Role of Fe(III) in preventing humic interference during As(III) detection on gold electrode: spectroscopic and voltammetric evidence

Voltammetric determination of inorganic arsenic in mildly acidified (pH 4.7) groundwaters from Mexico and India

Routine monitoring of inorganic arsenic in groundwater using sensitive, reliable, easy-to-use and affordable analytical methods is integral to identifying sources, and delivering appropriate remediation solutions, to the widespread global issue of arsenic pollution. Voltammetry has many advantages over other analytical techniques, but the low electroactivity of arsenic(V) requires the use of either reducing agents or relatively strong acidic conditions, which both complicate the analytical procedures, and require more complex material handling by skilled operators. Here, we present the voltammetric determination of total inorganic arsenic in conditions of near-neutral pH using a new commercially available 25 μm diameter gold microwire (called the Gold Wirebond), which is described here for the first time. The method is based on the addition of low concentrations of permanganate (10 μM MnO4-) which fulfils two roles: (1) to ensure that all inorganic arsenic is present as arsenate by chemically oxidising arsenite to arsenate and, (2) to provide a source of manganese allowing the sensitive detection of arsenate by anodic stripping voltammetry at a gold electrode. Tests were carried out in synthetic solutions of various pH (ranging from 4.7 to 9) in presence/absence of chloride. The best response was obtained in 0.25 M chloride-containing acetate buffer resulting in analytical parameters (limit of detection of 0.28 μg L-1 for 10 s deposition time, linear range up to 20 μg L-1 and a sensitivity of 63.5 nA ppb-1. s-1) better than those obtained in acidic conditions. We used this new method to measure arsenic concentrations in contrasting groundwaters: the reducing, arsenite-rich groundwaters of India (West Bengal and Bihar regions) and the oxidising, arsenate-rich groundwaters of Mexico (Guanajuato region). Very good agreement was obtained in all groundwaters with arsenic concentrations measured by inductively coupled plasma-mass spectrometry (slope = +1.029, R2 = 0.99). The voltammetric method is sensitive, faster than other voltammetric techniques for detection of arsenic (typically 10 min per sample including triplicate measurements and 2 standard additions), easier to implement than previous methods (no acidic conditions, no chemical reduction required, reproducible sensor, can be used by non-voltammetric experts) and could enable cheaper groundwater surveying campaigns with in-the-field analysis for quick data reporting, even in remote communities.

Arsenic speciation, oxidation and immobilization in an unsaturated soil in the presence of green synthesized iron oxide nanoparticles and humic acid

While the availability of arsenic (As) in soil is well known to be highly correlated with the presence of iron (Fe) oxides and humic acid (HA) in the soil, the relationship between Fe oxides and HA and As species in the soil is less well understood. In this study, As speciation in an unsaturated soil in the presence of external HA and green synthesized Fe oxide nanoparticles (FeNPs) showed that As(V) was mainly distributed to the specifically-bound (F2), amorphous and poorly-crystalline hydrous oxides of Fe, Al (F3) and the well-crystallized hydrous oxides of Fe and Al (F4). While As(III). This was the major component in unsaturated soil, and was mainly distributed to F4 and the residual fraction (F5). As bound to F3 and F5 was most sensitive to the addition of HA and FeNPs, while HA/FeNPs treatment increased the F3-bound As(V); however, it decreased the F5-bound As(III). Nonetheless the effect of HA on As is completely different to the HA/FeNPs treatment. The increase of As(V) in F3 resulted from F5-bound As(III) oxidation when treated by HA/FeNPs. Cyclic voltammetry confirmed that HA and Fe3+/Fe2+ redox enhanced As(III) oxidation, while FTIR revealed that HA-bound As(III) was the least available fraction in the soil. Finally, a mechanism involving a combination of HA and FeNPs was proposed for explaining the redistribution of As species in the soil.

Calix[4]arene Derivative-Modified Glassy Carbon Electrode: A New Sensing Platform for Rapid, Simultaneous, and Picomolar Detection of Zn(II), Pb(II), As(III), and Hg(II)

The glassy carbon electrode was fabricated with multifunctional bis-triazole-appended calix[4]arene and then used for the simultaneous detection of Zn(II), Pb(II), As(III), and Hg(II). Before applying the square-wave anodic stripping voltammetry, the sensitivity and precision of the modified electrode was assured by optimizing various conditions such as the modifier concentration, pH of the solution, deposition potential, accumulation time, and supporting electrolytes. The modified glassy carbon electrode was found to be responsive up to picomolar limits for the aforementioned heavy metal ions, which is a concentration limit much lower than the threshold level permitted by the World Health Organization. Importantly, the designed sensing platform showed anti-interference ability, good stability, repeatability, reproducibility, and applicability for the detection of multiple metal ions. The detection limits obtained for Zn(II), Pb(II), As(III), and Hg(II) are 66.3, 14.6, 71.9, and 28.9 pM, respectively.

Disposal of Fenton sludge with anaerobic digestion and the roles of humic acids involved in Fenton sludge

Fenton sludge (FS) generated from Fenton process with high contents of iron and organic contaminants is regarded as a hazardous waste that requires to be properly disposed. Considering that Fe(III) compounds could stimulate dissimilatory iron reduction (DIR) and enrich iron reducing bacteria (IRB) that utilized Fe(III) as electron acceptor to oxidize organic matters, FS was introduced in anaerobic digestion (AD) reactors for treating wastewater meanwhile disposing FS. Results showed that methane production and organic matters removal significantly increased with dosing FS from 0 to 2.4 g. Also, a majority of organic matters involved in FS was mineralized, including 38.5% of PAHs removal. Humic acids (HA) with redox-activity involved in FS might affect efficiency of DIR. After extracting HA from FS, the rate and the extent of Fe(III) reduction of FS decreased by 33.2% and 13.9%, respectively. Together with analysis of the electron exchange capacity of HA, it suggested that the HA involved in FS might serve as an electron shuttle to effectively promote DIR. The increase of sludge conductivity and the enrichment of IRBs in microbial communities with dosage of FS were in agreement with the above results.

