Análisis de resultados

The use of reclaimed wastewater for irrigation or the application of biosolids as fertilizer for soil amendment has the potential to transfer organic pollutants (such as BACs) into the soil environment. The fate of pollutants in the natural environment includes transport processes such as convection, diffusion and dispersion, adsorption and leaching, and persistence (biodegradation and chemical oxidation). Adsorption plays an important role in the behaviour of chemical compounds in soils and influences degradation, and leaching.

There is not much literature on the adsorption of BACs on sludge or soil; so far, only three studies report the adsorption of BACs on different types of sludge during the wastewater treatment process. García et al. (2006) studied the adsorption of BACs on sludge collected from a thickening tank, while Ismail et al. (2010) determined the adsorption of QACs (two different monoalkonium and benzalkonium halides) on four different sludges, primary, waste-activated, mesophilic and thermophilic digested sludge. The adsorption capacity of sludge was higher for benzalkonium halides compared to monoalkonium halides (Ismail et al., 2010). However, in both studies, the authors have indicated that the adsorption of BACs on sludge increases with the increasing carbon chain length in the BAC molecules. The sorption process is governed by both electrostatics as well as hydrophobic interactions between the surfactant molecules and the negatively charged sludge particles. Ren et al. (2011) studied the adsorption of tetradecyl benzyl dimethyl ammonium chloride (BDTA) on activated sludge at different pH, ionic strength and temperature. Adsorption equilibrium was achieved within 2 h of contact time, and the adsorption capacity increased with increasing solution pH up to pH 9.

Sarkar et al. (2010) were the first to report the sorption of different QACs (monoalkonium halides) on agricultural soils. They investigated the effect of three commonly used QACs, namely hexadecyl trimethyl ammonium bromide, octadecyl trimethyl ammonium bromide and Arquad, on dehydrogenase and potential nitrification activities in three different soils. The QACs inhibited the dehydrogenase activity and nitrification of the soils, and the toxic effect of QACs on soil microbial activities was influenced by the relative release of adsorbed QACs in soils. As a result, it is important to monitor the adsorption and leaching of these compounds to determine their bioavailability and the fate in soil environment, which are the key factors for the risk assessment of a micropollutant in soil. Therefore, reliable methods for the determination of leaching behaviour of contaminants are required.

Several different approaches have been used to determine desorption and leaching of organic compounds from contaminated soil. For example, stirred aqueous batch tests, introduction of a third phase as an adsorbent to the soil/water system, and column leaching tests are commonly adopted methods for leaching and desorption studies of organic

compounds from soil (Enell et al., 2004; García Frutos et al., 2011). Stirred aqueous batch tests have been applied by many authors to study the sorption and desorption of organic compounds in soil (Ismail et al., 2010); however, the effect of grinding of soils may overestimate the desorption rate of compounds when using this method (Enell et al., 2004). Column leaching methods are considered to be more realistic in determining the leaching and desorption of organic compounds occurring in the field (García Frutos et al., 2011).

The leaching of organic contaminants in soils depends on the adsorption equilibrium between the compounds in soil solution and adsorbed amount to the solid constituents of the soil, which in turn depends on available binding sites on the soil, mostly due to hydroxyl and carboxylic acid functionalities. The van der Waals attractions are important with soils with higher amount of clays as the compounds migrate between sheets in the clay structure or in binding sites on the edges of those sheets. The soils containing both higher clay and organic matter content have more potential binding sites for retention of the compounds consequently reducing the extent of leaching. The only study related to desorption/leaching of a BAC (hexadecyl benzyl dimethyl ammonium chloride with 16 carbon alkyl chain) from sewage sludge reported that about 5% of the BAC was desorbed in 24 h (Ismail et al., 2010). In another study, two different monoalkonium chlorides were found to desorb by 10 to 60%, depending on their molecular structure, from soil samples (Sarkar et al., 2010).

It is unavoidable to release organic pollutants (such as BACs) in the natural environment and it can happen in many ways, such as by the land application of biosolids, use of reclaimed wastewater for irrigation or by direct discharge of pollutants or due to soil erosion into sediments if BACs are present in agricultural land due to soil amendment with biosolids. The first two possibilities can be mitigated by redesigning/improving the conventional wastewater treatment technologies, which of course will take many years with the potential involvement of money. As a result, understanding on the sorption and leaching behaviour of BACs would be beneficial to characterize the risk associated to these compounds. For example, uptake of BACs from soil by plants is only possible if they are desorbed and become mobile in the rooting zone of soil environment.

To date, no study has been performed to determine the adsorption parameters of BACs on various types of agricultural soils and the leaching possibilities of BACs from biosolids or when soils are amended with biosolids. As a result, understanding the adsorption and leaching behaviour of BACs from soils amended with biosolids would be beneficial to characterize the risk associated with them.