Sulfate radical anion activated agro-industrial residues for Cr(VI) adsorption: is this activation process technically and economically feasible?

Abstract 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 ϕ 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 ϕ 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.