Preparation, modification and characterization of adsorbents from palm fatty acid distillates (PFAD) for dye removal from aqueous solutions

Abstract

Adsorption is an effective approach in the treatment of wastewater as it utilizes low cost adsorbent, no sludge, and is simple to operate. Palm fatty acid distillates is a suitable precursor of adsorbents due to its high carbon content, less commercial value and abundant availability. This work is aimed to evaluate the adsorption properties of adsorbents prepared from palm fatty acid distillates for methylene blue and congo red removal. Adsorbents with more yield were selected with the aid of response surface methodology (RSM) using Box Behnken design (BBD) approach. The resultant adsorbents were characterized for ultimate analysis, thermogravimetric analysis, surface area analysis, Fourier transform infrared spectroscopy analysis, surface morphology analysis, X-ray spectroscopic property, Boehm titration and point of zero charge of adsorbents. The surface area was between 212-222 m2/g, pore volume was 7.0 x10-7-2.8 x10-7 m3/g, pore size 51-122 A and the pore structure was majorly micropore, Boehm titration showed the presence of carboxylic, lactonic, phenolic and basic groups on the surface. The pH of adsorbent was 5 while the point of zero charge was 5.5. The adsorption of methylene blue and congo red were studied at varying concentrations (5 – 300 mg/L), contact time (10min – 72 h), temperature (30 -70 ?C) and pH (2 -10). RSM software using BBD approach was utilized to determine the optimum adsorption capacity of methylene blue and congo red by adsorbents and to know interrelationships and interactions among the factors that enhance maximum adsorption. The optimum adsorption of methylene blue was 47.8 mg/g at maximum initial concentration of 100 mg/L, temperature of 70 ?C, and contact time of 18 h and pH of 6 compared with experimental value of maximum methylene blue adsorption of 56 mg/g at concentration of 100 mg/g, temperature of 70 ?C, time of 36 h and pH of 5 was found to be suitable for methylene blue adsorption. Congo red adsorption was 189 mg/g at concentration of 300 mg/L, temperature of 70 ?C contact time of 15 h and pH of 6. The experimental value of congo red optimum is 237 mg/g at concentration of 300 mg/L, temperature of 70 ?C, time of 30 h and pH of 5. The model is suitable for congo red. The adsorption of methylene blue and congo red dyes by adsorbents was well described by Langmuir and Temkin models. The adsorption of dyes fitted into both pseudo-first order and pseudo-second order kinetic models, suggesting physiscochemical adsorption. Weber-Morris or intraparticle diffusion model reveals that the intraparticle diffusion is involved, but it is not the only rate-limiting step. Boyd model shows that film diffusion is the controlling mechanism for both methylene blue and congo red adsorption. The dyes adsorption is thermodynamically endothermic and spontaneous in nature.