Reuse of coal combustion ashes as dyes and heavy metal adsorbents: effect of sieving and demineralization on waste properties and adsorption capacity

Among the possibilities of limiting the disposal of coal combustion ashes (CCA), their reutilization as adsorbent materials is worthy of consideration. To this end, proper ashes beneficiation techniques can be put into practice. The adsorption of toxic compounds from industrial wastewaters is an effective method for both treating these effluents and recycling CCA. The aim of this paper is to give a contribution for understanding the relationships among beneficiation treatments, adsorbent properties and adsorption mechanism/efficiency. In this context, as-received CCA together with mechanically sieved and demineralized ashes were employed as adsorbents of methylene blue and cadmium from aqueous solutions. The maximum values of the specific adsorbate quantity captured by the adsorbent were fairly good, ranging from 23 to 110 mg/g, with removal efficiency close to 1 for a pollutant/solid initial ratio ranging from 10 to 20 mg/g. Moreover the influence of the beneficiation treatments on the adsorbent physico-chemical/microstructural properties was thoroughly discussed. These properties, in turn affect the adsorption mechanism (either cooperative/multilayer or high-affinity monolayer) and the capture efficiency. In this respect, when the operating conditions are those of greater practical interest, the different adsorbents can be ranked in the order demineralized ashes >mechanically sieved ashes > CCA, in excellent agreement with previsions based on materials characterization.