Removal Mechanisms Of Cr（Ⅵ） By HNO₃ Modified Biochar Under Different PH And O₂ Conditions

Heavy metal contamination of water has become a global environmental problem that needs to be solved urgently.Compared with other methods,the adsorption method has the advantages of low cost and environmental efficiency,which is important for the treatment of chromium-containing wastewater.Biochar has received widespread attention as a good adsorbent and reducing agent for heavy metals because of its rich porous structure and redox-active moieties（RAMs,i.e.,oxygen-containing functional groups and environmentally persistent free radicals（EPFRs））.Virgin biochars have relatively low sorption capacities.Therefore,different modification methods have been used to enhance the adsorption performance of biochar.In this study,corn straw biochar was modified by HNO₃ to prepare HNO₃ modified biochar（NBC）and a series of works was conducted to decipher the roles of HNO₃and the mechanism of Cr（Ⅵ）removal.In this study,biochars with different pyrolysis temperatures（300～600^°C）were firstly prepared,and the biochar prepared at 400°C was selected as the virgin biochar（BC）after comparing their performance in removing Cr（Ⅵ）.Then,the biochar modified with 0.8 mol·L^-1 HNO₃ was selected as the HNO₃ modified biochar（NBC）by comparing the effect of modification with different nitric acid concentrations.The results of the Cr（Ⅵ）adsorption experiments showed that the HNO₃ modification not only improved the removal rate of Cr（Ⅵ）by biochar,but also expanded the p H range of the reaction between biochar and Cr（Ⅵ）.The SEM-EDX and BET results showed that the NBC had a larger BET surface area(27.066 m²·g^-1)and pore volume(2.266×10^-2 cm³·g^-1)than those of BC(with 4.864 m²·g^-1 BET surface area and9.750×10^-3 cm³·g^-1 pore volume),and the average pore diameter has been lowered from21.435 nm（BC）to 4.383 nm（NBC）.The FTIR,Boehm analysis and the EPR spectrograms indicated that the modification process increased the content of RAMs of the biochar.The results of the zeta potential analysis showed that the BC was always negatively charged and the isoelectric point（p Hpzc）of NBC was 4.02.In this study,kinetic experiments for Cr（Ⅵ）removal by NBC were conducted under different initial p H（2.0,5.0 and 7.0）and oxygen conditions（oxic and anoxic）.The results revealed that the Cr（Ⅵ）removal by NBC was a hybrid adsorption-reduction process dominated by reduction and that the adsorption process was consistent with the pseudo-second-order model.p H and O₂ are important for the reduction of Cr（Ⅵ）.On the one hand,lower p H values are favorable to the adsorption and reduction of Cr（Ⅵ）by biochar,and about 89.12%（p H 2.0）,73.73%（p H 5.0）and 72.4%（p H 7.0）of chromium adsorbed on the NBC was in the form of Cr（Ⅲ）.On the other hand,NBC can directly and indirectly（activation of O₂ to produce·O₂^-for reduction）reduce Cr（Ⅵ）,where the direct reduction dominated the Cr（Ⅵ）removal and the contribution of indirect reduction increased with the increasing p H value.The RAMs of NBC played a key role in the reduction of Cr（Ⅵ）.In this study,the analysis of the characterization results of FTIR,XPS,Boehm titration and EPR revealed that both oxygen-containing functional groups and EPFRs were involved in the reduction of Cr（Ⅵ）.However,the main electron donors for the reduction of Cr（Ⅵ）by NBC at the same p H were different:oxygen-containing functional groups,especially–OH and C–O–C groups,played a dominant role in reducing Cr（Ⅵ）under acidic conditions,while the neutral condition was more favorable for EPFRs-dominated reduction of Cr（Ⅵ）.