Impact of Macromolecules on the Stability of MoS₂ Nanosheets and Understanding the Fate and Behavior Simulated in a Wastewater Treatment Plant Under Aerobic Conditions

The use of transition metal dichalcogenides (TMDCs) in energy, environmental remediation, and medicine presents a possibility for their release into the environment. To better understand the fate and behavior of TMDCs in the environment, their fate and behavior in a wastewater treatment plant were simulated. Molybdenum sulfide (MoS₂) nanosheets were used to mimic TMDCs in stability kinetics experiments in several electrolyte solutions (DI, NaCl, MgCl₂, and CaCl₂) while also investigating the effects of macromolecules (humic acid) on the nanosheets’ stability. The electrolyte type, pH, and the presence of humic acid (HA) impacted the agglomeration and zeta potential of the nanosheets. Significant agglomeration rates were reported near the point of zero charge pH (pH_pzc) in all electrolyte types. The Derjaguin–Landau–Verwey–Overbeek (DLVO) theory predicted that divalent cations reduced the electric double layer in both unspiked and HA-spiked samples. However, due to steric repulsions, the size of humic acid played a role in the stability of the nanosheets. The impact and fate of the nanosheets in a wastewater treatment plant (WWTP) were investigated using the 303A criteria of the Organisation for Economic Co-operation and Development (OECD). The analysis revealed that > 90% of the MoS₂ nanosheets were retained in the sludge, and only 10% were discharged with the effluent, which corresponded to the stability kinetics findings. Furthermore, the nanosheets did not affect WWTP functionality, which was attributed to the high percentage chemical oxygen demand (% COD) elimination at a concentration of 10 mg/L MoS₂ nanosheets. Graphical abstract: [Figure not available: see fulltext.].