Performance of Low-Temperature Torrefied Mango Seed Kernel for the Removal of Heavy Metals from Produced Water

This study evaluated the performance of a low-temperature torrefied and citric acid–activated mango seed biosorbent for removing Pb2+ and Cd2+ from produced water. Mango seed kernel (MSK) was torrefied at 200, 225, and 250°C temperatures and labeled T200, T225, and T250, respectively. Bulk density and hydrophobicity were determined by standard methods and used as the selection criteria for chemical activation. Ash, moisture, fixed carbon, and volatile matter contents were determined. FTIR was used to characterize the surface functional groups of torrified and torrefied-acid-activated MSK labeled as TAM. Batch biosorption experiments were designed to assess the removal efficiency of the biosorbent. Results showed that torrefaction improved hydrophobicity, ash content (AC) and fixed carbon content (FCC) while reducing bulk density, moisture content (MC), and volatile matter (VS) of MSK. The order of the degree of hydrophobicity is given as while the order of bulk density is indicating an inverse linear relation between hydrophobicity and bulk density. MC and VS were reduced by 68.64% and 18.8%, respectively, while increasing AC and FCC by 19.86% and 88.69%, respectively. FTIR spectra illustrated that torrefaction at 200 and 225℃ did not cause loss of functional groups; rather, it caused structural changes, unlike torrefaction at 250℃, which revealed loss of functional groups and structural changes. Chemical activation further revealed an intense narrow dip for the C=O group around 1650 – 1690 cm-1 wave number. TAM biosorbent was able to remove over 98% of dissolved solutes and ions (TDS and EC), including heavy metals (Pb2+ and Cd2+). The biosorbent worked better on Pb2+ than Cd2+, removing 99.5% and 98.57%, respectively. These results are indicative of the possibility of the use of low-temperature torrefied-acid-activated biomass in the wastewater treatment while encouraging the circular economy and sustainable manufacturing.