Evaluation of the effectiveness, safety, and feasibility of 9 potential biocides to disinfect acidic landfill leachate from algae and bacteria.

• Efficacy of different biocides for disinfection against algae and E. coli is evaluated. • Organohalogens are quantified following disinfection of landfill leachate. • Biocides have a threshold dosage under which organohalogens form below detectability. • Disinfection mechanism partly explains biocide action with organohalogen formation. • Metabisulfite is identified as a safe, eco-friendly, and effective biocide. This study evaluates 9 biocides as disinfectants against microbiological contaminants, specifically, microalgae and E. coli , while assessing their safety and environmental impact. Specifically, the biocide effectiveness and corresponding generation of halogenated compounds is assessed in a real contaminated groundwater receiving acidic leachate from a phosphogypsum landfill. Oxidizing agents are investigated, namely, hypochlorite, peracetic acid, hydrogen peroxide, chlorine dioxide, and persulfate, together with electrophilic biocides, namely, 2,2-dibromo-2-cyanoacetamide and (chloro-) methylisothiazolinone. In addition, a novel disinfection approach is assessed by applying reducing agents, namely, sulfite and metabisulfite. The disinfection mechanism and the formation of halogenated compounds are discussed on the basis of the mode of action and of the molecular structure of each biocide. Overall, the results show that an optimal dosage of the biocides exists to minimize the formation of harmful compounds in water while maximizing disinfection, especially for hypochlorite and peracetic acid. This dosage was between 0.03 mM and 0.15 mM depending on the biocide. The safety of electrophilic biocides is found to be associated to their molecular structure rather than their mode of action. Hydrogen peroxide, MIT, and metabisulfite are the most promising disinfectants in the contaminated groundwater matrix of interest since no halogenated by-products are detected upon successful disinfection, while they are able to completely inactivate bacteria and remove over the 80% of microalgae in the selected matrix. In particular, metabisulfite represents a highly promising biocide, owing to its low environmental and health impacts, as well as economic feasibility (estimated reagent cost ~0.002 € per cubic meter of treated water). Image, graphical abstract [ABSTRACT FROM AUTHOR]