Presence of Humic Acids in Landfill Leachate and Treatment by Flocculation at Low pH to Reduce High Pollution of This Liquid

Humic substances are abundant in landfill leachate, especially humic acids, which are insoluble at low pH in aqueous solutions. Focusing on the chemical properties of humic acids, we describe in this work a new method for a sustainable treatment of landfill leachate originated from solid wastes, which consists of the reduction of organic load (COD, chemical oxygen demand) and colour and is based in the gradual decrease in pH to the value in which HAs are insoluble in water solution. Zeta potential values mark the chemical stage of humic acids because of ionisation–protonation of the phenolic and carboxylic groups, and this parameter is monitored during flocculation, changing from −16.8 mV at pH 7.7 to 0.0068 mV at pH 2.0, when HAs precipitate. The final result is the reduction in the organic matter content (COD) and colour in the leachate, 86.1% and 84.7%, respectively. Solids produced by precipitation during the acidification treatment have been characterized by elemental chemical analysis and Fourier transform infrared spectrometry, concluding a high similarity in chemical composition with commercial and natural humic acids. Protonated humic acids at low pH can interact with other molecules by hydrogen bonds and form bigger molecular structures much more unstable in suspension, which conduct to precipitation. The mean diameter of the humic acids aggregates was measured, detecting the formation of big molecular structures at low pH. This process is analysed and compared economically with other processes proposed for landfill leachate treatment, resulting in a promising technique for the management of this residue.