Characteristics And Mechanism Of Aqueous Cd(?) And Pb(?) Sorption By Heat-treated Red Mud

Red mud is a mineral waste generated during the production of alumina.Nearly ten million tonnes of red mud are generated annually in China.Currently,various applications of red mud have not been widely applied on an industrial scale due to the higher cost and lower efficiency.Therefore,it is significant to develop applications of red mud and related theory.The objective of this study is to develop an application of red mud as a sorbent for the removal of aqueous metal ions.Heat treatment on red mud has been shown previously to increase its sorption capacity.However,contaminants used to test heat treatment as a potential strategy to improve sorption capacity by red mud were some metal cations with lower toxicity like Mn???,Ni???and Zn???.The effectiveness of heat treatment in improving the sorption of Cd???and Pb???,which are among the most hazardous and toxic heavy metal cations,has seldom been investigated.More,the studies about the effect of heat treatment on the sorption more focused on the sorption capacity.There is a lack of comprehensive evalution including sorption rate and sorption stability.More,the systematic investigation about sorption characteristics and sorption mechanism have seldom been explored.According to the critical scientific issue,the studied red mud was heated at different temperatures?200–900??.Based on the evaluation of sorption capacity,sorption rate and sorption stability,the best heating temperature and heat-treated red mud were optimized.The sorption characteristics and mechanism of Cd???and Pb???by optimized red mud were studied through batch sorption experiments,sequential extraction,XPS and VISUAL MINTEQ software.Additionally,the prediction models of respective Cd???and Pb???sorption by the optimized red mud were developed through response surface methodology.The conclusions are as follows?except for the fourth conclusion,other conclusions were obtained through sorption experiments under the system with single sorbate?:1.This study optimized the heating temperature and corresponding heat-treated red mud.The kinetics studies and leaching experiments showed red mud heat-treated at 500??RM500?exhibited higher sorption capacity,faster sorption rate and higher sorption stability,which attributed to its higher specific surface area and pore volume.The maximum sorption capacities of RM500 for Cd???and Pb???were 42.62 and196.08 mg?g^-1,respectively.Compared with raw red mud?RM?,the maximum sorption capacities of RM500 for Cd???and Pb???increased by more than 30%.The sorption rates of Cd???and Pb???by RM500 were 2–4 times as fast as the sorption rates of RM.The sorption stability of RM500 was about 2 times higher than it of RM in the acidic matrix or environment with higher ionic strength?0.9%NaCl?.2.The results suggested there was a positive correlation between higher pH and greater sorption.The results demonstrated there was a drastic competitive sorption between increased proton and sorbate cations at lower pH;Cd???and Pb???hydrolyzed at higher pH,with the pH more than 8,the fraction of Cd²⁺decreased,and the percentage of monovalent Cd???species?CdCl⁺and CdOH⁺?increased due to hydrolysis of Cd???,contributing to the stronger ion exchange and higher sorption capacity;decreased?G of specific sorption at higher pH as protons are being released as products contributing to higher sorption capacity.3.This study explored the effectof initial sorbates concentration and sorption temperature on the sorption.Results showed that Langmuir isotherm model was more suitable for fitting the sorption data of Cd???and Pb???,indicating that sorption corresponded to a monolayer formation of Cd???and Pb???onto the RM500 surface,respectively.The Cd???and Pb???sorptions by RM500 were two spontaneous and endothermic processes.More,an increase in the degree of freedom of these two adsorbing systems was detected.4.Analysing the competitive sorption between Cd???and Pb???elucidated that the competitive sorption depended on the initial concentration of metal ions.When the initial concentration of Pb???was higher than 10 mg?L^-1,Pb???had a competitive effect on Cd???sorption;when the initial concentration of Cd???was higher than 266mg?L^-1,Cd???had a competitive effect on Pb???sorption.However,the competitive sorption was not detected when the initial concentrations of metal ions were lower than mentioned concentrations.Based on X-ray photoelectron spectroscopy?XPS?analysis,Cd???enjoyed similar binding energy among 405–415 eV with Pb???indicating the similar electronic distributions of Cd???and Pb???,which resulted in competitive sorption.Competitive sorption experiments showed Pb???had a stronger competitive effect on Cd???sorption,and due to the properties of the elements,Pb???sorption capacity was more than Cd???sorption capacity.More,competitive sorption increased the fraction of water soluble species of Cd???and Pb???sorbed,resulting in decreasing Cd???and Pb???sorption stability of RM500.5.The sequential extraction analysis,XPS and VISUAL MINTEQ software were applied to enlighten the sorption mechanisms of Cd???and Pb???by RM500.The results showed that specific sorption was the predominant mechanism for Cd???and Pb???sorption by RM500,and some inner-sphere complexes like XCd?OH?and PbTiO₃ formed during specific sorption process.The physical sorption?electrostatic attraction?likely had a limited effect on sorption,because the final pH values?about 6?of the sorption systems were lower than the pHPZC?about 10.6?of RM500,leading to the positive charges on RM500 surface.The strength of specific sorption contributed to the higher sorption stability of RM500 due to the formation of chemical bonds during this process.More,ion exchange was identified as another Cd???sorption mechanism.6.The Box-Behnken experimental design and response surface methodology were applied in developing the sorption prediction model based on the relationships between the sorption process parameters and the respective sorption capacities of Cd???and Pb???by RM500.The validity of the sorption models suggested the accuracy of the developed models,which could be used as prediction models for the effectiveness of Cd or Pb-contaminated water treatment by RM500.Analyzing the model components identified pH as the most important process parameter?contribution rate was about 40%?.So,it is a critical indicator to regulate and monitor pH when the red mud is used to remediate the Cd or Pb-contaminated water.This study improved the sorption properties of aqueous Cd???and Pb???by red mud through heat treatment,and demonstrated the sorption characteristics and mechanism of Cd???and Pb???sorption by heat-treated red mud,and developed sorption prediction model.To a certain extent,this study enriched the theory of remediation of heay metal contaminated-water.More,these results were critical for the exploration of other sorption systems as a reference in the aspect of environment chemistry,and providing the theoretical support for the application of red mud.