Multidisciplinary approach on environmental contaminants in wastewater

ABSTRACT This doctoral thesis explores various strategies for environmental remediation and management, encompassing the use of biodegradable extracting agents for soil washing techniques to avoid the accidental release of persistent pollutants into the environment, the employment of affordable adsorbents to mitigate the spread of micropollutants (i.e., microplastics), and the mitigation of emerging contaminants such as methylisothiazolinone (MIT) through a biological process. Different aspects related to the soil washing techniques, in both ex-situ and in-situ configurations are here discussed, particularly focusing on the application and effectiveness of the remediation through biodegradable reagents. Nowadays, the use of these reagents is a key-factor to ensure a remediation process characterized by operating costs affordability and environmental sustainability. Low-molecular-weight organic acids (LMWOA), chelating agents, and biosurfactants can represent suitable extractant agents for the achievement of both high treatment efficiencies and minimized soil characteristics alteration. In this perspective, the characteristics of the main reagents involved in the washing process and the related mechanisms influencing the removal of Potentially Toxic Elements (PTEs) from soils are thoroughly analyzed. Indeed, the findings from previous studies highlight the need of preliminary investigations at lab scale to identify the soil properties and consequently select the best process operating conditions in order to perform feasible and effective treatments. Moreover, the literature review identifies the chelating agents as the most used extractants to date. Nonetheless, several research aimed at further deepening this topic also indicate the LMWOA and biosurfactants as promising and less impacting alternatives in the near future also due to their possible low-cost production through biological processes. Also, this thesis proposes the surgical mask as an affordable and sustainable adsorbent for the remediation of diesel–contaminated seawater to cope with the polymeric waste generated monthly in hospital facilities. This approach can also be helpful considering a possible future pandemic, alleviating the pressure on the waste management system by avoiding improper mask incineration and landfilling, as instead occurred during the previous COVID–19. Batch adsorption–desorption experiments revealed a complete diesel removal from seawater after 120 min with the intact laceless mask, which showed an adsorption capacity of up to 3.43 g/g. The adsorption curve was better predicted via Weber and Morris’s kinetic (R2 = 0.876) and, in general, with Temkin isotherm (R2 = 0.965 – 0.996) probably due to the occurrence of chemisorption with intraparticle diffusion as one of the rates–determining steps. A hysteresis index of 0.23 – 0.36 was obtained from the desorption isotherms, suggesting that diesel adsorption onto surgical masks was faster than the desorption mechanism. Also, the effect of pH, ionic strength and temperature on diesel adsorption was examined. The results from the reusability tests indicated that the surgical mask can be regenerated for 5 consecutive cycles while decreasing the adsorption capacity by only approximately 11%. Finally, this thesis aims to investigate, the effect of MIT on the nitrification-denitrification process in a moving-bed biofilm reactor (MBBR). After a preliminary study performed in a fed-batch reactor, two MBBRs were simultaneously operated in semi-continuous mode for 136 days evaluating a concentration of MIT in the influent of 0.1 and 1.0 mg/L. A parallel study was also conducted under anaerobic conditions to evaluate the effect of MIT on methane production and volatile fatty acids accumulation. At a mere concentration of 0.1 mg MIT/L, the nitrification process experienced an initial complete inhibition. Subsequently, nitrifying bacteria gradually recovered, reaching approximately 49% oxidation of ammonia nitrogen, a removal improved up to 64% at 1.0 mg MIT/L. Although the denitrification process faced temporary inhibition, ultimately achieved a 100% reduction in nitrite and nitrate concentrations when the MIT concentration was increased from 0.1 to 1.0 mg/L. The cumulative methane yield reached approximately 134 mL CH4/g VS for the control group and was similar to the methane production obtained in the presence of MIT. A MIT removal of 100% was achieved under aerobic, anoxic, and anaerobic conditions.