Circular Approach of Using Soybean Biomass for the Removal of Toxic Metal Ions from Wastewater

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Abstract
The discharge of industrial effluents containing toxic heavy metals in water sources has serious consequences for human health and the environment, and biosorption appears to be an environmentally-friendly and cost-effective alternative that can be used for wastewater treatment. The use of different types of agricultural waste as biosorbents for the removal of toxic heavy metals, although an alternative, is quite difficult to apply in practice because these wastes have many other uses. Based on these considerations, in this study, soybean biomass (SB), soybean waste biomass obtained from oil extraction (SBW), and biochar obtained from soybean waste pyrolysis (BC-SBW) were tested as biosorbents for removing Pb(II) and Cd(II) ions, in batch systems. Under optimal conditions (pH = 5.4, 4.0 g biosorbent/L, room temperature (25 ± 1 °C), contact time = 180 min), the biosorption capacity increases in the order SB < SBW < BC-SBW for both metal ions (Pb(II) ions (69.43 mg/g < 99.81 mg/g < 116.83 mg/g) and Cd(II) ions (25.63 mg/g < 36.12 mg/g < 49.10 mg/g)), indicating that BC-SBW has the highest efficiency in removing toxic heavy metals. In addition, experiments on wastewater samples have shown that, in addition to significantly reducing the content of heavy metals, BC-SBW also significantly improved other quality indicators (such as pH, TSM (total suspended matter), COD (chemical oxygen demand), BOD5 (biochemical oxygen demand)), compared to the other biosorbents (SB and SBW). Quantitative evaluation of the biosorption performance of each biosorbent (SB, SBW, and BC-SBW) shows that BC-SBW has a real chance of being used on an industrial scale for wastewater treatment. All these aspects allowed the development of a circular approach for the use of soybean biomass in the removal processes of toxic heavy metals. This approach minimizes the shortcomings of using biomass as a biosorbent and increases the chance of using these materials in industrial practice.

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