Removal of As(III) from Water Using the Adsorptive and Photocatalytic Properties of Humic Acid-Coated Magnetite Nanoparticles

Abstract

The oxidation of highly toxic arsenite (As(III)) was studied using humic acid-coated magnetite nanoparticles (HA-MNP) as a photosensitizer. Detailed characterization of the HA-MNP was carried out before and after the photoinduced treatment of As(III) species. Upon irradiation of HA-MNP with 350 nm light, a portion of the As(III) species was oxidized to arsenate (As(V)) and was nearly quantitatively removed from the aqueous solution. The separation of As(III) from the aqueous solution is primarily driven by the strong adsorption of As(III) onto the HA-MNP. As(III) removals of 40–90% were achieved within 60 min depending on the amount of HA-MNP. The generation of reactive oxygen species (•OH and 1O2) and the triplet excited state of HA-MNP (3HA-MNP*) was monitored and quantified during HA-MNP photolysis. The results indicate 3HA-MNP* and/or singlet oxygen (1O2) depending on the reaction conditions are responsible for converting As(III) to less toxic As(V). The formation of 3HA-MNP* was quantified using the electron transfer probe 2,4,6-trimethylphenol (TMP). The formation rate of 3HA-MNP* was 8.0 ± 0.6 × 10−9 M s−1 at the TMP concentration of 50 µM and HA-MNP concentration of 1.0 g L−1. The easy preparation, capacity for triplet excited state and singlet oxygen production, and magnetic separation suggest HA-MNP has potential to be a photosensitizer for the remediation of arsenic (As) and other pollutants susceptible to advanced oxidation.

Contribution of H-Abstraction to Photocatalytic Oxidation of As(III) by Bi2.15WO6

“…•− produced from the conduction band (CB) of Pt−TiO 2 was responsible for As(III) oxidation at pH 3.0 under visible light. Pham et al 12 suggested that 1 O 2 was also responsible for the oxidation of As(III) by the humic acid-modified magnetite nanoparticles at pH 6.0 under UV irradiation. Apart from aqueous ROS, the oxidizing species formed on the surface of photocatalysts also contribute to As(III) oxidation.…”

Nano-Photocatalytic Materials: Possibilities and Challenges

“…Photocatalysis is a technique that, on many occasions, needs to be combined with other physical processes to improve the pollutant removal efficiency. In this sense, P. Pham et al [ 12 ] synthesized materials with adsorptive and photochemical properties. These materials are based on magnetite (MNP) covered by humic acid substances (HA).…”

Uso de nanomateriales magnéticos y destilación por membrana para el tratamiento de iones fluoruro y arsénico en solución

“…La toxicidad de las especies de arsénico depende del estado de valencia, la especiación, la disponibilidad y las especies animales o tipos de células expuestas. La especie de arsénico trivalente es considerablemente más tóxica que la especie pentavalente, debido a su mayor capacidad de ligarse a tioles de biomoléculas, como ácido lipoico, residuos de cisteína de enzimas o proteínas 133 . La biodisponibilidad del As está determinada principalmente por la especiación, por lo tanto, la oxidación de As (III) a As (V) es necesaria para la remediación eficaz del arsénico 28 .…”

“…En un trabajo reciente Pham et al 133 estudiaron la capacidad de nanopartículas de magnetita recubiertas de ácidos húmicos para actuar como materiales adsorbentes y fotocatalizadores de la oxidación de As (III). Estos autores realizaron ensayos de fotocatálisis en presencia de sorbato de potasio, un desactivador de estados triplete mediante procesos de transferencia de energía.…”

“…Adicionalmente, Pham et al 133 propusieron mediante ensayos indirectos para la oxidación de As (III) (fotocatalizada por nanopartículas de magnetita recubiertas de ácidos húmicos) la mediación del oxígeno singlete molecular, generado a partir de los estados triplete de la materia orgánica. Esta especie reactiva podría también estar involucrada en nuestros casos.…”