Study On Microwave-assisted Preparation Of Rice Husk-based Adsorbent And Control Of Cadmium Water Source Pollution

At present China is at a high speed in the process of urbanization,and sudden heavy metal water source pollution incidents occur frequently,posing a serious threat to regional ecological environment and water security of residents.Traditional technologies have such obvious bottlenecks as limited removal performance,hard-to-collect heavy metal ions and complex operations during the emergency disposal of sudden heavy metal contamination incidents in water source.To address these gaps,this work selected the cadmium ion,which caused sudden water source pollution frequently,as the target pollutant and focused on the fast microwave-assisted preparation of xanthate-modified adsorbent material that was derived from low-cost rice husk.Moreover,an adsorption-coagulation treatment system on the basis of the synthesized adsorbent was developed for emergent Cd?II?removal.Besides,an interval-valued trapezoidal fuzzy group decision-making model was developed,in order to identify the most favorable technology for the disposal of waterworks sludge generated from the system.This work explored the fast microwave-assisted synthesis of xanthate-modified adsorbent(RH_MW-Mg).The rice husk powder?RH?was carbonized with sulphuric acid and was fast modified by sodium hydroxide,carbon disulfide?CS₂?and magnesium sulfate through alkalization,xanthation and substitution processes under microwave irradiation.Response surface methodology was used to optimize the preparation parameters and the optimal synthesis conditions were obtained,i.e.,300 s of alkalization contact time,2.0 of CS₂-RH ratio and 90 s of xanthation contact time.Compared to water-bath heating,the processing time could be decreased from 2.5 h to 0.11 h by using microwave irradiation,and the Cd?II?adsorption capacity of the product could be increased from 137.16 mg/g to 191.32 mg/g.Fourier transform infrared spectroscopy,scanning electron microscope,x-ray photoelectron spectroscopy,etc,were used to characterize the RH_MW-Mg.The results showed that the surface of rice husk turned out to be rougher with the formation of many well-developed pores after modification.The point of zero charge of RH_MW-Mg was identified as 7.91,and the adsorption mechanism of Cd?II?ions on RH_MW-Mg was believed to be a combination of ion exchange and chelation.The applicability of the RH_MW-Mg for emergent Cd?II?removal from waterworks was verified from various aspects.The RH_MW-Mg could be operated over an extensive pH range of 5.0-8.0,revealing that the Cd?II?ions could be efficiently removed without adjusting the water pH ahead of time.Moreover,the adsorption could fast achieve the equilibrium within 30 min,indicating that the RH_MW-Mg could complete the Cd?II?uptake in water pipelines before these hazardous ions swarmed into waterworks.The RH_MW-Mg had a good Cd?II?removal performance at relatively low temperature,which was consistent with the practical emergency disposal process.The RH_MW-Mg possessed outstanding adsorption capacities for the heavy metals?e.g.Pb,Ni and Sb?which may coexist with cadmium,indicating that the RH_MW-Mg could be employed to achieve the one-step adsorptive elimination of targeted heavy metals in sudden cadmium pollution,thus decreasing the emergent disposal complexity to a much lower level.The co-ions such as Na⁺,K⁺,Mg²⁺and Ca²⁺exhibited limited inhibitory effect on the Cd?II?uptake by RH_MW-Mg,and the adsorption performance was not obviously affected by storage time.All these results indicated that the RH_MW-Mg is appropriate for the emergent disposal process of sudden cadmium water source pollution.This work investigated the emergent Cd?II?removal performance of the RH_MW-Mg based adsorption-coagulation system.The results showed that the maximum dosage of RH_MW-Mg in the absence and presence of polymeric aluminum chloride?PAC?increased from 385 to 770 mg/L,while the RH_MW-Mg dosage higher than this limitation will lead to the water turbidity higher than 3NTU.The technical parameters,i.e.,the dosages of RH_MW-Mg and PAC,of the hybrid system that could treat the Cd?II?concentration different times higher than the standard value were determined.Under the premise of meeting the turbidity threshold of sand filter and the threshold value of Cd?II?concentration in drinking water,the hybrid system could achieve the maximum removal of the Cd?II?concentration 186.6 times higher than the standard value by using 760 mg/L RH_MW-Mg combined with 30 mg/L PAC.The application safety of the adsorption-coagulation system was then assessed.The results showed that the residual concentrations of Cd?II?,Na⁺,Mg²⁺andSO₄^2-were far below their corresponding limit values in drinking water,even with the incorporation of natural water background values of these ions,indicating that the application of this adsorption-coagulation system would not produce adverse impact on the quality of the drinking water supply.In order to effectively dispose the Cd-containing sludge generated from the system,an interval-valued trapezoidal fuzzy group decision-making model was developed to identify the most favorable technology among the feasible technologies,i.e.,cement solidification-secure landfill,rotary kiln incineration,cement kiln co-processing and hydrometallurgy metal recovery.Group-G1 was used to determine the indexes weights of the technique evaluation index system.Generalized interval-valued trapezoidal fuzzy numbers were employed for expressing experts'evaluation information,and experts'weights were determined through data standardization and similarity calculation.A hamming-based ordered weighted aggregating?HOWA?operation was applied to the objective aggregation of different experts'opinions,and Fuzzy-TOPSIS was used to treat the aggregation data for identifying the ranking sequence of technical alternatives.The results showed that the rotary kiln incineration was perceived as the most appropriate disposal measure for Cd-containing waterworks sludge.The generalized applicability of this adsorption-coagulation system and the designed evaluation framework were also discussed.The results showed that the hybrid system possessed enormous potentials in the emergent remediation research of many other heavy metal contaminations,and the developed evaluation model could be applicable to many environmental decision processes,which could help experts inform intelligent environmental management decisions.In conclusion,the research results of this work enriched the technical alternatives for emergency disposal of sudden heavy metal water source pollution and provided a new model for solving such environmental management group decision-making problems as technique selection,which are strongly significative both on theory and practice.