Partitioning of major and trace inorganic contaminants in fly ash acid mine drainage derived solid residues

Abstract

Acid mine drainage was reacted with coal fly ash over a 24 h reaction time and species removal trends evaluated. The evolving process water chemistry was modeled by the geochemical code PHREEQC using WATEQ4 database. Mineralogical analysis of the resulting solid residues was done by X-ray diffraction analysis. Selective sequential extraction was used to evaluate the transfer of species from both acid mine drainage and fly ash to less labile mineral phases that precipitated out. The quantity of fly ash, volume of acid mine drainage in the reaction mixture and reaction time dictated whether the final solution at a given contact time will have a dominant acidic or basic character. Inorganic species removal was dependent on the pH regime generated at a specific reaction time. Sulphate concentration was controlled by precipitation of gypsum, barite, celestite and adsorption on iron-oxy-hydroxides at pH > 5.5. Increase of pH in solution with contact time caused the removal of the metal ions mainly by precipitation, coprecipitation and adsorption. PHREEQC predicted precipitation of iron, aluminium, manganese-bearing phases at pH 5.53-9.12. An amorphous fraction was observed to be the most important in retention of the major and minor species at pH > 6.32. The carbonate fraction was observed to be an important retention pathway at pH 4-5 mainly due to initial local pockets of high alkalinity on surfaces of fly ash particles. Boron was observed to have a strong retention in the carbonate fraction.

Suitability of Fly Ash Vermicompost as a Component of Pine Bark Growing Media: Effects on Media Physicochemical Properties and Ornamental Marigold (Tagetes spp.) Growth and Flowering

“…South Africa alone generates more than 28 million tons of this fly ash at power stations annually, of which 95% of this is not used but disposed of at sites near power stations (Gitari et al 2010). Due to the processes involved in coal formation, fly ash contains both beneficial plant nutrients like phosphorus, together with potentially toxic heavy metals.…”

Challenges in Recovery of Valuable and Hazardous Elements from Bulk Fly Ash and Options for Increasing Fly Ash Utilization

“…Ca-Na-Ni-Mg-Pb-Mn-Cu-Zn-Al-Fe-Cr (most released at high pH) (most released at low pH) ), oxy-anions of Se, As, Mo, B and Cr and cations (Al, Fe, Na, K, Ca, Sr, Ba, Zn, Cu, Cd and Mg) are leached from the fly ash heaps by the wastewater derived from the ash slurry which is to be collected for further concentration [17].…”

Analytical Relevance of Trace Metal Speciation in Environmental and Biophysicochemical Systems

“…Soil, sediments or precipitates are known reservoirs or sinks of trace metals in the environments, and the metals may be present in several different physicochemical forms/phases [25][26][27]. These phases are water soluble, exchangeable; specifically adsorbed; carbonate; secondary Fe and Mn oxides; organic matter; sulphides and silicates [28]. This procedure that was originally developed as a pivotal heavy metal speciation scheme using extracting agents has evolved as the foundation for recent advances in fractionation speciation schemes.…”

“…The sequential extraction or fractionation schemes are a very useful method, for characterizing solid phases associated trace elements in soils, sediments or particulates [29][30][31][32][33]. It must be pointed out that the extracting reagents employed in speciation analysis are chosen based on their selectivity and specificity towards a particular physicochemical species of trace metal [28]. The reagents cocktail for respective extraction steps are capable of disrupting the binding agents between individual element and the sediments thus allowing possible release of metal species into the solution.…”