Study On Preparation,Modification Of Tea Residue Activated Carbon And Its Application In Dye Wastewater Treatment

The production process of instant tea derived from Pu'er fermented tea make the tea residue(tea residue)generated from this process difficult to be applied in many regions.Because of high fixed carbon content,large output,easy collection and low cost,tea residue is considered to be an ideal raw material for fabrication of activated carbon.In this study,a novel kind of biomass-based activated carbon prepared from such tea residue was investigated.Furthermore,the method of surface modification by nitric acid was employed to improve its absorption on dye and heavy metal ions in dye wastewater,in which the content of surface acidic functional groups can be increased except the damage of its pore structure.The modified activated carbon was applied to the simulated cationic dye wastewater and anionic dye wastewater respectively,for investigation of the kinetics and mechanism of its absorbing the dyes and heavy metal ions usually presented in such two types of dye wastewater.The main research contents of this article are as follow as:In the fabrication process of the activated carbon,KOH and ZnCl₂ were used as activators to activate the tea residues.The effects of factors,such as activation temperature,activator dosage and activation time on the adsorption performance of the activated carbon obtained in the two preparation process were studied.At heating rate 5?/min,when carbonization temperature of 600?,activation temperature of 800?,impregnation ratio(liquid-solid ratio of 80% KOH/bio-char wt/wt)of 5:1,and activation time of 60min,the activated carbon has the relatively well adsorption performance attributed to its microporous structure with a high specific surface area of 2414m²/g,and a pore volume of 0.969 cm³/g.However,the obtained ZnCl₂-activated carbon at optimal preparation conditions,which activation temperature was 600?,impregnation ratio(Liquid-solid ratio of 80% ZnCl₂/tea residue wt/wt)was 3.75:1,and activation time was 120min,were mesoporous activated carbon with specific surface area of 1211 m²/g,and a total pore volume of 1.090 cm³/g.Due to the poor adsorption performance of activated carbon on heavy metals,nitric acid was used to oxidize activated carbon(AC)prepared by KOH method to increase the content of acidic functional groups on the surface of AC,which may improve the adsorption performance for heavy metal ions.The effects of nitric acid concentration,temperature and reaction time on the microstructure and surface functional groups of the activated carbon were discussed.The method of BET,Boehm titration,and zero charge point determination were employed to characterize the microstructure and surface physiochemical features of AC before and after modification.From the results of measurement,the pore collapse which led to decrease of specific surface area and pore volume was apt to happen when AC was treated by nitric acid with high concentration,although the surface acid groups of it would increase.Whereas,the modified AC with increased amount of surface acidic functional groups and well microstructure could be obtained after the treatment by nitric acid with appropriate concentration at the set temperature within the relatively long reaction time.Under the conditions of 120?,0.5mol/L nitric acid and 4h treatment,the total amount of acidic functional groups on the surface of AC increased from 0.246mmol/g to 1.416mmol/g,the specific surface area increased from 2414m²/g to 2576m²/g,and pore volume increased from0.969cm³/g to 1.154cm³/g.The modified activated carbon(MAC)obtained under these conditions shows well adsorption on methylene blue(MB),indigo carmine(IC)and Cd²⁺.Batch adsorption experiments for cationic dye MB,the anionic dye IC,and heavy metal ion,Cd²⁺ were employed to assess adsorption performance and then find out the adsorption mechanism of MAC and AC.Owing to the difference of pore structure and surface chemistry,the adsorption performance of MAC and AC for two dyes are not identical,and their maximum theoretical adsorption capacity for MB were 2000mg/g and 1667mg/g,1111mg/g and 1250mg/g for IC,respectively.The adsorption of two adsorbents on two dyes were consistent with the Langmuir model,fitted well with pseudo-second-order model,and suggested the adsorption behavior were monolayer physical adsorption.The increasing of surface acidic functional groups results MAC has better adsorption performance than AC on four heavy metals,especially on Cd²⁺,Cu²⁺and Pb²⁺.The adsorption performance of MAC for high toxicity ion Cd²⁺ is higher than AC,and their maximum theoretical capacity were82.64mg/g and 44.84mg/g,respectively.The adsorption experiments of two adsorbents on Cd²⁺ were consistent with the Langmuir model,fitted well with pseudo-second-order model,and illustrated the adsorption behavior were monolayer adsorption and a combination of chemical and physical adsorption.Besides,the adsorption performance and adsorption mechanism of MAC on dye and heavy metal ion when they were simultaneously present in the simulated wastewater were studied.From the bath experiments,it was found that the competitive adsorption and electrostatic repulsion between MB and Cd²⁺ would lead to the decreasing adsorption performance of MAC on two pollutants.While the adsorption mechanism of MAC on MB and Cd²⁺ both existed in the stimulated anion wastewater was still consistent with the Langmuir model,fitted well with pseudo-second-order model,and showed the adsorption behavior were monolayer adsorption and a combination of chemical and physical adsorption,respectively.There is a synergic adsorption between IC and Cd²⁺ at mixed solution,which improves the adsorption capacity of MAC for two pollutants.The adsorption experiments of MAC on IC and Cd²⁺ were still consistent with the Langmuir model,fitted well with pseudo-second-order model,and demonstrated the adsorption behavior were monolayer adsorption and a combination of chemical and physical adsorption,respectively.From the research,it can be suggested that the activated carbon based on tea residue has the potential application for removal on dyes and heavy metals from dye wastewater.