Porous vaterite and cubic calcite aggregated calcium carbonate obtained from steamed ammonia liquid waste for Cu2+ heavy metal ions removal by adsorption process

Highlights: • Porous vaterite and cubic calcite aggregate CaCO₃ was produced successfully using steamed ammonia liquid. • The maximum adsorption Cu²⁺ capacity was 1152.07 mg·g⁻¹. • The removal of Cu²⁺ contains adsorption and ion exchange chemical precipitation two simultaneous processes. Removal of copper ions (Cu²⁺) from wastewater has garnered significant interest due to the growing environmental concerns. In order to address such issues, porous vaterite and cubic calcite aggregated CaCO₃ is successfully prepared from steamed ammonia liquid waste (CaCl₂) and (NH₄)₂CO₃ (Ca²⁺/CO²⁻_3M ratio = 5) via a simple precipitation method conducted at room temperature. The physicochemical properties of CaCO₃ are studied using conventional material characterization techniques. Furthermore, the possible formation mechanism of the as-obtained CaCO₃ is also discussed in this work. The effects of the CaCO₃ quantity, pH, temperature, reaction duration, and initial Cu²⁺ solution concentration on the Cu²⁺ adsorption by CaCO₃ are also investigated. A high Cu²⁺ removal rate, i.e., as high as 99%, is achieved with the use of CaCO₃ (1000 mg·L⁻¹). After the adsorption process, the residual Cu²⁺ concentration is less than 0.5 mg/L (from an initial Cu²⁺ concentration of 500 mg·L⁻¹), which meets the discharge standard for industrial waste. The adsorption kinetics of the process can be expressed by a pseudo-second-order model, with a R² of 0.9992. Langmuir adsorption model can be used to express the equilibrium adsorption of the as-prepared CaCO₃, with a R² of 0.9967. The obtained sediment after adsorption process is characterized with XRD, XPS, SEM, SEM-EDS, TEM, TEM-SEAD, EDS, and EDAX elemental mapping. Based on the collective results, Cu²⁺ removal mechanism by the aggregated CaCO₃ in aqueous solution is based on two simultaneous process, i.e., adsorption and ion exchange chemical precipitation.