Sustainable and selective capture of Ca2+/Mg2+ from seawater desalination brines via hierarchical chitosan-based biochar aerogel adsorption

Abstract

To address the challenge of removing high-concentration calcium and magnesium ions (Ca²⁺/Mg²⁺) from seawater desalination brines (SDB), this study introduces a novel chitosan-based biochar aerogel (CBA) system specifically engineered for hypersaline water treatment. The innovative adsorbent material utilizes high-temperature pyrolysis chitosan (85 % and 95 % deacetylation degree (DD)) generate biochar and then crosslinked into three-dimensional porous aerogels (CBA85 and CBA95), which were compared with conventional activated carbon aerogel (ACA). Comprehensive characterization revealed that CBA95 exhibited a high density of free amino groups and a well-developed mesoporous structure, with a specific surface area of 107.04 m²/g, approximately 3.14 times that of ACA. The maximum adsorption capacities of CBA95 for Ca²⁺ and Mg²⁺ were 5.15 mg/g and 9.37 mg/g, respectively. Selectivity adsorption experiments demonstrated CBA95 exhibited high selectivity for Ca²⁺ and Mg²⁺, with distribution coefficient (K_d) of 164.25 mL/g and 42.73 mL/g, respectively. The equilibrium data were aligned with the Langmuir isotherm model, suggesting a monolayer chemical adsorption mechanism, and further the electrostatic and complexation adsorption mechanism was identified and proposed. Most notably, fixed-bed reactor of CBA95 maintains over 87 % of Ca²⁺ and 91 % of Mg²⁺ removal efficiency after 9 adsorption-desorption cycles. The pilot-scale experiment of CBA95 in Qingdao SDB achieved 72.3 % Ca²⁺ and 74.6 % Mg²⁺ removal, confirming high industrial potential. Thus, this research not only establishes a theoretical foundation for applying chitosan-based materials in SDB softening but also provides a solution for large-scale engineering-scale applications due to their environmental sustainability and potential.