Sulfate Radical Anion Activated Agro-Industrial Residues for Cr(VI) Adsorption: is This Activation Process Technically and Economically Feasible?

Three low-cost and abundant agro-industrial residues have been used as hexavalent chromium adsorbents. Blackberry residues and spent ground coffee were the most efficient adsorbents with 10.6 +/- 5.9 and 9.7 +/- 3.0 mg g(-1) maximum adsorption uptake, respectively. With a capacity of 1.4 +/- 0.1 mg g(-1), avocado seeds showed the lowest adsorption efficiency, all at pH = 5.5. These three solids were treated with a new sulfate radical anion and low temperature-based oxidation process. The particles' characterization showed no modification in the surface area nor the textural properties but the surface chemistry changes. The electrophoretic mobility, infrared, and energy-dispersive X-ray spectroscopies showed modifications in the surface charge and functional groups due to the oxidation-mediated activation process. Upon activation, not all materials increase their adsorption capacity, and experimental correlations were made to understand this behavior. However, the activated avocado seeds have a maximum adsorption uptake of (31.6 +/- 7.4) mg g(-1), comparable to activated carbons' adsorption efficiency. Regardless of the material used, experimental confirmation of the reduction-complexation Cr(VI) adsorption mechanism at pH = 5.5 was made. Finally, the technical-economic parameter is introduced to facilitate a proposed activation method's economic feasibility evaluation. Interestingly, raw and activated avocado seeds under some studied scenarios showed similar maximum adsorption uptake values but with different phi values of 1.55 and 27.2 g Cr(VI)/USD. Thus, although the processes obtain similar yields, there are dissimilar regarding their economic feasibility. (C) 2021 Elsevier Ltd. All rights reserved.