Modification of Rice Straw Biochar via Co-pyrolysis with Polyethylene Terephthalate: A Valorization Approach to Lead(II) Adsorption

Lead (Pb) contamination remains a critical and persistent environmental threat in the Philippines, with toxic concentrations persisting in domestic water supplies. This study presents a circular economy solution by upcycling rice straw (RS) and polyethylene terephthalate (PET) into a functional biochar composite for Pb(II) removal. Utilizing co-pyrolysis at 400°C, the research evaluated the influence of varying feedstock ratios (1:0, 2:1, 1:1, 1:2 w/w) and deionized (DI) water washing post pyrolysis. Thermogravimetric (TGA), Scanning Electron Microscopy (SEM), and Fourier Transform Infrared (FTIR) analyses revealed that while the RS provided a stable lignin-derived carbon structure, PET acted as a modifying agent, introducing active carbonyl (C=O) groups. Adsorption behavior was modeled using Langmuir and Freundlich isotherms, comparing unwashed and deionized water-washed variants. The unwashed 2:1 RS-PET ratio demonstrates a near-perfect statistical fit with the Freundlich model (R² = 0.999), a high adsorption intensity (n=3.46), and Freundlich adsorption capacity (K_f) of 0.929. Notably, DI- water washing significantly diminished performance—reducing K_f of the PET-rich samples by at least 94%—indicating that the initial mineral layer is the primary driver for Pb(II) uptake via ion exchange. By transforming dual waste streams into high-affinity adsorbents, this study provides a scalable and potential pathway for heavy metal remediation and sustainable waste management in the Philippines.