Efficient removal of nickel ions from aqueous solutions using dicalcium phosphate Dihydrate: Synthesis and valorization into trinickel phosphate octahydrate

Abstract

Wastewater contaminated with nickel poses a significant threat to the ecosystem due to its toxicity and bioaccumulation in living organisms. This work explores the removal of nickel ions from aqueous solutions through the formation of trinickel phosphate octahydrate (Ni₃(PO₄)₂.8H₂O) using dicalcium phosphate dihydrate (DCPD). The experimental method adopted consist to introduce DCPD into aqueous solutions of Ni²⁺ at room temperature and stirred for different durations. The X-ray diffraction, Infrared Spectroscopy, Differential thermal and thermogravimetric analysis, Scanning Electron Microscopy coupled with X-ray Energy Dispersive Spectroscopy, X-ray photoelectron spectroscopy X, Inductively Coupled Plasma and Impedance Spectroscopy were employed. The results indicate that DCPD exhibits a sorption capacity of 293.467 mg/g for Ni²⁺, while the recovered phosphates are crystallized into (Ni₃(PO₄)₂.8H₂O). The removal mechanism occurs in two distinct stages. The first stage, for a processing time of less than 1200 min, involves an ion exchange between nickel ions in the solution and calcium ions in the phosphate, achieving a maximum exchange of 25 %. The second stage, occurring between 1200 and 8740 min, entails the dissolution of the intermediate precipitated phosphate (Ca_0.75Ni_0.25HPO₄.2H₂O) and the concurrent formation of Ca_3-xNi_x(PO₄)₂.8H₂O. This stage culminates in a second ion exchange process, leading to the formation of (Ni₃(PO₄)₂.8H₂O). Impedance spectroscopy results further confirm the structural transformation by revealing a two-step electrochemical response. The Nyquist and Bode plots show a transition from surface adsorption at high frequencies to diffusion-controlled ion incorporation at lower frequencies. Electrical circuit modeling highlights the progressive charge transfer and conduction evolution, correlating with the formation of a stable nickel phosphate phase. The final product exhibits high grain boundary resistance, indicating a dense and well-structured material with potential applications in electrochemical systems.