One-step synthesis and adsorptive characteristics of composite magnetic activated carbon for rhodamine B removal

Abstract

Most of adsorption research has been carried out with dyes since their existence affects not only the quality of water but also changes the aquatic ecosystems as well as reduces the light penetration. Fine activated carbons are effective in dye adsorption, but they are extremely difficult to be separated from the solution when the carbons become exhausted. Magnetic activated carbon (MAC) has an advantage for the separation of spent activated carbon due to its excellent magnetic properties. However, complicated and multiple steps in the preparation process, and smaller adsorption capacity as compared to conventional activated carbon are the significant drawbacks of magnetic activated carbon. A simple synthesis method which simultaneously involves the activation and magnetization in a single step was introduced. This work was aimed at evaluating the adsorptive properties of composite magnetic activated carbons prepared from palm kernel shell. The activated carbons were prepared at various impregnation ratios of ZnCl2 and FeCl3, activation temperatures ranging between 300 and 800oC and times of 1 to 3 h. ZMAC-2.5 prepared at 600oC and 2 h with impregnation ratios of ZnCl2:FeCl3:PKS = 1.5:1.0:1.0, which has BET surface area of 1775 m2/g and mesoporosity of 93.8 % endowed a higher rhodamine B (RB) adsorption of 371 mg/g. For the purpose of comparison, MACs prepared by conventional magnetization methods and non-magnetic activated carbon (ZAC-1.5) were also employed in adsorption studies. MACs were characterized based on proximate analysis, elemental analysis, textural characteristics, chemical properties and magnetic properties. The batch adsorption was evaluated for equilibrium isotherm, kinetics and thermodynamics properties of RB by MACs. Langmuir isotherm model gave the best conformity of equilibrium data indicating a monolayer adsorption of RB onto activated carbons. The kinetics data were fitted better with pseudo-second-order model. The intraparticle diffusion and Boyd models revealed that both film and pore diffusion may be involved in the adsorption process, but none is the sole rate-limiting step. The positive values of enthalpy change and entropy change indicate that the adsorption process is endothermic and spontaneous at high temperature. The activation energy of 24.1 - 28.9 kJ/mol suggested that the adsorption of RB onto ZMAC-2.5 is a physisorption process. In fixed bed adsorption, the effect of initial RB concentration, flow rate and bed height were evaluated. The increase in initial concentration or flow rate, or the decrease in bed height could result in increasing adsorption capacity but decreasing breakthrough time. The column data fitted well with Thomas, Yoon-Nelson and Bohart-Adams models with high coefficient of determination (R2) and low sum of squared errors (SSE). These models describe that the adsorption process is controlled by the interaction between RB molecules and ZMAC-2.5 surface. In hot water regeneration studies, ZMAC-2.5 showed a higher recovery than ZAC-1.5 for three consecutive regeneration cycles due to its magnetic properties. The optimum conditions were found to be 68.3 oC and 1 h to achieve the regeneration efficiency of 36.8 % and recovery of 93.7 %. Magnetic activated carbon showed a great potential to ease the separation of exhausted activated carbon using permanent magnet in wastewater treatment.