Novel integrated acidic and microbial modification strategy to improve adsorption performance of water hyacinth

Industrial wastewater often contains high concentrations of Cr(VI), posing serious environmental and health threats. This study introduces a novel approach to enhancing the adsorption performance of lignocellulosic materials through thermal chemical modification of water hyacinth using dilute H₂SO₄ under mild (low-temperature) conditions, and then a microbial treatment step by a cellulolytic bacterium strain. The effects of modification time (0-50 h), acid volume/material ratio (3-11 mL/g), temperature (25-60 °C), and acid concentration (0.5-3% v/v) were systematically evaluated using a one-factor-at-a-time (OFAT) method and further optimized via Box-Behnken Design (BBD) coupled with response surface methodology (RSM). Under the optimal conditions (30 h, 5.5 mL/g, 45 °C, and 1.5% v/v H₂SO₄), the treated biomass achieved a maximum Cr(VI) adsorption capacity of 3.22 mg/g. Characterization using scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), and Brunauer-Emmett-Teller (BET) surface area analysis confirmed that the mild acid treatment effectively reduced cellulose crystallinity, and introduced abundant C = O and O-H functional groups, while the surface area (1.86 m²/g) remained unchanged. The zero point of charge (pH_pzc) of treated water hyacinth (TWH) was determined to be 5. Adsorption followed the Langmuir isotherm (R² = 0.972) and pseudo-second-order kinetic model (R² = 0.9982), involving both intra- and extra-particle diffusion. Furthermore, the results revealed that microbial treatment with Alcaligenes sp. KHM19 for 6 days achieved a Cr(VI) removal efficiency of 95.42% and an adsorption capacity of 3.85 mg/g, which was approximately 1.2 times higher than that obtained by thermal-acidic modification alone. This integrated method, combining biological treatment with thermal chemical modification, represents a novel and sustainable strategy for efficient Cr(VI) removal from industrial wastewater.