Effect of inorganic salts on dissolved organic matter adsorption using water hyacinth leaves and graphite in saline water

Dissolved organic matter (DOM) removal from saline water is essential for producing potable water, yet conventional activated carbon is costly and often derived from non-renewable precursors. This study evaluates water hyacinth leaves (WHL) as a waste-biomass biosorbent and coal-based graphite (GR) as a carbon adsorbent for DOM removal from saline water containing sodium chloride (NaCl) and magnesium chloride (MgCl₂). DOM was extracted from potting soil (initial dissolved organic carbon ≈ 19–34 mg L⁻¹), and batch experiments were conducted over a salinity range of 0–45 g L⁻¹ to assess the effects of salt type, contact time, adsorbent dosage, DOM concentration, and temperature. Characterization (BET, SEM-EDS, FTIR, XRD) showed that WHL is a micro-mesoporous, chemically active biomass with abundant oxygen-containing functional groups, whereas GR is a meso-macroporous, highly crystalline carbon with fewer surface functionalities but larger pore volume. GR achieved consistently high DOM removal (>80%) for all NaCl and MgCl₂ concentrations, with a maximum of about 90% at 1 g L⁻¹ and 35 g L⁻¹ NaCl, indicating robust, salt-tolerant adsorption. WHL performance decreased from approximately 75–80% DOM removal at low salinity to below 50% at the highest MgCl₂ concentration, suggesting stronger ionic competition and active-site blockage by divalent cations. Adsorption kinetics for both adsorbents followed the pseudo-second-order model (R² > 0.98), and equilibrium data were best described by the Freundlich isotherm, with GR exhibiting more favorable 1/n values. Overall, GR is more suitable for DOM removal from saline waters, while WHL is better suited to low-salinity applications. • Effect of inorganic salts on DOM adsorption from saline water was evaluated using water hyacinth leaves and graphite. • Graphite achieved >80% DOM removal and resisted salinity effects. • Water hyacinth showed lower, salt-sensitive removal. • Adsorption followed pseudo-second-order and Freundlich models.