Research On The Removal Of Uranium By Sewage Sludge And Sewage Sludge-based Biochar

The quick increase in population has brought forth a large requirement for water,and the consequent large increase in sewage treatment plants and the improvement of treatment efficiency have led to the production of a large amount of excess sludge.If these sludges cannot be treated safely and effectively,serious secondary pollution problems will occur.In recent years,some researchers have found that biochar prepared from sludge can effectively remove heavy metals.The conversion of sludge into biochar by slow pyrolysis and hydrothermal carbonization(HT)as adsorbents for pollutants in water bodies is safe and effective.The biochar made from sludge meets the goal of sustainable resource recovery and utilization and promotes a wastewater-based recycling economy.This paper explored the adsorption behavior of U(Ⅵ)by the hydrothermal pretreated anaerobic sewage(HT-AD)sludge,the residual sludge(S1)and the compost sludge(C1).In addition,hydrochars(at 100,150,and 190 0C)and pyrochars(250℃)(S-HC100,S-HC150,S-HC190,S-BC250 and C-HC100,C-HC150,C-HC190,and C-BC250)were prepared from S1 and C1 as adsorbents for U(VI).By using various characterization techniques,such as scanning electron microscopy(SEM),Fourier transform infrared spectrometry(FTIR),N2 adsorption and desorption isotherm,13C nuclear magnetic resonance(NMR),etc,the surface morphological characteristics,functional groups,and surface area of the adsorbents,and the mechanism of U(Ⅵ)removal is obtained.This paper mainly concludes that:(1)HT-AD sludge has a flocculent structure and a large specific surface area.Solution pH and ionic strength affected the adsorption rate of U(Ⅵ)by HT-AD sludge.The inner-sphere and out-sphere complexation between the UO22+ and the HT-AD sludge,S1 and C1 dominated the adsorption when pH<6.0 and pH>6.0,respectively.The adsorption of U(Ⅵ)by HT-AD sludge was more in line with the pseudo-second-order kinetic model,and the adsorption isotherm corresponded well with the Langmuir model,indicating single-layer chemical adsorption.At 278 K,pH=3.2,the maximum adsorption capacity(qmax)of HT-AD sludge for U(Ⅵ)is 117.13 mg/g,which is higher than most unmodified biochars and activated carbons.-OH,-COOH and P-O functional groups may participate in the adsorption of U(Ⅵ)on HT-AD sludge.HT-AD sludge also shows good reusability and low risk of heavy metal leaching.(2)S-BC250 contains more aromatic carbon and less polar carbon,while S1-based hydrochars have higher polarity and lower aromaticity.The adsorption dynamics of S1,S-HC190,and S-BC250 were better fitted by pseudo-second-order dynamic model.S-BC250 adsorption isotherm data were more in line with the Langmuir model,while S1,S-HC100,S-HC150,and S-HC190 complied with the Freundlich model,their gmax values were compared as S1<S-BC250<S-HC100<S-HC150<S-HC190.The hydroxyl,carboxyl,and phosphoryl groups of S1 and S1-based biochars played an important role in the adsorption of U(Ⅵ).The adsorption capacity of S1-based biochars were higher than that of S1,which may be due to the increase of specific surface area and polar functional group content of the material after hydrothermal treatment.More than 86%U(Ⅵ)were desorbed by 0.01M HCl or Na2CO3 solution.The reduction in adsorption efficiency of S-HC190 during the cyclic adsorption experiment were lower than that of S1.The leaching of heavy metals in S1 decreased after hydrothermal or pyrolysis treatment,indicating the fixation of heavy metals.(3)The adsorption kinetics of Cl,C-HC100,and C-BC250 were more consistent with the pseudo-second-order kinetic model.The adsorption isotherm data of C1 and Cl-based hydrochars were more consistent with the Freundlich model,while C-BC250 was more consistent with the Langmuir model.According to the Langmuir model,the order of qmax of C1,C-HC100,and C-BC250 was C-HC100>C-HC150>C-BC250>OHC190>C1.The decrease of adsorption capacity of Cl-based hydrochar with the increase of preparation temperature was likely due to the destruction of the stable organic matter structure of C1 during the hydrothermal treatment,which reduced the content of polar functional groups in the material.During hydrolysis and pyrolysis treatment,the destruction of the organic matter structure also lead to the re-release of insoluble heavy metals,which increases the risk of heavy metal leaching.