Development of aerated concrete waste/white cement composite for phosphate adsorption from aqueous solutions: Characterization and modeling studies

Adsorbing phosphate (PO4-3) from aqueous solutions onto concrete-based construction waste represents a low-cost, nontoxic, and sustainable technique. The novelty of this paper focuses on the adsorption and desorption of PO4-3 from aqueous solutions using autoclaved concrete (AC) chemically activated with white cement (AAC), representing an innovative adsorbent. Pseudo-second-order model better represent adsorption for AC (0.29 mg g-1; pH 6.35) and AAC (0.54 mg g-1; pH 6.53). The experimental data adjusted better with Freundlich and Langmuir models for AC (8.87 mg g-1) and AAC (3.90 mg g-1), suggesting physisorption and chemisorption, respectively. The presence of calcite could promote ligand exchange onto adsorbent surfaces. For desorption, pseudo-first-order model better explained for AC (0.19 mg g-1) and pseudo-second-order model for AAC (0.39 mg g-1). Sips model better fitted the equilibrium data (0.45 mg g-1 AC; 1.97 mg g-1 AAC), related to ionic exchange and electrostatic repulsion. Phosphate adsorption tends to be more reversible in AC. The chemical activation with white cement enhanced the phosphate removal and recovery by the alternative low-cost auto-claved aerated concrete. Finally, AC and AAC presented potential of reducing phosphate with 64.7% and 92.6% for raw wastewater and 83.1% and 84.8% for primary-treated effluent of a corn flour processing, respectively.

Automated summary

The study found that concrete-based construction waste can adsorb phosphate (PO4-3) from aqueous solutions, providing a low-cost, nontoxic, and sustainable technique. The novelty of this study lies in the use of autoclaved concrete (AC) chemically activated with white cement (AAC) as an innovative adsorbent for the adsorption and desorption of PO4-3 from aqueous solutions. The adsorption process was better represented by the pseudo-second-order model for AC (0.29 mg g-1; pH 6.35) and AAC (0.54 mg g-1; pH 6.53). The experimental data fit well with the Freundlich and Langmuir models for AC (8.87 mg g-1) and AAC (3.90 mg g-1), indicating physisorption and chemisorption, respectively. The presence of calcite facilitated ligand exchange on the adsorbent surfaces. In the desorption experiment, the pseudo-first-order model was more suitable for AC (0.19 mg g-1), while the pseudo-second-order model was better for AAC (0.39 mg g-1). The Sips model provided a better fit for the equilibrium data (0.45 mg g-1 AC; 1.97 mg g-1 AAC), indicating ionic exchange and electrostatic repulsion. The adsorption of phosphate tended to be more reversible in AC. The chemical activation with white cement enhanced the removal and recovery of phosphate by the alternative low-cost autoclaved aerated concrete. Finally, AC and AAC showed potential for reducing phosphate, with removal rates of 64.7% and 92.6% for raw wastewater, and 83.1% and 84.8% for primary-treated effluent from corn flour processing, respectively.