O. Karki, S. Mandal
Texas State University,
United States
Keywords: waste valorization, effluent treatment, avocado biochar, dye adsorption, heavy metal removal
Summary:
Carbon-based materials have emerged as sustainable and efficient solutions for environmental remediation due to their tunable surface chemistry and high adsorption capacities. Among environmental contaminants, synthetic dyes such as Methyl Violet (MV), Malachite Green (MG), Methylene Blue (MB), and Crystal Violet (CV) are widely used in textile and dyeing industries but pose severe ecological and health risks due to their toxicity and persistence. Similarly, heavy metals like lead [Pb(II)] and organic pollutants such as phenol represent serious environmental hazards, capable of causing carcinogenic, neurological, and organ-specific disorders even at trace concentrations. Among various treatment strategies, adsorption has proven to be the most effective and economical method for removing such pollutants. This study explores the use of fruit waste-derived carbon composites, specifically Avocado Biochar Composites (ABC), as adsorbents for cationic dyes (MV, MG, CV) and Pb(II) ions. To enhance adsorption performance, the ABC was chemically modified with sodium borate (Na₂B₄O₇·10H₂O) to produce Modified Avocado Biochar Composites (MBC), which exhibited increased surface area and enriched functional groups. Adsorption experiments demonstrated that MBC consistently outperformed ABC and raw avocado biomass (ABM) in removing both dyes and heavy metals, highlighting the role of borate modification in improving surface reactivity and binding efficiency. These results establish MBC as a low-cost, sustainable, and high-performance adsorbent for wastewater treatment applications. Avocado seed waste was collected, thoroughly washed with deionized water, and sun-dried. The dried material was pyrolyzed in a tubular furnace at 500 °C for 2 hours under oxygen-free conditions to prevent combustion. The resulting biochar (ABC) was ground, sieved to a uniform particle size, and stored in airtight containers. Single-component adsorption experiments were conducted to evaluate the effects of contact time (10–50 minutes). For each test, 0.5 g of ABC or MBC was added to 250 mL aqueous solutions containing individual dyes or Pb(II) ions, shaken at 200–210 rpm at 20 ± 5 °C, and maintained at pH ~7. Dye concentrations were analyzed via UV–VIS spectrophotometry at λ_max values of 590 nm (MV, CV), 618 nm (MG), and 600 nm (MB), while Pb(II) levels were measured using an Agilent 8900 Triple Quadrupole ICP-MS. Contact time studies revealed that adsorption efficiency increased rapidly within the first 10 minutes and reached equilibrium around 40–50 minutes. MBC consistently achieved the highest removal efficiencies: ~97% for MG, ~92% for MV, and ~92% for CV, compared to ABC and ABM. For Pb(II), MBC achieved ~88% removal at lower concentrations (5–10 mg/L), while efficiency decreased at higher concentrations due to active site saturation, demonstrating concentration-dependent adsorption behavior. The enhanced performance of MBC is attributed to the increased number of active sites and improved functional group availability introduced by borate modification.