Carboxylic acid-mediated sustainable synthesis of calcium carbonate from oyster shells: Mechanistic insights into calcium extraction and phase transformation

Abstract

Oyster shell waste, containing 94.0% CaCO3, represents an underutilized biogenic source for precipitated calcium carbonate (PCC). This study demonstrates that carboxylic acids (acetic, formic, and citric acid) can serve as dual-functional agents for both sustainable calcium extraction and additive-free phase-controlled precipitation under alkaline conditions. High-purity PCC (>98% CaCO3) was obtained under most conditions, with calcium recovery strongly dependent on acid type and concentration: citric acid reached a maximum recovery of 98.7% at 1.5 M, formic acid 92.3% at 2.0 M, and acetic acid ∼80% at 2.0 M, comparable to HCl. Unlike HCl, which produced exclusively calcite, acetic and formic acids generated metastable vaterite and aragonite at low concentrations (≤1.0 M). These phase transitions were governed by ligand-assisted surface complexation and the acid-mediated distribution of ionic species (Ca²⁺, CO₃²⁻, COO⁻), where the addition Na2CO3 induced rapid nucleation, and residual carboxylates facilitated the precipitation of metastable phases. This carboxylic acid-mediated approach is consistent with previous reports suggesting up to seven times greater economic viability than conventional methods. Furthermore, the synthesized vaterite from oyster shells aligns with high-performance benchmarks, such as over 99% adsorption capacity for heavy metals (e.g., Pb, Cd). These findings provide mechanistic insights into acid-controlled PCC crystallization and highlight carboxylic acids as environmentally benign, tunable agents for high-value valorization of oyster shell waste, offering a sustainable approach for high-value applications such as CO2 sequestration and specialized paper coatings.