Isotherm models and kinetics of copper adsorption by using hydrochar produced from hydrothermal carbonization of faecal sludge

Low cost adsorbents have been extensively reported for use as a promising substitution for commercial adsorbents for pollutant removal in water and wastewater treatment. In this study, hydrochar produced from the hydrothermal carbonization (HTC) of faecal sludge (FS) (called HTC-hydrochar) was further chemically modified with KOH (called KOH-hydrochar) to improve its surface functional groups, which were suitable for copper (Cu) removal. The adsorption of Cu was conducted using the produced HTC-hydrochar and KOH-hydrochar as absorbents. Experimental results showed the KOH-hydrochar could adsorb Cu at the maximum adsorption capacity of 18.6 mg-Cu/g-hydrochar with Cu removal efficiency of 93%, relatively higher than the HTC-hydrochar and a commercial powdered activated carbon. The quantity of the surface functional groups of the adsorbents was more effective in Cu removal than the surface area. The Cu adsorption mechanism was found to follow the pseudo-second order and intra-particle diffusion models and fit well with Freundlich and Langmuir isotherms. Application of hydrothermal carbonization could be a novel candidate to convert FS into hydrochar which is pathogen free, and to employ the produced hydrochar as an adsorbent to remove Cu from industrial wastewaters.