Purification Of Se(Ⅳ)-Containing Wastewater With Zero-Valent Iron Enhanced By Nitrogen-Doped Shrimp Shell Biochar

With the development of modern industrial activities such as mining,smelting and the development of selenium-enriched industries,the excess selenium in water has become one of the toxic pollutants that cannot be ignored.Selenium coexists with other pollution such as antibiotics and heavy metals,forming a complex polluted water and threatening the human health.Nano-zero valent iron（nZVI）with low toxicity and high reductive activity can reduce the toxic selenite（Se（Ⅳ））into the nontoxic Se（0）,but it suffers from the easy agglomeration and deactivation by oxidation when using for water treatment.In this paper,the nitrogen-doped porous biochar（NBC）derived from a marine waste biomass,shrimp shell was used to support the nano zero valent iron,resulting in a composite（NBC-nZVI）.The NBC with excellent electrochemical performance promoted the reactivity and electron utilization of nZVI and enhanced the removal capacity of Se（Ⅳ）.Meanwhile,the NBC,as carbon material,could capture the organic pollutions such as antibiotics.Therefore,NBC-nZVI was expected to be the candidate for simultaneously removing the Se（Ⅳ）and antibiotics in the potential wastewater of se-enriched culture.In addition,the sulfurized nano zero valent iron/NBC（NBC-S/nZVI）was prepared to offer the composite with active sites of S^2-and Fe（0）,which could simultaneously sequestrate the Cd（II）and Se（Ⅳ）from the potential waste water from the Se-enriched plant.The contents and the conclusions for this thesis are as follows.（1）NBC-nZVI was in situ synthesized on NBC by a liquid phase reduction.The composition and structure of NBC-nZVI were characterized by XRD,SEM,TEM,BET and XPS,which proved that nZVI was successfully loaded on NBC.The removal performance of Se（Ⅳ）by NBC-nZVI and nZVI was compared,and their changes of physicochemical properties in the Se（Ⅳ）removal process were investigated by Zeta potential and electrochemical analysis.The results showed that NBC increased the surface potential of the composite to promote the Se（Ⅳ）enrichment and provided protons to promote the reaction of Se（Ⅳ）and nZVI.In addition,NBC provided an additional electron transport pathway for the electron transfer between nZVI and Se（Ⅳ）,which simultaneously improved the reactivity and electron utilization of nZVI for Se（Ⅳ）sequestration.At the initial p H of 3.0,the reaction rate and electron availability of NBC-nZVI were increased by 143.4%and 10.1%compared to nZVI,which leaded to an increase of（26.40%）in the removal capacity for the unit Fe（0）in NBC-nZVI in comparison to the unit Fe（0）in nZVI when capturing Se（Ⅳ）removal capacity.（2）In order to address the potential co-contamination of Se（Ⅳ）and antibiotics in the Se-enriched culture,NBC-nZVI was used to simultaneously remove Se（Ⅳ）and ciprofloxacin（CIP）from water.The results showed that the adsorption capacity of NBC-nZVI for CIP was higher than that of NBC and nZVI（at p H₀=7.0,the adsorption capacity of NBC-nZVI was 1.21 and 1.55 times higher that of NBC and nZVI,respectively）.At the same time,in the two-component system,CIP did not significantly impact the removal of NBC-nZVI toward Se（Ⅳ）,while the introduction of Se（Ⅳ）could promote the adsorption of NBC-nZVI for CIP.The system containing NBC-nZVI,CIP and Se（Ⅳ）was further treated by H₂O₂.Because H₂O₂ enhanced the renewal of nZVI surface to generate more Fe³⁺/Fe²⁺,which was conducive to the reduction of Se（Ⅳ）and the degradation of CIP.Therefore,more than 90%CIP and Se（Ⅳ）were removed within 45 min.（3）In view of potential co-contamination of Se（Ⅳ）and Cd（II）in selenium-enriched planting,NBC-S/nZVI was prepared by the combination of sulfur modification and nano zero valent iron synthesis.The structure of the product was characterized by technologies of XRD,SEM,Raman,XPS,and so on.The adsorption behavior and effects of Se（Ⅳ）and Cd（II）in single component system and binary component system were systematically studied.The results showed that S^2-was the main site for Cd（II）capture,and the removal capacity of Cd（II）was improved by increasing the degree of sulfuration.However,the increase of sulfuration resulted in a decrease of the content of zero valent iron,which caused a discount in the Se（Ⅳ）removal capacity of NBC-S/nZVI.NBC-S/nZVI prepared with S/Fe molar ratio of 0.3revealed an excellent removal capacity for Se（Ⅳ）and Cd（II）,and its adsorption capacity for Se（Ⅳ）and Cd（II）in single metal system could reach 223.2 and 164.3 mg/g,respectively.Compared with the mono-metal system,Se（Ⅳ）in the bimetallic system could increase the adsorption rate of NBC-S/nZVI toward Cd（II）,but reduced its adsorption capacity;however,Cd（II）could promote both of the adsorption rate and capacity of NBC-S/nZVI for Se（Ⅳ）.This thesis proposed to use NBC as the carrier of nZVI to construct the composite with the function of NBC and nZVI,and realized the synchronous and efficient remediation of Se（Ⅳ）and CIP in water.In addition,the nZVI in NBC was sulfurized to offer S^2-active site for the Cd（II）capture.Combining the Fe（0）toward Se（Ⅳ）,this composite could simultaneously sequester Se（Ⅳ）and Cd（II）from water.These studies took the seafood waste,the shrimp shells as raw materials to deal with potential pollution of Se-rich industry,which not only provided a new way for the high-value utilization of seafood waste biomass,but also provided theoretical and technical support for potential pollution control of Se-rich industry.