Study On Microwave-assisted Preparation Of Rice Husk-based Adsorbent And The Removai Of Heavy Metals In Wastewater

With the rapid development of the world economy,a large number of heavy metal industrial wastewater is discharged every year,and organisms can not destroy and degrade heavy metal pollutants.Once discharged into the environment,it will become permanent pollution.Adsorption is considered to be one of the best methods for heavy metal wastewater treatment and recovery.Rice husk is rich in hydroxyl groups,which can react with carbon disulfide（CS₂）in alkaline medium by adding sulfur group xanthan ester,and then adsorb heavy metals in water effectively.In this paper,xanthan ester modified rice husk was prepared rapidly under microwave irradiation.Single factor experiment and orthogonal experiment were used to optimize the preparation parameters of the modified material.The optimum preparation conditions were as follows:the ratio of rice husk to carbon disulfide was1:2（g/mL）,the alkalization time was 5 min,yellowing time 90 s,which was much lower than that of modified rice husk prepared by water bath method（165 min）).The adsorption capacity of Cd（II）was 151.51 mg/g,higher than that of water bath modified rice husk（124.35 mg/g）.By means of scanning electron microscope（SEM）and Fourier transform infrared spectroscopy（FTIR）,the results of rice husk before and after modification were studied.The results showed that the xanthan ester group had been successfully grafting into the rice husk,and there were more adsorption sites on the surface of the modified rice husk.The effects of pH value,adsorption time and temperature on the removal efficiency were analyzed.the results showed that the modified rice husk was not conducive to the removal of lead,cadmium and nickel ions under the condition of too low pH.For heavy metals in single ion system,the correlation coefficient of quasi-second-order kinetics is larger than that of quasi-first-order kinetic model,which indicates that chemical adsorption is the main adsorption of single ion by modified rice husk.In most cases,the quasi-second-order kinetics can better fit the adsorption process except for some two-ion and three-ion systems.The order of preferential adsorption of heavy metal ions by modified rice husk is Pb²⁺>Cd²⁺>Ni²⁺.Langmuir adsorption isotherm model for single ion system and The linear correlation coefficient R² of most biion and triion system models is higher than that of Freundlich model,but the Freundlich adsorption isotherm model is more suitable for biion systems(such as Ni²⁺/Cd²⁺)and triion systems(Ni²⁺/Cd²⁺/Pb²⁺).The results show that the adsorption of metals in double ion system and triion system by rice husk is not only related to one adsorption mechanism.It is inferred that the adsorption mechanism of metal ions in water by modified rice husk is chelating coordination and ion exchange.The waste water of a certain electric plating plant and the electroplating waste water in the electroplating workshop of the Harbin Boiler Plant were treated with modified rice hull to reach the emission standard.After five"adsorption-regeneration"cycles,the removal rate of modified rice hull to Pb²⁺,Cd²⁺and Ni²⁺remained above 96%.The adsorption of modified rice hull to Pb²⁺,Ni²⁺and Cd²⁺is stable within 48 h,and the effect of co-existing cations on the efficiency of three heavy metal ions is consistent,from large to small:Ca²⁺>Mg²⁺>K⁺>Na⁺.The technology and suggestion for the effective treatment of the adsorbent after adsorption are analyzed.The modified rice hull is treated by the incineration method.In this study,modified rice husk was used to adsorb heavy metals in water,which provided theoretical support for efficient and safe removal of heavy metals in water.The adsorption process of single ion,double ion and triion system is studied,which provides certain guiding significance for the treatment of heavy metal wastewater.