Highly Sensitive and Selective Detection of Arsenic Using Electrogenerated Nanotextured Gold Assemblage

Arsenic is considered as a toxic heavy metal which is highly detrimental to ecological systems, and long-term exposure to it is highly dangerous to life as it can cause serious health effects. Timely detection of traces of active arsenic (As³⁺) is very crucial, and the development of simple, cost-effective methods is imperative to address the presence of arsenic in water and food chain. Herein, we present an extensive study on chemical-free electrogenerated nanotextured gold assemblage for the detection of ultralow levels of As³⁺ in water up to 0.08 ppb concentration. The gold nanotextured electrode (Au/GNE) is developed on simple Au foil via electrochemical oxidation-reduction sweeps in a metal-ion-free electrolyte solution. The ultrafine nanoscale morphological attributes of Au/GNE substrate are studied by scanning electron microscopy. Square wave anodic stripping voltammetry (ASV) response for different concentrations of arsenites is determined and directly correlated with As³⁺ detection regarding the type of electrolyte solution, deposition potential, and deposition time. The average of three standard curves are linear from 0.1 ppb up to 9 ppb (n = 15) with a linear regression coefficient R ² = 0.9932. Under optimized conditions, a superior sensitivity of 39.54 μA ppb^-1 cm^-2 is observed with a lower detection limit of 0.1 ppb (1.3 nM) (based on the visual analysis of calibration curve) and 0.08 ppb (1.06 nM) (based on the standard deviation of linear regression). Furthermore, the electrochemical Au/GNE is also applicable for arsenic detection in a complex system containing Cu²⁺, Ni²⁺, Fe²⁺, Pb²⁺, Hg²⁺, and other ions for the selective and sensitive analysis. Au/GNE substrate also possesses remarkable reproducibility and high stability for arsenic detection during repeated analysis and thus can be employed for prolonged applications and reiterating analyses. This electrochemically generated nanotextured electrode is also applicable for As³⁺ detection and analysis in a real water sample under optimized conditions. Therefore, fabrication conditions and analytical and electroanalytical performances justify that because of its low cost, easy preparation method and assembly, high reproducibility, and robustness, nanosensor Au/GNE can be scaled up for further applications.

An adsorbent based on humic acid and carboxymethyl cellulose for efficient dye removal from aqueous solution

Adsorbents from sustainable precursors are highly desirable for practical applications. In this study, carboxymethyl cellulose (CMC) and humic acid (HA) were adopted to fabricate a composite immobilized on precipitates of calcium hydroxide (PCH) and ferric hydroxide (PFH). The as-obtained adsorbent, denoted as HA-CMC/PMH (PCH and PFH were referred to as PMH), was analyzed by infrared spectroscopy, scanning electron microscopy, X-ray photoelectron spectroscopy and thermo gravimetric analysis. When applying for a typical organic pollutant of methylene blue (MB) removal from aqueous solution, a high adsorption capacity of 666.67 mg/g was achieved over HA-CMC/PMH. The adsorption mechanisms of MB involved ion exchanging and π-π interactions, this adsorption process mainly occurred on both anionic and aromatic groups. Adsorption behavior analysis indicated that Langmuir isotherm model and pseudo-second order kinetic model fitted experimental data well, suggesting a mono-layer adsorption of MB onto the adsorbent. Further thermodynamic analysis proved that the adsorption of MB onto HA-CMC/PMH was an exothermal spontaneous process. The developed adsorbent is also reusable with 79.93% of adsorption capacity remaining in the fifth recycle runs. Therefore, the adsorbent of HA-CMC/PMH is suggested to be a promising candidate for adsorption applications.

Direct Quantification of Cd2+ in the Presence of Cu2+ by a Combination of Anodic Stripping Voltammetry Using a Bi-Film-Modified Glassy Carbon Electrode and an Artificial Neural Network

Abstract: In this study, a novel method based on a Bi/glassy carbon electrode (Bi/GCE) for quantitatively and directly detecting Cd²⁺ in the presence of Cu²⁺ without further electrode modifications by combining square-wave anodic stripping voltammetry (SWASV) and a back-propagation artificial neural network (BP-ANN) has been proposed. The influence of the Cu²⁺ concentration on the stripping response to Cd²⁺ was studied. In addition, the effect of the ferrocyanide concentration on the SWASV detection of Cd²⁺ in the presence of Cu²⁺ was investigated. A BP-ANN with two inputs and one output was used to establish the nonlinear relationship between the concentration of Cd²⁺ and the stripping peak currents of Cu²⁺ and Cd²⁺. The factors affecting the SWASV detection of Cd²⁺ and the key parameters of the BP-ANN were optimized. Moreover, the direct calibration model (i.e., adding 0.1 mM ferrocyanide before detection), the BP-ANN model and other prediction models were compared to verify the prediction performance of these models in terms of their mean absolute errors (MAEs), root mean square errors (RMSEs) and correlation coefficients. The BP-ANN model exhibited higher prediction accuracy than the direct calibration model and the other prediction models. Finally, the proposed method was used to detect Cd²⁺ in soil samples with satisfactory results.

Shape dependent stripping behavior of Au nanoparticles toward arsenic detection: evidence of enhanced sensitivity on the Au (111) facet

This work reports a comparative study of gold cubes {100}, octahedra {111}, and rhombic dodecahedra {110} toward the detection of arsenic for the first time. Au octahedral nanoparticles were found to exhibit the highest sensitivity.