Gilberto Abate

Comparison of methodologies for determination of carboxylic and phenolic groups in humic acids

Abstract A nonlinear method for fitting acid–base potentiometric titration data was applied to commercial and vermicompost humic acids in order to determine the concentration and conditional p K a s of the ionizable sites. Data was also treated by a linear method based on modified Gran functions. In both cases the discrete site model was adopted, which means that the humic acid was treated as a mixture of monoprotic acids. Electrostatic effects were evaluated by performing titrations in media of 0.010, 0.10, and 1.0xa0molxa0l −1 NaCl. Results were compared with those obtained by conductimetric titration with NaOH and Ba(OH) 2 , infrared spectra of humic acid with different grades of protonation, and classical Ca-acetate and Ba(OH) 2 exchange methods. Models with five and six binding sites were the best to represent experimental data of potentiometric titration using both the linear and nonlinear techniques.

Heavy metal distribution in recent sediments of the Tietê-Pinheiros river system in São Paulo state, Brazil

Abstract The concentrations and possible chemical associations of Al, Fe, Mn, Ca, Cu, Pb, Cd, Zn, Ni and Cr in sediments of the Tiete-Pinheiros river system in Sao Paulo state, Brazil, were studied using a 3-step sequential extraction protocol recommended by the Standards, Measurements and Testing Programme (SM&T, formerly BCR). A single extraction with 0.1 mol l−1 HCl was applied in parallel to anoxic and air-dried samples. The river system crosses the metropolitan area of Sao Paulo (MASP), which houses a population of nearly 17 million people, and receives a large load of industrial and domestic wastes. Samples were collected from reservoirs in the surroundings of MASP, named Billings, Pirapora and Rasgao, and from the Barra Bonita reservoir, that is located in the Tiete river, 270 km downstream from Sao Paulo city. The distribution of metals indicates the recent pollution characteristics for samples from the Billings, Pirapora and Rasgao reservoirs. In these sediments the metals are associated to a large degree with reactive forms such as sulphides and carbonates, or adsorbed to amorphous oxyhydroxides of Fe and Mn. In samples from Barra Bonita, heavy metals are mainly associated with the residual fraction, suggesting that their concentrations are controlled significantly by transport processes with fine particles as carriers from diffuse pollution sources.

Determination of atrazine using square wave voltammetry with the Hanging Mercury Drop Electrode (HMDE).

This paper presents the optimization of instrumental and solution parameters for determination of atrazine in river waters and formulation by square wave voltammetry (SWV) using a hanging mercury drop electrode. The best sensitivity (35.2+/-0.4muAmlmug(-1)) was achieved using a frequency of 400Hz and a medium composed of 40mmoll(-1) Britton-Robinson (BR) buffer at pH 1.9. The detection limit was 2mugl(-1) with a linear dynamic range between 10 and 250mugl(-1). Application of the method to real samples of river waters fortified with 10mugl(-1) of atrazine resulted recoveries between 92 and 116%. Additionally, good agreement was observed between results obtained by the proposed method and by HPLC for river water samples spiked with 25mugl(-1) of atrazine. The determination was not affected by the presence of humic acid at concentration of 5mgl(-1), indicating that interactions of the herbicide with this class of compounds are fully labile. The stability of the voltammetric signal for samples spiked with 250mugl(-1) atrazine was evaluated over a period of 14 days in four samples. For two samples, no systematic variation was observed, while for the other two, a decrease of peak current between 3 and 15% occurred, suggesting that the stability is dependent on the sample nature. HPLC analyses suggest formation of deethylatrazine during the second week of storage in the samples for which the SWV peak current had the more intense decrease.

