Calcinated sea urchin shell waste for rapid phosphate removal from greywater for application to nature-based systems

Abstract

Nature-based decentralised solutions (NBDS) have been trialled for greywater treatment, but their adoption for this is limited due to their inefficient phosphorus removal. To enhance NBDS like green walls, and closely align their engineered designs with Sustainable Development Goals, repurposing waste materials within them is important. This study evaluated the effectiveness of using sea urchin waste shells to remove phosphate from greywater. Modifying the raw waste shells into calcinated shells at 800 °C substantially increased their phosphate adsorption capacity (q_e) from 0.69 to 13 mg P/g and phosphate removal equilibrium was achieved within three minutes. The maximum phosphate adsorption capacity was 41.98 mg P/g with a removal efficiency of 98 %. Material characterisation of the shells from XRD, FTIR, SEM, and EDS analyses revealed that phosphate removal primarily occurred through co-precipitation with Ca^2 +. The effects of shell adsorbent dosage, initial solution pH, calcination temperature, and presence of co-existing pollutants on phosphate adsorption capacity, were investigated. A Langmuir adsorption isotherm and pseudo-first-order kinetic best described phosphorus removal behaviour. Chemical equilibrium modelling using Visual MINTEQ confirmed the phosphate precipitation process. These results demonstrate waste sea urchin shells' potential for rapid, effective phosphate removal in NBDS for greywater treatment.