The removal of fluoride from aqueous solutions using biomass ash derived from power industry

Abstract

a b s t r a c tThe ability of biomass ash to remove fluoride from aqueous solutions has been described. Seven samples of biomass ash from Polish power stations and combined heat and power plants using only biomass were tested as adsorbents for fluoride removal. After preliminary research, two of ash samples were chosen for further examination. Selected ash samples were examined in terms of the specific surface area and the pore size distribution. Their pH values of the point of zero charge were also determined. The examination of changes in the crystallographic phases of the sorbent before and after the sorption process was done using the XRD technique. Changes in ash samples before and after contact with the fluoride solution using the FTIR method were also analyzed. The effect of initial concentration of fluoride solutions, ash dose, contact time, temperature, pH, agitation rate and particle size of ash samples on fluoride removal efficiency has been examined. The percentage removal of the fluoride described as removal efficiency and the amounts of fluoride adsorbed per mass unit of ash vs. time and after reaching equilibrium was calculated. It was found that only in the case of one of the ash samples, CaF 2 was identified, and that probably has been precipitated as the result of that ash dissolution in water with the content of fluorides. The presence of fluorides in the residual solid phase bounded with compounds containing potassium, calcium sulphates and phosphates indicates that precipitation of CaF 2 is not the major mechanism responsible for fluoride removal. Therefore, an attempt was undertaken to describe the mechanism of the adsorption phenomenon. The kinetic profile of fluoride uptake was determined and the fitting to adsorption isotherm models was examined. The exact fitting to the pseudo-second-order model pattern of this model was confirmed for both of ash samples. It was found out that the fluoride adsorption process is well-described by Temkin isotherm plot.