Mechnism Study On The Removal Of Acid Orange And Hexavalent Chromium Through Biochar From Water

The rapid industrialization in China significantly increased the risk of heavy metal and organic pollution of water environment.Thereinto,chromium-containing organic wastewater discharged from electroplating,tanning,pigment,textile,steel and metal processing industries is the most representative one.It has great practical significance to develop the simultaneous removal process of chromium and organic pollution.Biochar is a low-cost,easy-to-operate,and environmentally friendly material.It has been widely used in various types of wastewaters and exhibited excellent pollutant removal efficiency.In this study,a series of porous biochar（BC700,BC800,BC900）were prepared by using aquaculture solid waste as precursors for the removal of hexavalent chromium[Cr（Ⅵ）]and dye（Acid Orange,AO7）from wastewater.The relevant conclusions are listed as follows:（1）The preparation strategy of biochar with large specific surface area was carefully proposed by using aquaculture solid waste as precursor and potassium carbonate as pore-forming agent.The prepared biochar materials were analyzed by N₂adsorption-desorption,Fourier transform infrared spectroscopy（FTIR）,X-ray photoelectron spectroscopy（XPS）,and Raman spectroscopy.The results evidenced well deceloped specific surface area and porous structure of the prepared biochar.The specific surface area of BC900 reached 1679 m²/g and the pore volume was 1.55 cm³/g,which provided sufficient mass transfer interface and active sites for interface reactions.FTIR analysis showed that the prepared biochar contained heteroatom oxygen groups such as C-O-C,C=O and-OH.Raman spectroscopy revealed that the defect degree of biochar changed with the pyrolysis temperature,and the biochar obtained at 800℃had the highest defect degree.When the pyrolysis temperature is 900℃,the defect degree decreased,indicating that extreme high temperature influenced the structural reconstruction of biochar.（2）The prepared biochar exhibited excellent adsorption removal performance toward Cr（Ⅵ）and AO7.At reaction conditions of 25℃and p H=2.0,the maximum adsorption capacity of Cr（Ⅵ）and AO7 onto BC900 reached 214.13 and 666.67 mg/g,respectively.The adsorption and removal process of the two pollutants fitted the pseudo-second-order kinetic model and Langmuir model,demonstrating monolayer adsorption was dominant and accompanied by non-uniform surface adsorption.The major rate-controlling step of the adsorption process might be the intraparticle diffusion process,but not the only limiting step.In addition,the increase of temperature also influenced the removal of Cr（Ⅵ）and AO7.Biochar also exhibited certain selectivity toward different pollutants.（3）Biochar could exhibit reducibility under normal temperature.XPS analysis showed that more than 75%adsorbed chromium on the biochar surface existed in the formation of Cr（III）,suggesting that biochar played the role of reductant and greatly reduced the toxicity of chromium pollution.Density functional theory calculations further showed that the topological defects of biochar can effectively adjust the electronic structure inertia of the carbon matrix,rendering better adsorption of Cr（Ⅵ）through electrostatic attraction,and further causing the reduction of adsorbed Cr（Ⅵ）through subsequent electron transfer.Finally,coexistence of low concentration of AO7promoted the removal of Cr（Ⅵ）,indicating that there was an electron transfer process between the adsorbed AO7 and Cr（Ⅵ）,which was worthy of further study.This study provided a systematic and feasible strategy for the preparation of biochar with large specific surface area,and the prepared biochar shows excellent removal performance toward Cr（Ⅵ）by AO7.The results of this research provided new method for the preparation of biochar and shed light on the mechanism analysis for pollutant removal.