Comprehensive characterization of maize lime-cooking wastewater with a prospective approach for Ca-P minerals recovery: Implications for waste valorization

The production of maize-based products relies on an initial step of alkaline maize-cooking (nixtamalization), which employs important amounts of lime (Ca(OH)2). The wastewater generated during this process, known as nejayote, constitutes one of the most challenging agro-industrial effluents to treat due to its complex and highly variable physicochemical characteristics, including high concentrations of dissolved Ca2+ and highly alkaline pH values (>10). Such characteristics impose detrimental effects in systems devoted to water treatment and compound separation systems applied to this residue. In this paper, several batches of nejayote from a full-scale facility producing maize flour were thoroughly characterized to decipher the variability and magnitude parameters relevant for its treatment and potential mineral recovery. Then, a methodology for removing dissolved Ca2+ from nejayote was implemented by inducing the precipitation of calcium-phosphate (Ca-P) minerals. The results confirmed that the main chemical components of nejayote were total suspended solids (3.7 ± 0.8 gTSS/L), total chemical oxygen demand (COD) (16.8 ± 5.4 gCODtot/L), total organic carbon (TOC) in the liquid fraction (4.1 ± 1.6 gTOCliq/L), and dissolved Ca2+ (1.5 ± 0.2 g/L), and that statistically significant differences in these and other physicochemical parameters of nejayote (pH, N species, metals and ions content, etc.) were found within the same maize-processing factory. Furthermore, it was documented that ∼ 99.9 % of the dissolved Ca2+ in nejayote can be recovered as carbonate-substituted high-purity forms of apatite and hydroxyapatite between 170 and 380 nm mean particle size. The implications of valuable Ca-P minerals recovery for a circular economy approach that allows the enhancement of water treatment and resource recovery processes are also discussed.