Study On Removal Of Heavy Metal Ions In Wastewater By Induced Crystallization

As the economy and society prosper and the industrialization process continues to accelerate,industrial pollution is increasing.Heavy metal pollution has become an urgent problem to be solved in the international community due to its characteristics of high toxicity,difficulty in degradation and easy accumulation,which seriously pollutes the environment and threatens human health.However,from the perspective of recycling,it is of great significance to recover heavy metals from wastewater.Induced crystallization process is a new method developed in recent years on the basis of traditional precipitation method for the treatment of pollutants in water.By adding the crystal nucleus to the water,the crystallization reaction can be induced,and the crystallization product can grow on the inducing crystal carrier,so that the pollutant can be separated from the solution and removed.The induced crystallization process not only has the high efficiency of the traditional precipitation method,but also has the advantages of no pollution,small occupation area and recyclability.Based on the latest development of induced crystallization technology in water treatment,combined with the chemical characteristics of heavy metal ions,this study applies the induced crystallization technolo,gy to the treatment of heavy metal wastewater,which is mainly applied to the chromium-containing and nickel-containing wastewater with a large proportion and high toxicityFirstly,the operation conditions of induced crystallization process are studied,including:fluid operation mode,mixture mode of reagents and waste water(crystal nucleation mode),hydraulic load and residence time.The results showed that the optimal operating conditions of the induced crystallization reactor were as follows:fluidized bed crystallization reactor,internal mixing mode of reagents and wastewater,hydraulic load of 35 m·h-1and residence time of 70 min.Furthermore,the effect of induced crystallization process on the removal of single chromium-containing and nickel-containing wastewater was investigated under optimal operating conditions,and the effects of reaction pH and influent concentration were also investigated.The results showed that the induced crystallization process was effective for the treatment of wastewater containing chromium and nickel,and the crystallization product had clear crystal shape and compact structure.In addition,when the inlet pH of chromium-containing wastewater is between 7.0 and 8.0,sodium carbonate is used as the crystallization agent,and the ratio of[CO32-]/[Cr3+]=2.0:1,the treatment effect of chromium-containing wastewater is better,and the highest removal rate can reach 95.54%.The optimal inlet pH of nickel-containing wastewater is between 10.0 and 11.0,and the average nickel content of effluent can meet the national standard of<1.0 mg·L-1.Then,the experimental results of chromium and nickel removal by induced crystallization were investigated and optimized.The results showed that the single induced crystallization reactor had poor treatment effect on Cr3+ and Ni2+,and the average effluent concentrations of Cr3+ and Ni2+ were 2.495 mg·L-1 and 2.258 mg·L-1 respectively in the stable phase of the experiment,which could not meet the emission standards.A two-stage series experiment was designed,in which a fluidized bed crystallization reactor with identical size and shape was added on the basis of the original single fluidized bed crystallization reactor.The results showed that the average removal rate of Ni2+ increased from 88.28%to 96.07%,and the average removal rate of Cr3+ increased from 86.5%to 97.96%.The experimental results were greatly improved.The average effluent concentrations of Ni2+ and Cr3+ were 0.786 mg·L-1<1.0 mg·L-1 and 0.408 mg·L-1<1.0 mg·L-1,both of which could meet the effluent standards.In order to improve the treatment effect of induced crystallization process,modified active magnesia was used to treat chromium-containing wastewater by combining with the theory of crystal formation nuclear dynamics about surface tension of induced crystallization carrier.The experimental results showed that modified active MgO,as a crystallization inducing carrier,could shorten the reaction start-up time,prolong the longest operation period of the experiment and improve the removal effect of the experiment,with an average removal rate of 94.37%.At the same time,the modified active MgO also showed good reproducibility.Through roastling treatment,the removal performance remained high after regeneration,and the removal effect of chromium after three times of regeneration was maintained above 90%.Therefore,it is possible to use active magnesia in induced crystallization process.In conclusion,it is possible to treat wastewater containing chromium and nickel by induced crystallization,and has a broad application prospect.