| With the fast development of modern industry,heavy metal containing wastewater produced from metallurgy,mining,electroplating,and metal finishing become a serious environmental problem.Chromium(Cr)is a common heavy metal,which could be enriched in human body after ingestion,causing diseases such as central nervous system,liver and kidney damage.Cr generally existed in water in hexavalent and trivalent forms,of which Cr(Ⅵ)could form chromate and can easily dissolved in water with higher toxicity.Thus the efficient removal of Cr(Ⅵ)has long been a research hotspot.Compared to conventional methods for Cr(Ⅵ)removal,photocatalysis is method with faster reaction rate and higher removal percentage,which can reduce Cr(Ⅵ)to less toxic Cr(III).However,current photocatalysts are mainly noble metal or metal oxide nanoparticles,which has weak adsorption performance that limited the applications in high concentrated Cr(Ⅵ)solutions.Moreover,the high capital cost of these materials has restricted the industrial production and applications.Compared to inorganic materials,organic materials including cellulose,chitosan,chitin,and sodium alginate have abundant functional groups which can interact with Cr(Ⅵ)via adsorption.By modification or crosslinking,variety functional groups could be introduced to these materials,which could further enhance the adsorption behavior towards Cr(Ⅵ).In addition,these organic materials are abundant in nature and can be easily acquired from waste biomass.In this study,sodium alginate and biochar was selected as the base matrix to synthesize efficient and environmental friendly adsorbents for the removal of Cr(Ⅵ)ions.(1)Firstly,sodium alginate was crosslinked by using glutaraldehyde as crosslinking agent,and then thiourea grafted on the molecular surface to introduce amino group.The resultant solution was solidified in calcium chloride solution to acquire GTSA material.Protonated amino groups on GTSA can generate electrostatic attraction with Cr(Ⅵ)in acidic solution and adsorb Cr(Ⅵ)efficiently.Moreover,Cr(Ⅵ)could be reduced to Cr(III)under UV light exposure,and the reduced Cr(III)ions were released to the solution by electrostatic repulsion,which provided free adsorption sites for further adsorption,so as to improve the reduction ability of Cr(Ⅵ).Within 3 h,the removal rate of 50 mg/L Cr(Ⅵ)solution reached 99%,and the reduction rate of Cr(Ⅵ)reached 79%.Cyclic experiments show that GTSA has good stability.After five photocatalytic tests,the removal rate for 10 mg/L Cr(Ⅵ)solution remained 98.23%,suggesting that GTSA could be regenerated successfully.(2)Based on GTSA,biomass activated carbon was introduced to increase the adsorption capacity of adsorbent.Sodium alginate was also modified with glutaraldehyde and thiourea,and biomass activated carbon was blended with the modified solution to increase the specific surface area and oxygen-containing functional groups of the adsorbent.Then T-BSA was prepared by solidification in calcium chloride solution,and its adsorption behavior for Cr(Ⅵ)solution was investigated.According to the fitting of adsorption kinetics and thermodynamics,the adsorption process conforms to pseudo-second-order kinetics and Langmuir isothermal model.Within 12 h,the Cr(Ⅵ)removal rate of 50 mg/L was 91.36%.The results indicated that T-BSA has a good adsorption capacity for Cr(Ⅵ).(3)After the preparation of T-BSA,its photocatalytic behavior in Cr(Ⅵ)containing solution was investigated.Compared with GTSA,the specific surface area of T-BSA is greatly increased,which increases the reaction sites of photocatalyst.After photo-reduction of Cr(Ⅵ)from the solution,the reduced Cr(III)ions can also be adsorbed and removed from the solution,enhancing the overall removal of total chromium.By model fitting the photocatalytic process,it can be seen that the photocatalytic process is more in line with the quasi-first-order kinetics.Under UV irradiation,the removal rate of Cr(Ⅵ)at 50 mg/L was 99.98% within 3 h,and the content of Cr(III)in the solution was 49%. |
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