Chrysotile asbestos treated with phosphoric acid as an adsorbent for ammonia nitrogen

The purpose of this study was to find an alternative application for chrysotile asbestos, given that there is a complete structure of extraction and production of this class of serpentine minerals, but its use is banned for many applications. The idea was to obtain a compound that could immobilize phosphate by triggering a reaction between the magnesium oxide and hydroxide contained in the mineral, without causing phosphate leaching. To this end, chrysotile (Mg₃SiO₅(OH)₄) was treated with phosphoric acid (H₃PO₄) in a molar ratio of 1:3 in an aqueous medium at 85 °C until the solvent evaporated, resulting in two different solid compounds, which were prepared in a similar manner. The first compound (cri/H₃PO₄ 1:3)₁, was obtained by rinsing and then heat-treating it at 150 °C for 6 h, while the second one, (cri/H₃PO₄ 1:3)₂, was rinsed after the heat treatment. Compound (cri/H₃PO₄ 1:3)₁ underwent partial leaching, while compound (cri/H₃PO₄ 1:3)₂ showed a mass increase of 48%, with the formation of crystalline magnesium pyrophosphate mixed with amorphous SiO₂. The latter compound adsorbed N-NH₃ at pH 10, following the pseudo-first-order model (activation energy = 8329 ± 1696 J mol^-1). Equilibrium experiments, which were performed following Hill's sigmoidal type S2 isotherm model, indicated that the adsorption phenomenon was governed by two processes, i.e., complexation up to the inflection point (K_H between 10.0 mg L^-1 at 40 °C and 13.6 mg L^-1 at 25 °C) followed by adsorption. The q_max varied from 18.0 to 19.6 mgN g^-1 and the adsorbent was reusable, maintaining its initial adsorbent capacity during its first reuse. This material, which was tested on real effluents, presented a N-NH₃ removal rate similar to that shown by the test solutions. The treatment of chrysotile with H₃PO₄ conducts it to a composite that adsorbs ammoniacal nitrogen at pH 10 and it is reusable maintaining the adsorption capacity.