???was another important factor influencing Cr???removal.Cr???could be entirely removed at a concentration of 1–50mg L^-1.After the reaction,Cr distributed on biochar primarily as Cr?III?,and mainly distributed on the edge of biochar,which was related to the uneven distribution of surface functional groups of biochar.The reaction was considered to be a combination of multiple procedures,including electrostatic force between Cr???and the biochar,Cr???reduction,and ion exchange between Cr?III?and the cations,and complexation between Cr?III?and the C=O group on the biochar.?2?The removal of Cr???by pine needle biochar was in accordance with the pseudo second-order kinetic model,and the correlation coefficient R² was 0.998.The existing form of Cr???in aqueous solution and the surface charge of pine needle biochar influenced Cr???removal.The best Cr???removal by pine needle biochar was achieved at pH 2.0 with 83.9%Cr???removal from aqueous solution.The initial Cr???concentration also showed a significant effect on Cr???removal.When the initial concentration was in the range of 0-25 mg L^-1,99.9%Cr???was removed.After the reaction,Cr mainly distributed on the edge of pine needle biochar in the form of Cr?III?,the functional groups on the edge played a significant role on Cr???removal.The removal process of Cr???by pine needle biochar included electrostatic attraction between Cr???and biochar,reduction of Cr???,complexation between Cr?III?and C=O functional group.?3?Biochar derived from apple wood was an effective media for removing Cr???from water.The pseudo-second order kinetic model was the superior model for Cr???removal.Cr???removal was highly pH-dependent with the highest Cr???removal efficiency?99.9%?at pH 2.0.The initial Cr???concentration greatly affected its removal using biochar.More than 99.9%of Cr???was removed using biochar when the initial Cr???was within the range of 0–50 mg L^-1.After the reaction,Cr was heterogeneously distributed on biochar,mainly on the surface of particle.Most of Cr was existed as Cr?III?.Cr???removal using apple wood biochar included multiple processes,including electrostatic attraction,Cr???reduction to Cr?III?,complexation between Cr?III?and C=O group,and ion exchange between Cr?III?and cations.?4?TP–nZVI–OB is a potentially superior composite for Cr???treatment of contaminated aqueous solution.The Cr???removal performance of the TP–nZVI–OB was optimized at an Fe/C mass ratio of 2:1.The kinetic data were best fitted by the pseudo-second order kinetic model.The Cr???elimination was maximized at pH 2.0with the Cr???removal of 99.9%.The Cr???removal efficiency was insensitive to low Cr???concentrations(0–50 mg L^-1)and remained at 99.9%.The reaction between Cr???and TP–nZVI–OB?2:1?involved multiple processes?sorption,reduction and co-precipitation?.TP–nZVI–OB also exhibited a superior Cr???removal from actual electroplating wastewater,with the removal efficiency of 99.9%.The interferences of Cr???removal by coexisting heavy metals?Zn,Cu and Pb?and coexisting anions(SO₄^2-,Cl^-and NO₃^-)were weak,,and these ions could be removed by TP–nZVI–OB to some extent.The TP–nZVI–OB synthesized using this method has certain effectiveness and feasibility in the application of Cr???polluted wastewater treatment.?5?The Fe and N co-doping was achieved via carbothermal method followed with NH₃ treatment.The synthesized Fe/N-OB could effectively remove Cr???from aqueous solution.The Fe/N-OB obtained at 1000°C?Fe/N-OB1000?possessed the highest proportion of Fe and displayed a more fast Cr???removal with 99.9%removal efficiency within 10 min.The best Cr???removal was achieved at pH 2.0.Cr???removal by Fe/N-OB1000 was markedly dependent on Fe/N-OB1000 dosage and initial Cr???concentration.The doped Fe and N played a key role on Cr???removal which involved adsorption,reduction along with Cr?III?complexation.Fe/N-OB can also achieve a rapid and effective removal of Cr???from actual electroplating wastewater.It was found that the presence of coexisting heavy metals?Zn,Cu and Pb?and coexisting anions(SO₄^2-,Cl^-and NO3^-)had no obvious influence on the Cr???removal.Fe/N-OB1000 could also remove these ions to some extent.The Fe/N-OB synthesized by this method is a low cost and effective material for removing Cr???from polluted water.