Solar radiation effect on the complexation capacity of aquatic humic substances with metals

Abstract

Este estudo teve como principal objetivo avaliar o efeito da radiação solar sobre a capacidade de complexação das substâncias húmicas aquáticas (AHS) com as espécies metálicas Al 3+ , Cr 3+ , CrO 42-e Ni 2+ . Microcosmos foram montados com AHS extraídas do Rio Preto com concentração de 5,0 mg L -1 de carbono orgânico dissolvido e diferentes concentrações de metais, sendo expostos à radiação solar in situ. As constantes de estabilidade condicional foram de 2,0 × 10 -2 , 1,1 × 10 -5 e 2,6 × 10 -2 para os complexos AHS-Cr 3+ , AHS-Ni 2+ e AHS-CrO 4 2-, respectivamente. Os resultados mostraram que a radiação solar foi responsável por um decréscimo na capacidade de complexação de 15-26% para AHS-Al 3+ , 15-72% para AHS-Cr 3+ , 12-18% para AHS-CrO 4 2-e de 13-42% para AHS-Ni 2+ . Conclui-se que a radiação solar altera a disponibilidade de metais previamente complexados às AHS, tornando-os disponíveis no corpo aquático.This study had as main objective to assess the effect of solar radiation on the capacity of the aquatic humic substances (AHS) to complex Al 3+ , Cr 3+ , CrO 4 2-and Ni 2+ . Microcosms were prepared with AHS from Preto River containing 5.0 mg L -1 of dissolved organic carbon and different concentrations of metal species, being exposed to solar radiation, in situ. Conditional stability constants obtained for the AHS-Cr 3+ , AHS-Ni 2+ and AHS-CrO 4 2-complexes were 2.0 × 10 -2 , 1.1 × 10 -5 and 2.6 × 10 -2 , respectively. The results showed that solar radiation is responsible for decreasing capacity complexation in 15-26% for AHS-Al 3+ , 15-72% for AHS-Cr 3+ , 12-18% for AHS-CrO 4 2-and 13-42% for AHS-Ni 2+ . We can conclude that solar radiation alters metals' availability previously complexed to AHS, making them available in the aquatic body.Keywords: aquatic humic substances, metal complexation, solar radiation, fluorescence IntroductionAquatic humic substances (AHS) have been intensively studied because they play an essential role in biogeochemical processes. 1,2 They can participate in the complexation reactions with metal ions thereby influencing the transport and availability of them in the environment, 2-4 may suffer interactions with organic compounds and thus, reduce or increase its toxicity to the biota, 4,5 can act in redox 6 and photochemical reactions. 7 Aquatic HS are products of the decomposition of vegetal and animal remains from microbiological transformations. They are normally dark colored organic macromolecules with a high molecular weight, a complex structure consisting of aromatic and aliphatic compounds, which have a large number of functional groups containing oxygen, e.g., carboxylates (-COOH), phenolics (aromatics-OH), alcoholics (-OH), quinones (aromatics=O), among others. 1,2 Depending on the water solubility, AHS can be operationally separated into three fractions: fulvic acids that are soluble in all pH range, humic acids that are soluble in alkaline pH and insoluble in acid pH, and humin that is insoluble in all pH range. It is worth noting that the AHS comprise about 40-60% ...

Characterization of typical aquatic humic substances in areas of sugarcane cultivation in Brazil using tetramethylammonium hydroxide thermochemolysis

“…These values are characteristic of water bodies within the region where the samples were obtained (Melo et al, 2012;Pantano et al, 2014). The average total organic carbon values for samples from the Preto river were 1.77 and 3.13 mg L −1 for the dry and rainy period, respectively.…”

“…AHSs from Preto River display characteristics of recently degraded organic matter. This finding was confirmed by the presence of predominantly aliphatic structures with a low degree of humification (Melo et al, 2012). A characterization of HSs from sediments of the Preto River using off-line TMAH-thermochemolysis-GC-MS showed contributions from compounds rich in lipids and fatty acid methyl esters (FAMEs), plant waxes, plant biopolyesters and a large amount of FAMEs that are related to the large amount of HA in the structure.…”

“…The dissolved organic carbon (DOC) content was determined using a total organic carbon analyzer (Shimadzu TOC-VCSN). Additional details regarding the sample analyses can be found in the study conducted by Melo et al (2012). The collection of surface water samples for SHA extraction was performed according to the recommendations of USEPA (2000USEPA ( , 2007 and method 1669(USEPA, 1996.…”

Interaction of arsenic species with tropical river aquatic humic substances enriched with aluminum and iron

“…Signals in region I could not be seen in the spectra, while low intensity signals in regions II and IV were indicative of low protein contents of these AHS samples. Higher intensities were observed in region III, for both AHS samples, characteristics of fulvic acids that are of lower molecular weight and possess more carboxylic groups and a greater number of complexation sites, compared to humic acids (Mak and Lo 2011;Melo et al 2012;Winter et al 2007). Higher intensities in region III were shown by AHS-I rather than by AHS-S.…”

Lability of natural organic matter in freshwater: a simple method for detection using hydrogen peroxide as an indicator

“…2 cycling (McKnight, et al 1985;Melo et al 2012;Nebbioso and Picollo 2013;Thurman et al 1982;White;Vaughan;Zeep, 2003;Westhorpe;Mitrovic, 2012).…”