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

Abstract

At an industrial scale, significant quantities of solid activated sludge were produced as a by-product 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 contaminated 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 20 μm in size with a porous structure. The outcomes of surface analysis indicated that D-BC200 had abundant surface 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 modelling the chemisorption process involved in removal of Mo(VI). Langmuir model better described adsorption isotherm, and the maximal adsorption capacity at 298 K was 82.1 mg/g, which was comparatively higher among the adsorbents reported. The enthalpy and entropy of the adsorption process are –39.3 kJ/mol and –125.6 J/mol K, respectively. This indicated that the adsorption process 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.