Redox Conversion of Chromium(VI) and Arsenic(III) with the Intermediates of Chromium(V) and Arsenic(IV) via AuPd/CNTs Electrocatalysis in Acid Aqueous Solution

Abstract

Simultaneous reduction of Cr(VI) to Cr(III) and oxidation of As(III) to As(V) is a promising pretreatment process for the removal of chromium and arsenic from acid aqueous solution. In this work, the synergistic redox conversion of Cr(VI) and As(III) was efficiently achieved in a three-dimensional electrocatalytic reactor with synthesized AuPd/ CNTs particles as electrocatalysts. The AuPd/CNTs facilitated the exposure of active Pd{111} facets and possessed an approximate two-electrontransfer pathway of oxygen reduction with the highly efficient formation of H 2 O 2 as end product, resulting in the electrocatalytic reduction of 97.2 ± 2.4% of Cr(VI) and oxidation of 95.7 ± 4% of As(III). The electrocatalytic reduction of Cr(VI) was significantly accelerated prior to the electrocatalytic oxidation of As(III), and the effectiveness of Cr(VI)/As(III) conversion was favored at increased currents from 20 to 150 mA, decreased initial pH from 7 to 1 and concentrations of Cr(VI) and As(III) ranging from 50 to 1 mg/L. The crucial intermediates of Cr(V) and As(IV) and active free radicals HO• and O 2 •− were found for the first time, whose roles in the control of Cr(VI)/As(III) redox conversion were proposed. Finally, the potential applicability of AuPd/CNTs was revealed by their stability in electrocatalytic conversion over 10 cycles. ■ INTRODUCTIONThe combined pollution of heavy metals in acid wastewater has become a formidable problem to environmental scientists and engineers for decades. As a consequence of improper treatment, effluents containing abundant heavy metal ions such as lead, nickel, copper, arsenic, or chromium may inflow into surface water or seep into groundwater, which pose great threats to aquatic life and drinking water sources.1,2 Thus, it is urgent to remove the heavy metals prior to their discharge into the environment.Arsenic and chromium, as priority pollutants, are highly toxic and frequently coexist with highly acidic wastewater (e.g., acid mine drainage).3 The valence states of arsenic or chromium have decisive impacts on their environmental fate and toxic effect.4,5 For example, As(III) is the most prevalent form of arsenic in the natural environment, which is more toxic than As(V) to mammals. 6 Additionally, As(III) is also more mobile than As(V), because As(III) is quite easier originated from the oxidative weathering or reductive dissolution of As-containing minerals, suggesting the immobilization of As(III) in acid solution more strictly depends on the redox conditions. 7 Therefore, it is advisible to preoxidize As(III) to As(V), coupled with a following adsorption or coprecipitation of the As(V) by using adsorbents or coagulants, to finally enhance the removal of arsenic species in acid aqueous solution. 8,9 Similarly, Cr(VI) is quite soluble and mobile in acid wastewater, 10 whileCr(III) is less toxic and insoluble, and can be effectively immobilized as the hydroxide in a wide pH range. 11,12 Thus, the prereduction of Cr(VI) to Cr(III) is conducive to the complete removal of ch...

Visible-light photocatalysis accelerates As(III) release and oxidation from arsenic-containing sludge

“…The pathway of As(III) being oxidized into As(V) is proposed as follows (eqs 10-13): [37,38] As(III) + •OH → As(IV) + OH¯ k = 9×10 9 M -1 s -1 (10) As(III) + •O2¯ + H + → As(IV) + HO2¯ k = 3.6×10 6 M -1 s -1 (11) As(IV) → As(III) + As(…”

Synthesis of highly fluorescent Cu/Au bimetallic nanoclusters and their application in a temperature sensor and fluorescent probe for chromium(iii) ions

“…26,27 As Cr 3+ might be the ultimate reason for toxicity, it is of great signicance to explore rapid, simple and selective strategies for the highly sensitive detect it. To date, many methods have been developed for the quantitative determination of chromium, such as electrochemical sensing methods, 28,29 colorimetric assays, 30,31 inductively coupled plasma-mass spectrometry, [32][33][34] uorescence detection methods, [35][36][37][38] organic molecule probes detection methods. 39,40 Among these methods, the uorescence method is highly focused on because of its ideal sensing performance such as simplicity, short response time and lower cost.…”

Electrochemical oxidation of As(iii) on Pd immobilized Pt surface: kinetics and sensing performance

“…[18][19][20][21] Its noteworthy that the principle of the oxidation process is based on the conversion of As(III) into a stable product As(V) by releasing two electrons. [22][23][24] However, since As(III) species are more soluble and mobile than As(V) species, development of electrode materials for the conversion of As(III) to As(V) regarding detection, determination, and removal is still a challenge. To date, several articles have been published on the electrochemical and electrokinetic studies of As(III) electro-oxidation using Pt, Au, Hg, indium tin oxide (ITO) coated glass and carbon-based materials.…”

“…To date, several articles have been published on the electrochemical and electrokinetic studies of As(III) electro-oxidation using Pt, Au, Hg, indium tin oxide (ITO) coated glass and carbon-based materials. 16,17,[22][23][24][25][26][27][28][29][30][31] In this context, to the best of our knowledge, very few research groups have worked on the applicability of palladium as an electrocatalyst for As(III) electro-oxidation. 22 Moreover, contemporary scientists and technologists are paying much attention to minimizing the usage of precious metals as catalysts by developing porosity in the catalytic matrix since this approach can improve the active catalytic area.…”