Repurposing the natural resources of soybean hull waste–dolomite for the phosphate adsorption

The occurrence of phosphate in water is a pivotal compound that influences water bodies. Therefore, the excessive amount of phosphate in wastewater must be reduced and controlled to inhibit its convolution. In this work, we repurposed the utilization of natural resources, including dolomite and soybean hull (SH) waste, as advanced adsorbent materials to reduce phosphate concentrations in water. The different pyrolysis temperatures exhibit the different features of SH–dolomite biochar (SD) composite. At a pyrolysis temperature of 400 °C, the dolomite phase is still retained with the low defect of carbon formation. The dolomite phase is transformed into CaCO₃ when the pyrolysis temperature increases to 600 and 800 °C. The defect and disordered carbon are more pronounced in SD 600 °C and SD 800 °C. Intriguingly, such features exhibit multilayer adsorption in SD 400 °C and monolayer adsorption in SD 600 °C and SD 800 °C, according to the isotherm analysis result. SD 800 °C exhibits the highest adsorption capacity compared with SD 600 °C and SD 400 °C. This can be attributed to the occurrence of CaCO₃ and MgO in SD 800 °C, which enhances the interaction between phosphate and SD 800 °C. There is no significant change of SD features after the adsorption, implying good stability of the SD composite for the adsorption process. This finding provides a concept for developing adsorbents from natural resources with good stability and performance.