Avaliação da formação de trialometanos e ácidos haloacéticos decorrentes da cloração de águas de abastecimento contendo cianobactérias

Abstract

Despite the benefits of using chlorine for pathogens control, studies show that the use of this substance can contribute to the formation of unwanted and carcinogenic halogenated organic by-products (HOS), such as Trihalomethanes (THMs) and Haloacetic Acids (HAAs). Algogenic organic matter (AOM), composed of algae and cyanobacteria, that usually occur due to blooming events in water bodies, inclunding Microcystis aeruginosa and Cylindrospermopsis raciborskii, can be the precursor of THMs and HAAs formation. This study was divided in four stages: It was proposed that in the first stage, an analytical method by liquid liquid extraction (LLE), followed by gas chromatography (GC) linked to a mass spectrometry (MS) detector, in substitution of an eletron capture detector (ECD) for the quantification of THMs and HAAs. In the second phase, chlorine demand was determined for varied dosages of samples containg species of Microcystis aeruginosa and Cylindrospermopsis raciborskii in density of 1.0 x 104 and 1.0 x 106 cell.mL-1. In the third phase, the formation of THMs and HAAs in function of chemical oxidant doses (calcium hypochlorite and chlorine gas) were evaluated, using the following parameters: water temperature (25°C and 30°C), pH (6.5 and 8.5) and contact time (30 min and 24 h), recurrent conditions in Brazil. In the fourth phase, the use of granular activated carbon (GAC) and agitation were evaluated as removal strategies, in case the formation occurs after the contamination of samples for different concentrations (THMs = 100 and 150 g.L-1; HAAs = 58 and 116 g.L-1). For THMs, LLE was effective in association with GC-MS, so it was possible to dispense onerous and automated extraction steps. For HAAs, chromatograms were obtained with few interferences in haloaceticos acids with less sensitivity to the MS detector. The chlorine demand assays at the highest concentration for both oxidants showed great consumption values and low residual chlorine concentrations. With regard to the parameters studied, the formation of THMs and HAAs occurred only in the concentration of 1.0 x 106 cell.mL-1. Trichloromethane was the specie of THMs that presented in the maximum concentration (45,88 g.L-1) for Microcystis aeruginosa in the following conditions: 5.0 mg.L-1 of chlorine gas, 30°C water temperature, pH of 8.5, and detention time of 24 h. For HAAs, 2.5 mg.L-1 dose of calcium hypochlorite, 30°C of temperature, pH 8.5 and 24 h to Cylindrospermopsis raciborskii, resulted in the highest concentration among the analyzes (4.81 g.L-1). No formations were observed in the concentration of 1.0 x 104 cell.mL-1, as the majority of the results were below of the detection limit of the validated method. According to the literature, metabolites released by cells can interfere in the formation of THMs and HAAs. We found that THMs (93,74%) and HAAs (84,66%) were removed in satisfactory conditions by adsorption in granular activated carbon, but agitation resulted in less removal of THMs (83,74%) and HAAs (10,08%)