Effective removal of pollutants from sea cucumber aquaculture tailwater by two-stage adsorption and its mechanism

Sea cucumber aquaculture tailwater(SCAT) contains pollutants such as antibiotics, humic acids, and amino acids. Direct discharge can deteriorate water quality and have toxic effects on aquatic organisms. In the present study, a two-stage granular activated carbon (GAC) adsorption approach was adopted to treat SCAT to eliminate pollutants. The reduction values for florfenicol and neomycin sulfate from the tailwater were 82.38% and 86.51%, respectively, whereas those for chemical oxygen demand (COD Mn ) and total nitrogen (TN) were 64.80% and 88.43%, respectively. The COD Mn and TN concentrations after the adsorption met the “Discharge standard of tailwater for mariculture” requirements (COD Mn  ≤ 20 mg/L, TN ≤ 8 mg/L) in China. The adsorption mechanisms mainly involved hydrophobic interactions, hydrogen bonding, electrostatic interaction, and π-π conjugation between surface functional groups on the GAC and the florfenicol and neomycin sulfate molecules, the large specific surface area and abundant microporous and mesoporous structures of GAC provided a substantial number of adsorption sites, environmental conditions such as temperature and pH were also critical factors. The adsorption isotherm and kinetics followed the Langmuir and pseudo-second-order models, respectively, indicating that the adsorption process was monolayer and chemical adsorption. Thermodynamic analysis revealed that the adsorption of antibiotics onto GAC was a non-spontaneous, exothermic, and entropy-reducing process. The adsorption rates of FF and Neo were controlled by both of film diffusion and particle diffusion. The findings of the present study confirm the effectiveness and applicability of the two-stage GAC adsorption process for treating tailwater, which is a feasible technical approach for implementation.