Influence of pH and ionic strength on removal processes of a sedimentary humic acid in a suspension of vermiculite

Understanding the interactions of humic acids and clay minerals is important for modeling the geochemical fate and transport of nutrients and pollutants in soil and natural waters upon changes of environmental parameters such as ionic strength and pH. This paper describes the adsorption/precipitation properties of a sedimentary humic acid under different conditions of pH and ionic strength in presence of the clay mineral vermiculite. The clay suspension was prepared to have a distribution of grain size composed mainly of particles <63xa0μm, which is believed to play a major role on transport and fate of adsorbed materials in aquatic environments. The pHs studied were 5.0, 6.0 and 7.0 (±0.1 pH units), in ionic medium of 0.002 and 0.020xa0molxa0l−1 KNO3, using initial HA concentrations from 0 to 100xa0mgxa0l−1. The maximum removal of HA from solution occurred at pH 5.0 in medium of 0.020xa0molxa0l−1 KNO3. This removal is mostly due to precipitation, which occurred at similar rates in both ionic strength, so that the additional HA removal at higher ionic strength (0.020xa0molxa0l−1 KNO3) was attributed to adsorption. Adsorption processes intensified at higher ionic strength suggest the occurrence of chemical interactions between the interfaces since under these conditions the electrostatic interactions are minimized. A similar effect was observed at pH 6.0, but in this case adsorption prevailed as the removal mechanism in medium of 0.020xa0molxa0l−1 KNO3. At pH 7, adsorption is the predominant process removing HA from solution, with increase of adsorption upon decreasing the ionic strength, suggesting that electrostatic attractions play the major role in the adsorption mechanism under these conditions. Comparison of the results obtained using a re-precipitated commercial HA with low N content suggested that the presence of positively charged amine groups increase the adsorption and decrease desorption.

Complexation of Cd(II) and Pb(II) with humic acids studied by anodic stripping voltammetry using differential equilibrium functions and discrete site models

Abstract Binding of Cd(II) and Pb(II) to two humic acids was studied by differential pulse anodic stripping voltammetry using continuous differential equilibrium functions and linear Scatchard plots. One of the humic acids was a commercial sample from Aldrich (AHA) and the other was a sedimentary humic acid isolated from a Brazilian water reservoir (BBHA). The complexation study was performed at pH between 5 and 6 in 0.020 mol l −1 KNO 3 . The complexes were considered fully labile in the range of the metal to ligand concentration ratios studied, having diffusion coefficients ( D ML =1.3×10 −7 and 2.7×10 −7 cm 2 s −1 , for BBHA and AHA, respectively) smaller than the free cation [ D M =8×10 −6 and 7×10 −6 cm 2 s −1 for Cd(II) and Pb(II), respectively]. Differential equilibrium functions and the discrete site model led to similar interpretations of the experimental data. Binding of Pb(II) to both HA samples was stronger than for Cd(II), with a greater heterogeneity of binding sites. This was verified by the heterogeneity parameter, Γ, obtained from the differential equilibrium functions, as well as from the number of classes of binding sites used to fit the titration data to Scatchard plots.

Spectrophotometric determination of acid volatile sulfide in river sediments by sequential injection analysis exploiting the methylene blue reaction.

This paper demonstrates the application of sequential injection analysis to perform sulfide determination using the methylene blue chemistry, based on two reagents: 3.63 mmol l(-1)N,N dimethyl-p-phenylene diamine hydrochloride in 1.1 mol l(-1) HCl solution and 19 mmol l(-1) FeCl(3), also in 1.1 mol l(-1) HCl. These solutions are aspirated inside the holding coil of the sequential injection system as two reagent zones sandwiching the sample zone. Under optimized conditions, the detection limit was calculated at 40 mug l(-1) S(2-), with a linear dynamic range from 0.05 to 2 mg l(-1) S(2-). This linear range can be extended up to 32 mg l(-1) using in-line dilution for sulfide concentrations greater than 2 mg l(-1). The robust characteristic of the SI system with syringe pump leads to very stable analytical curves (precision of 4%), minimizing the laborious preparation of sulfide standards. The method was applied in the determination of acid volatile sulfide in river sediments.

