Fernando Henrique do Nascimento

Complexation of Hg(II) by humic acid studied by square wave stripping voltammetry at screen-printed gold electrodes

This paper reports the development of a sequential injection (SI) method to study the complexation of Hg(II) by humic acid (HA) using square wave anodic stripping voltammetry at a screen-printed gold electrode (SPGE). The SI system injected samples (in 0.020 mol L(-1) NaNO(3) and pH 6.0) to the flow cell during the deposition step and exchanged the medium to 0.050 mol L(-1) HCl to perform the stripping step under stopped flow conditions. For sample volume=750 μL, flow rate=15 μL s(-1) (deposition step), and square wave frequency=100 Hz, the detection limit was 20 nmol L(-1) and the sampling throughput was 17 analyses per hour. Complexation of Hg(II) was studied using a 25.0 mg L(-1) suspension of HA (72.5 μmol L(-1) of ionizable sites), and total Hg(II) concentrations from 1 to 20 μmol L(-1). The diffusion coefficient of the complex was (1.3±0.2)×10(-7) cm(2) s(-1). A complexing capacity of 537 μmol g(-1) was found. The log of the differential stability constant (logK(DEF)) decreased from 7.0 to 5.3 as the log of the degree of site occupation (logθ) increased from -1.6 to -0.5. The values of logK(DEF) were consistent with complexation of Hg(II) by carboxylic and phenolic groups.

Sequential Injection Assisted Stripping Chronopotentiometry at Screen Printed Gold Electrodes for Determination of Hg(II) in Adsorption Studies

ABSTRACT The quest for low cost adsorbents to remove highly toxic mercury(II) from aqueous solutions is currently a worldwide research area. Evaluation of kinetic and thermodynamic parameters related to adsorption requires serial analyses, usually made by cold vapor atomic absorption spectrometry, a sensitive but expensive technique. In the present paper, stripping chronopotentiometry with sequential injection analysis was developed to investigate the adsorption of mercury(II) on sodium homoionic vermiculite. Mercury was deposited at 0.3 V vs. a Ag/AgCl pseudo-reference electrode under flow conditions (30 µL s−1) and reoxidized by a current of 0.10 µA after the flow was stopped. The limits of detection and quantification were 14 and 47 nmol L−1, respectively. The linear dynamic range extended to 1.0 µmol L−1, and the relative standard deviations of the blank, 0.10, 0.20, and 0.30 µmol L−1 solutions (n=3) were 4.6, 1.1, 0.9, and 1.1%, respectively. The accuracy was evaluated by analyzing a certified estuarine sediment (ERM-CC580), recovering 103 ± 1% of the certified mercury(II) concentration (132 ± 3 mg kg−1). Adsorption of mercury(II) on vermiculite followed pseudo second order kinetics. The adsorption isotherms (25.0 ± 0°C) were properly fitted by the Langmuir equation resulting in an adsorption capacity 21 ± 2 µmol g−1 and an adsorption constant of 0.28 ± 0.05 L g−1.

POR QUE MARCAS CORPORATIVAS? A PERCEPÇÃO DE EXECUTIVOS BRASILEIROS SOBRE OS MOTIVOS PARA ADOTAR CORPORATE BRANDING

O conceito de Marca Corporativa apresenta ampla aplicacao e escopo da gestao, gerando diversos motivos que levam a opcao por esta estrategia. O objetivo do estudo e entender quais sao os motivos percebidos como relevantes para adotar a marca corporativa, bem como quais as abordagens mais proeminentes entre executivos brasileiros. Este estudo, de natureza descritiva e quantitativa, envolveu o mapeamento dos principais motivos abordados na literatura e uma survey com 275 gestores de empresas de segmentos e portes variados. Os resultados indicam a predominância da visao da marca corporativa como meio de comunicar os valores, seguida da escola voltada para a construcao da imagem positiva. Entre os motivos mais relevantes, o fortalecimento das marcas dos produtos e o foco nos consumidores chama mais atencao, seguido de fortalecimento de identidade em multinacionais. O estudo contribui com o mapeamento abrangente dos motivos para esta estrategia e a visao sobre quais as perspectivas predominantes no mercado brasileiro.

Adsorption/Desorption of Hg(II) on FDU-1 Silica and FDU-1 Silica Modified with Humic Acid

Incorporation of humic acid in the FDU-1 mesoporous silica improved Hg(II) adsorption and did not destroy the ordered network, providing surface area of 500 m2 g−1, pore volume of 0.9 cm3 g−1, and mean pore diameter of 10 nm. Carboxylic and phenolic groups increased the affinity by forming surface complexes with Hg(II). Isotherms (25.0 ± 0.1°C) were fitted to the Freundlich equation, exhibiting Kf values that increased with pH and 1/n that decreased to values < 1 with the heterogeneity of sites. Hg(II) desorption in 0.10 mol L−1 acetic acid was lower than 10%, suggesting that chemisorption processes govern the Hg(II) removal.

An electrochemical sequential injection method to investigate the adsorption of selenite on Fe(III) polyhydroxy cations intercalated vermiculite

A sequential injection - square wave anodic stripping voltammetry (SI-SWASV) method for determination of Se(IV) at a gold working electrode was developed to investigate the adsorption of Se(IV) onto vermiculite intercalated with Fe(III) polyhydroxy cations. The limits of detection and quantification were 0.060 and 0.20 μmol L-1, respectively (4.7 and 15.7 μg L-1). The linearity was up to 1.0 μmol L-1, and the sampling throughput was 18 analyses h-1. The proposed approach is a low-cost alternative to more expensive spectrometric methods. Adsorption onto vermiculite intercalated with Fe(III) polyhydroxy cations removed 93% of Se(IV) from a 1.0 μmol L-1 solution (250 mL) after 5 min of contact time with 625 mg of adsorbent. Adsorption isotherms (25.0 ± 0.5 °C) were fitted by the Freundlich equation resulting in 1/n = 0.51 ± 0.03 and Kf = (1.584 ± 0.002) × 103 μmol1-1/n g-1 L1/n (r2 = 0.995). Fitting by the Langmuir equation resulted in an adsorption constant of 0.026 ± 0.008 L g-1 and adsorption capacity of 47 ± 5 μmol g-1 (3.7 ± 0.4 mg g-1) (r2 = 0.97). This capacity was higher than that found for several other iron oxides, but lower than that obtained for oxide/hydroxide-based Fe(III) nanoparticles.