Fertilising properties of materials based on opoka and waste concrete after sorption of humic substances from raw reject water

The treatment and reuse of wastewater in a strategic approach to mitigate water scarcity and its declining quality. Natural organic matter, particularly humic substances (HS) from decomposed plant and animal material, often determined the quality of treated wastewater. This study integrates HS recovery into conventional wastewater treatment, aligning with global trends towards carbon capture and storage. The efficacy of opoka (OP) and waste autoclaved aerated concrete (AAC) to sorb HS from the real reject water (RW) was evaluated. The analysis showed that fulvic acids comprised 89% of the organic matter in RW. OP and AAC demonstrated HS sorption capacities of 200 mg g⁻¹. The metal concentrations in the post-sorbents remained within the regulatory limits set for fertilizers by Polish and EU standards. The materials were further tested for phytoactivity in hydroponic systems at different doses (0.05–25 g L⁻¹).A more balanced ratio of ionic forms in the post-sorbents resulted in lower conductivity, improving their chemical stability and phytoactivity. Phytotoxicity studies indicated a that a concentration range of post-sorbents from 0.1 to 2.5 g L⁻¹ was optimal for promoting the growth of most of the tested plants. This research highlights the potential of using post-sorbents as fertilizers, advancing sustainability by recovering HS from RW and supporting a circular economy approach.