Acid mine drainage treatment by coagulation process using a synthesized sulphate-based coal fly ash coagulant

Abstract

The study focused on the synthesis of a composite coagulant containing polymeric sulphates of. Thereafter, the study assessed the performance of the produced composite coagulant in the treatment of acid mine drainage. The CFA, a by-product of coal combustion rich in Fe, Al, Si, Mg, and Ca oxides was used in this investigation to synthesize a composite coagulant. This was performed using direct pressure H2SO4 leaching process. The results of the study found that the recoveries of Fe, Al, Si, Mg, and Ca from CFA were influenced by the concentration of H2SO4, temperature, solid to liquid ratio, and time. Subsequently, the extraction process of metals and silicon were optimized using the concentration of H2SO4, temperature, solid to liquid ratio, and time process variables. The raw and processed CFA samples were analysed. The results showed that particle size distribution of the leach residues compared to raw changed during the leaching process. The SEM analysis revealed that new phases were present in the treated CFA sample compared to raw CFA. The morphology of CFA residues revealed that porous and stem-like structures were formed after leaching.

The optical microscopy and SEM divide EDS investigations of the floc formation revealed that pollutants adsorption onto hydroxide precipitates occurred during the coagulation process. A comparison of the conventional iron and aluminium sulphate coagulants showed that the produced composite coagulant performed better than conventional coagulants in terms of removal of Mg, Ca, Si, Zn, and TDS. However, the performance of the conventional iron and aluminium sulphate coagulants was quite similar for the removal of Al, Fe, Mn and Ni. In general, little information is available about the equilibrium capacities and adsorption kinetics of CFA-based coagulant for Al, Fe, Mn, Mg, Ca, Si, Zn, and Ni in aqueous solution. Furthermore, very limited information is available on the thermodynamic and kinetics aspects of adsorption of other element compounds onto other hydroxide precipitate-based adsorbents. Therefore, in order to better understand the adsorption process equilibrium and kinetics, a comprehensive study was conducted. The adsorption isotherm study showed that Temkin and BET equations best fitted the coagulation process. Both, Temkin and BET equations revealed that the adsorption energy was less than zero exothermic behaviour. The chemical reaction kinetic model study suggested the applicability of the pseudo second-order kinetic model. The molecule diffusion kinetic model investigation showed that multi adsorption stage occurred during the process. The intraparticle diffusion coefficients for the different pollutants were calculated from Weber Morris and Boyd’s models and the results showed that the intraparticle diffusion coefficients, of the specific pollutants are in the range of