| Mercury mainly existed in the form of Hg2+ ion in aqueous solution.It can entered the human body through the food chain,inactivating important proteins in human body and causing human disease.Therefore,Hg2+ ion must be removed from containing-Hgig wastewater,so that the wastewater can meet the emission standard.Common methods for removing H2+ ions were chemical precipitation,adsorption,ion exchange,extraction,reverse osmosis and so on.In which biomass carbon had become a hot research topic for many scholars to adsorb Hg2+ ions.Taking rice husk as the raw material,biomass carbon was prepared at low temperature in this paper.The effects of preparation temperature,sulfuric acid concentration and soaking time on adsorption effect of the biomass carbon were discussed by orthogonal experiment.The optimum conditions were as follows:the preparation temperature 180 ℃,the concentration of sulfuric acid 0.05 mol·L-1,and the soaking time 12 h.Finally,the biomass carbon(BC)with iodine adsorption of236 mg·g-1 was obtained under these conditions.Then the biomass carbon(BC),activated carbon(AC),and high temperature biomass carbon(HC)were used to adsorb Hg2+ ion in solution.The results showed that the removal rates of AC and HC reached 82.19%and 82.91%,respectively,at the conditions of pH=6,temperature 25 ℃,the dosage 2.8 g·L-1 and the initial concentration of Hg2+ ion 10 mg·L-1.The adsorption capacity were 2.94 mg·g-1,2.96 mg·g-1,respectively.For BC,the best adsorption conditions were pH=4,temperature 25 ℃,the dosage 2.8 g·L-1.The removal rate reached 78.11%and the adsorption capacity were 2.79 mg·g-1.So BC prepared in this paper had a certain adsorption ability on Hg2+ ion.In order to improve the adsorption performance of BC,HC and AC,they were modified by KOH and NaHS.And BC modifided by potassium hydroxide(BC-P),HC modifided by potassium hydroxide(HC-P),AC modifided by potassium hydroxide(AC-P),BC modified by Sodium hydrosulfide hydrate(BC-S),HC modified by Sodium hydrosulfide hydrate(HC-S),and AC modified by sodium hydrosulfide hydrate(AC-S)were obtained,respectively.Fourier transform infrared spectroscopy(FT-IR),scanning electron microscope(SEM),Boehm titration and elemental analysis were used to characterize the adsorbents being modified.Then these adsorbents were applied to adsorb the Hg2+ ion in solution.The effects of adsorption time,pH of solution,dosage of adsorbents,temperature of solution and the initial concentration of Hg2+ ion on the removal rate were studied.It was found that the maximum removal rate of AC-P,HC-P and BC-P were 94.23%with a adsorption capacity 4.71 mg·g-1,92.66%with a adsorption capacity 4.59 mg·g-1 and 96.30%with a adsorption capacity 4.81 mg·g-1,respectively,under the condition of adsorption time of 240 min,the optimum adsorption temperture 25 ℃,initial concentration of 10 mg·L-1,pH value of 6 and dosage of 2.0 g L-1.While for adsorbents modified by NaHS,AC-S,HC-S and BC-S,the maximum removal rate were 97.48%with a adsorption capacity 8.12 mg·g-1,96.06%with a adsorption capacity 7.60 mg·g-1,and 98.01%with a adsorption capacity 8.49 mg·g-1 in turn at the conditions of the condition of adsorption time of 240 min,the optimum adsorption temperture 25℃,initial concentration of 10 mg·L-1,pH value of 4 and dosage of 1.2 g·L-1.It were showed that the modified low temperture biomass carbon had a better performance on adsorbing Hgr,ion in the solution tha1 other adsorptions after being modified.Finally,the optimum adsorption conditions of BC-S determined by orthogonal experiment were adsorption time of 240 min,temperture of 25OCC solution pH of 4,dosage of 1.2 g·L-1 and initial concentration of Hg2+ 10.0 mg·L-1.And the adsorption rate of Hg was 98.01%and the adsorption capacity was 8.49 mg·g-1 under these conditions.After adsorption tests,the adsorption kinetics were fitted with the data obtained from the experiments.It was found that the quasi-second-order kinetic model could explain the adsorption process well.The adsorption process were consistent with the Langmuir adsorption isotherm equation.Adsorption thermodynamics fitting found that Gibbs free energy was less than zero in the entire adsorption process.So the adsorption process could be carried out spontaneously.Heating was conducive to the occurrence of adsorption between 15 ℃ and 25℃,because Enthalpy change was positive.While Enthalpy change was negative and cooling was conducive to adsorption between 25℃ and 35℃.At last desorption experiments were carried by deionized water,HCl,HNO3 and EDTA.Analysis showed that the desorption effect of EDTA was the best.The adsorbents could be reused for 6 times,and the adsorption percentage of Hg2+ ion was still above 90%,The recirculation of adsorbent was realized.Then orthogonal experiments were taken to screen the desorpton conditions.Results showed that the desorption efficiency of BC-S was the best when the desorption time was 12 h,the desorption temperature was 35 ℃,the EDTA concentration was 0.05 mol·L-1,and the solution pH was 4.For BC-P,the best desorption were the desportion time of 12 h,the desorption temperature of 35 ℃,the EDTA concentration of 0.05 mol·L-1,and the solution pH was 6. |
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