Modification Of Activated Carbons And Experimental Research On The Removal Of Representative Heavy Metals

The using of water that contaminated with heavy metals dramatically threatened people’s health. Adsorption has already become one of the most common methods for the removal of heavy metals from water. Due to the simple operation process and a wide range of raw material sources, adsorption with activated carbon was often used in advanced treatment of drinking water and emergency treatment of water pollution accidents. However, the conventional activated carbons, in a general way, along with relatively poor adsorption performances, which including the low adsorption capacity, slow adsorption rate, narrow p H range and so forth. To improve the removal efficiency of heavy metals, surface modification of conventional activated carbons was conducted by an oxidation-loading process. The physicochemical properties of modified activated carbons have been characterized, meanwhile, the adsorption performance of chromium(Ⅵ) and cadmium(Ⅱ) was investigated, as well as the adsorption mechanism.The main contents and conclusions in this study are as follows:1) Three traditional activated carbons with different materials(wood, coconut and coal) were used in the experiment, and the effect of characteristic parameters on the adsorption performance of Cr(Ⅵ) was studied. Comparison of surface properties and economic practicality of three activated carbons was conducted. The results indicated that impurities on the wooden activated carbon is lower, and the wooden activated carbon has a moderate price, the biggest p HPZC value, and the highest content of surface acidic functional groups, which are beneficial to the adsorption of negatively charged Cr(Ⅵ). The wooden activated carbon was selected as the substrate for modification.2) The modification of AC-0(the pretreated wooden activated carbon) was investigated using HNO3(ACNF) and H3PO4(ACPF) as oxidants, and Fe Cl3 as supported substance. The physicochemical properties of modified activated carbons were characterized with SEM, BET, FT-IR, XRD and Boehm titration. The results suggested that the pore structure of activated carbon was slightly changed by oxidation corrosion and iron loading. Magnitudes of the specific surface area, micro and total pore volume and average pore size submit to the order of AC-0>ACPF>ACNF. The iron loaded on ACNF maybe hematite, and probably goethite on ACPF. The iron content of ACNF and ACPF was 0.231 mg/g and 0.144 mg/g, respectively. The surface chemical properties(functional groups) of activated carbons were significantly changed by oxidation of acids and Fe Cl3. The total surface acidity of ACNF and ACPF had increased averagely 1.2275 mmol/g when compared to AC-0, and the increment was exceeded 81%. Meanwhile, the total surface acidity of ACNF had increased 0.946 mmol/g when compared to ACNF, which modified with HNO3 oxidation, and the increment was 52.7%. After modification, the developed pore structure of activated carbon retained its original state, on the other hand, the values of functional groups and oxides had dramatically improved.3) For the removal of Cr(Ⅵ), the orthogonal experiments were established to get the optimal modification conditions in different oxidation processes, static adsorption experiments were conducted to figure out the influence of circumstance factors on the adsorption performance of Cr(Ⅵ), and the experimental data of Cr(Ⅵ) adsorption were analyzed with equilibrium isotherms, kinetics and thermodynamics. The results indicated that the optimal modification conditions of both oxidants were as follows: oxidant concentration 10%, ferric chloride concentration 0.05mol/L, reaction temperature 60℃, reaction time 6h. The p H of water determined the adsorption efficiency, a higher adsorption of Cr(Ⅵ) could be received under condition of low p H value. The adsorption processes of Cr(Ⅵ) onto AC-0 and ACNF were better described with homogeneous adsorption model, while the heterogeneous adsorption model was more suitable for describing the adsorption data of Cr(Ⅵ) on ACPF. Because of the increase of acidic functional groups and the addition of attraction force from loaded iron oxides, the adsorption capacity of modified activated carbons had remarkably improved. The maximum adsorption capacities(qm) were 7.087, 66.890 and 19.279mg/g for AC-0, ACNF and ACPF, respectively. After 30 minutes adsorption, the removal efficiency of Cr(Ⅵ) on AC-0, ACPF and ACNF reached 10.3%, 33.3% and 29.7%, respectively.4) For the removal of Cd(Ⅱ), the same research methods were used to study the adsorption behavior of heavy metals in cationic state onto modified activated carbons, which modified with HNO3 and Fe Cl3. The optimal modification conditions were: HNO3 concentration 10%, Fe Cl3 concentration 0.05mol/L, reaction temperature 80℃, reaction time 6h. The adsorption data of Cd(Ⅱ) on AC-0 could be described with the multilayer coverage process, while the adsorption equilibrium data of Cd(Ⅱ) onto ACNF was better fitted to the monolayer adsorption. The maximum uptake capacity(qm) of AC-0 and ACNF were 3.398mg/g and 8.065mg/g, respectively. The adsorption of Cd(Ⅱ) on AC-0 and ACNF is a spontaneous exothermic nature, as well as ion-exchange reaction.