Sorption of atrazine, propazine, deethylatrazine, deisopropylatrazine and hydroxyatrazine onto organovermiculite

The interaction of atrazine (AT), propazine (PROP), deethylatrazine (DEA), deisopropylatrazine (DIA) and hydroxyatrazine (HAT) was studied with the clay mineral vermiculite saturated with K+ (VTK), and with the mineral modified by insertion of hexadecyltrimethyl-ammonium (HDTMA-VT) in the interlayer of VTK. The crude VTK exhibited negligible interaction with AT, PROP and DEA, moderate interaction with DIA and strong interaction with HAT. For the HDTMA-VT materials, removal percentages from aqueous medium for initial concentrations between 0.05 and 1.00 mg L-1 were 56 to 63% for AT, 43 to 45% for DEA, 12 to 19% for HAT and 77 to 78% for PROP. Sorption of AT, DEA and PROP onto HDTMA-VT was significantly enhanced in comparison to VTK. Ionic strength and pH had no significant influence on the sorption process. Desorption of all compounds from HDTMA-VT after 24 h of contact time was between 20 and 30% of the amount initially adsorbed.

Construction and evaluation of a flow-through cell adapted to a commercial static mercury drop electrode (SMDE) to study the adsorption of Cd(II) and Pb(II) on vermiculite

This paper describes the construction and application of a robust flow-through cell for use with the capillary of a commercial static mercury drop electrode. Linearity of peak current was observed up to 0.50 mumol l(-1) for Cd(II) or Pb(II) in anodic stripping voltammetry experiments performed under continuous flow during the deposition step, using 120 s of deposition time and flow rate of 4.0 ml min(-1). Under these conditions the limits of detection for Cd(II) and Pb(II) were 13 and 17 nmol l(-1), respectively. An analytical throughput of 20 analyses per h was possible using 10 s for cleaning the cell between two samples and including the time needed for the potential scan, which was performed with the flow stopped, using the differential pulse mode for current sampling. The linear dynamic range can be extended up to 5 mumol l(-1) for both cations if the deposition time is decreased to 30 s, a condition in which the sampling throughput is 35 analyses per h. The proposed manifold was used to study the adsorption rates of Cd(II) and Pb(II) onto vermiculite at different pHs, allowing one to perform high sensitivity measurements at high sampling frequency, using low cost instrumentation.

Acid-basic and complexation properties of a sedimentary humic acid. A study on the Barra Bonita reservoir of Tietê river, São Paulo State, Brazil

Acid-base and complexation properties of humic acid (HA) isolated from a river sediment were studied by potentiometric titration, adopting the discrete site distribution model and the modified Gran functions for data fitting. Six classes of titratable groups were characterized, with pKa values between 2.4 and 10.2. Carboxylic groups accounted for 66% of the total of ionizable sites. The complexing properties were studied with regard to Cu2+, Pb2+, Cd2+ and Zn2+ ions by potentiometric titration using Cu ion selective electrode, or amalgam electrodes (Pb, Cd and Zn). The data treatment by the Scatchard method revealed two binding sites for copper and lead and one binding site for cadmium and zinc. The average stability constants were in the following order: log KHA-Cu > log KHA-Pb > log KHA-Cd @ log KHA-Zn, while the complexing capacity order, Cc, was: Pb > Cu > Cd @ Zn.

Utilização de eletrodos potenciométricos de amálgama em estudos de complexação de substâncias húmicas

Potentiometric amalgam electrodes of lead, cadmium, and zinc are proposed to study the complexation properties of commercial and river sediment humic acids. The copper complexation properties of both humic acids were studied in parallel using the solid membrane copper ion-selective electrode (Cu-ISE). The complexing capacity and the averaged conditional stability constants were determined at pH 6.00 ± 0.05 in medium of 2x10-2 mol L-1 sodium nitrate, using the Scatchard method. The lead and cadmium amalgam electrodes presented a Nernstian behavior from 1x10-5 to 1x10-3 moles L-1 of total metal concentration, permitting to perform the complexation studies using humic acid concentrations around of 20 to 30 mg L-1, that avoids colloidal aggregation. The zinc amalgam electrode showed a subnernstian linear response in the same range of metal concentrations. The Scatchard graphs for both humic acids suggested two classes of binding sites for lead and copper and one class of binding site for zinc and cadmium.