Process-scale evaluation of biochar for calcium removal and mineral recovery in industrial wastewater treatment

Abstract

Increasing calcium (Ca²⁺) concentrations in water systems, largely attributed to industrial effluents and agricultural runoff, present serious environmental and health concerns, including scaling, infrastructure degradation, and elevated risk of kidney stone formation. Conventional calcium removal techniques such as chemical precipitation and reverse osmosis are often energy-intensive and economically unsustainable. This study explores engineered biochars derived from algae and wood as alternative, sustainable materials for Ca²⁺ remediation from aqueous environments. The performance of each biochar was evaluated through a combination of atomistic simulations, batch and continuous-flow adsorption experiments, and gas–liquid–solid phase carbonation tests. Algae-based biochar reduced Ca²⁺ concentrations by up to 47 percent, from 802.4 to 425 mg per liter, outperforming wood-based biochar, which reduced concentrations from 839.5 to 574.3 mg per liter. Both biochars demonstrated significantly higher Ca²⁺ uptake than inert glass bead controls. Carbonation experiments further confirmed the ability of biochar to facilitate calcium carbonate (CaCO₃) precipitation, with algae biochar yielding 762 mg of CaCO₃ and exhibiting substantial surface-scale deposition, suggesting enhanced nucleation and crystal growth. Density Functional Theory (DFT) calculations indicated that nitrogen- and oxygen-containing surface functional groups, particularly amine, pyridine, and carboxyl moieties, are key contributors to Ca²⁺ binding. The adsorption energy analysis supported the superior performance of algae biochar, driven by stronger Ca²⁺ affinity. These results demonstrate the potential of designed biochars as efficient, low-cost materials for water treatment, while supporting sustainable resource management and circular bioeconomy initiatives.