Natural calcium phosphates from circular economy as adsorbent phases for the remediation of textile industry waste-waters

Adsorption is one of the most important techniques for the remediation of textile industry wastewaters containing high concentrations of toxic dyes. Calcium phosphates (CaPs) have been advocated among the most promising adsorbent phases in virtue of their high dye adsorption potential, low production cost, and possible recovery from circular economy sources. In this paper, we investigated the application of synthetic CaPs (CaP-S) and two CaPs extracted from food by-products, namely from bone meal (CaP-BM) and fish bones (CaP-FM) for the removal of a model dye, copper phthalocyanine tri-sulfonic acid, also known as acid blue 185 (AB-185). All the materials were characterized from the physical-chemical point of view and used for the study of adsorption kinetic and isothermal adsorption of AB-185 at neutral, acidic and basic pH values. The best adsorption capacity was obtained for CaP-S, followed by CaP-BM and CaP-FM. Nonetheless, results showed that all the materials can be employed for removing AB-185 from wastewaters of textile industry, and we correlated dye removal efficiency to the adsorbent phase specific surface area (SSA) and ?-potential, as well as to dye concentration and pH of wastewaters. Finally, as the process used to produce the studied CaPs affects their main final physical-chemical parameters and performance in terms of dyes removal, it is important to consider the variables of CaPs production process (e.g., temperature, time of treatment, grinding etc.), especially when CaPs are recovered from circular economy sources by thermal treatments of food-byproducts.