Desenvolvimento de metodologias para a especiação de arsênio em águas e matrize salinas empregando voltametria adsortiva de redissolução catódica
PILAU, Eduardo Jorge. Development of methodologies for the arsenic speciation in waters and saline matrices by adsorptive cathodic strtipping voltammetry. 2005. 116 f. Dissertação (Mestrado em Química) - Universidade Federal de Santa Maria, Santa Maria, 2005.
Pilau, Eduardo Jorge
The present work reports the development of methodologies for the arsenic speciation in waters and saline matrices by adsorptive cathodic stripping voltammetry (AdCSV). The determination of As(III) by AdCSV was investigated and optimized using the organic ligand sodium diethyldithiocarbamate (DDC) in acidic medium. The main experimental parameters that have influence on the As(III) determination by AdCSV were systematically investigated. The reaction between As(III) and DDC allowed the sensitive determination of inorganic As(III). The detection limit 0,5 μg L-1 and the quantification limit 1,2 μg L-1 were calculated for As(III), with a standard deviation of 1,6% for As(III) 20 μg L-1 (n = 5). The voltammetric method was compared with other existing stripping voltammetric methods, wich employ the ligand pyrrolidine dithiocarbamate (PDC) and Cu(II) ions for the As(III) determination. For the As(III)/As(V) speciation, reduction steps of As(V) to As(III) were investigaded using two different methods: chemical reduction with Na2S2O5/Na2S2O3 and photochemical reduction with UV radiation. The chemical reduction step using the mixture Na2S2O5/Na2S2O3 under heating at 80 °C was optimized regarding the heating time of the solution and concentration of reducing agent. The reducing agent Na2S2O5/Na2S2O3 in the concentration 2,5/0,25 mg mL-1, respectively, was shown to be efficient for the As(V) reduction after 45 min heating of the solution. The photochemical reduction of As(V) by UV irradiation in the presence of reducing sugars such as glucose and frutose was also investigated for the As(V) determination by AdCSV in the presence of PDC. The experimental parameters irradiation time and glucose concentration were systematically investigated. The reduction of As(V) to As(III) occurs with ∼100% efficiency in the presence of 60 mg L-1 glucose added to the As(V) solution irradiated for 90 min at 90 °C. This way, total arsenic was determined as As(III) after photochemical reduction of mixtures containing As(III) and As(v), which allowed arsenic speciation studies of inorganic arsenic. The photochemical reduction allowed also the determination of the arsenic organic species monometilarsenic acid (MMA) and dimethylarsenic acid (DMA) as As(III) after UV irradiation in the presence of glucose as well as the determination of total arsenic (organic + inorganic). The methodology based on the voltammetric determination of the complex formed between As(III) and DDC was applied to the determination and speciation of As(III) and As(V) in samples of dialysis water, mineral water, model sea water and hemodialysis saline concentrates. In view of the high saline concentration of the samples, the voltammetri determination of As(III) in the presence of DDC, as well as the chemical reduction step of As(V) with Na2S2O5/Na2S2O3, were optimized regarding experimental parameters such as pre-concentration time of the complex at the HMDE, reduction time and concentration of reducing agent in the saline samples. Recovery experiments for As(III) e As(V) were carried out in spiked samples. The recovery results for total arsenic between 82 and 105% assure that the methodology can be applied for the arsenic speciation in real samples of waters and matrices with saline concentrations.