Adsorption of molybdenum(VI) by low-temperature biochar derived from activated sludge and application in reservoir water purification

At an industrial scale, significant quantities of solid activated sludge were produced as a by-prod-uct in wastewater treatment plants. In this contribution, activated sludge biochar was synthesized under oxygen-limited conditions with the purpose of adsorptive removal of Mo(VI) from contam-inated water. Our experiments show that the sludge biochar have a better performance in Mo(VI) removal compared to the raw sludge. Additionally, we report that demineralized biochar (pyrolyzed at 200°C, D-BC200) performs better than sludge biochar samples pyrolyzed at other temperatures. From microscopic point of view, D-BC200 particles are shown to be smaller than 20μm in size with a porous structure. The outcomes of surface analysis indicated that D-BC200 had abundant sur-face functional oxygen-containing groups. This facilitated the uptake of Mo(VI). We performed both non-linear and linear pseudo-second-order kinetics modelling to describe the experimental kinetics observed in our experiments. We conclude that both approaches are suitable for model-ling the chemisorption process involved in removal of Mo(VI). Langmuir model better described adsorption isotherm, and the maximal adsorption capacity at 298K was 82.1mg/g, which was comparatively higher among the adsorbents reported. The enthalpy and entropy of the adsorption process are –39.3kJ/mol and –125.6J/molK, respectively. This indicated that the adsorption pro-cess was spontaneous and endothermic. Both coexisting anions and natural organic matter could inhibit adsorption of Mo(VI) to some extent. When D-BC200 was applied for the removal of Mo(VI) from natural reservoir water, complete removal of Mo(VI) was observed.