Quantification and the sources identification of total and insoluble hexavalent chromium in ambient PM: A case study of Aktobe, Kazakhstan

A colorimetric and electrochemical dual-mode system for identifying and detecting varied Cr species based on fungus-like porous CoS nanosensor

The differentiation of valence states plays a crucial role in determining the toxicity of chromium (Cr) in environmental samples. In this work, two modes of colorimetric and electrochemical analytical methods based on a fungus like porous CoS (FP CoS) nanosensor were developed for rapid, specific, and portable detection trace/ultra-trace chromium species (Cr(VI) and Cr(III)). The FP CoS exhibited peroxidase activity as a nanozyme for the colorimetric detection of Cr(VI), catalyzing the oxidation of 3,3',5,5'-tetramethylbenzidine (TMB) to a blue oxidation product (oxTMB) in the presence of Cr(VI) instead of unstable H2O2 as an oxidizer at room temperature over existing methods. Based on the promotion of colorimetric reaction by increasing in Cr(VI) concentration, an effective colorimetric detection of Cr(VI) method was established with a detection limit (LOD) low to 3.93 μg L-1 and unique selectivity for Cr(VI) over 21 interfering ions (containing 15 metal ions and 6 anions). Innovatively, Cr(VI) could be reduced to Cr(III) without TMB, then selectively enriched by redox reaction with FP CoS. Hence, Cr (III) can be selectively and effectively enriched by FP CoS applying voltage, and then detected using cyclic voltammetry, with a lower LOD of 0.116 μg L-1 and high sensitivity ignoring background interferences. By integrating the dual-mode detection channel, the FP CoS nanosensor offers a convenient and flexible method for simultaneously determining Cr(VI), Cr(III), and total chromium in diverse samples.

Assessment of pollution and health risks from exposure to heavy metals in soil, wheat grains, drinking water, and atmospheric particulate matter

Heavy metal pollution is a global environmental issue that poses serious threats to human health. In the Bian East Industrial Cluster Zone of Kaifeng City, prolonged sewage irrigation and industrial activities have led to severe heavy metal contamination in various environmental media and crops. However, the health risks posed by heavy metals in different environmental media in this area are yet to be assessed. Thus, we measured the concentrations of multiple heavy metals in samples from different environmental media (soil, drinking water, and atmospheric particulate matter) and crops (wheat grains). A health risk assessment model was used to evaluate the health risks posed by heavy metals through different exposure pathways and estimate the contribution of each medium and heavy metal to multi-media comprehensive health risks. The results indicated that in most sample sites within the study area, the soil was contaminated with Cu, Zn, Cd, Pb, As, and Hg, whereas atmospheric particulate matter contained elevated levels of Cr and Cd. However, the heavy metal contents in the wheat grains and drinking water did not exceed national standards. Health risk assessment revealed high levels of non-carcinogenic and carcinogenic risks (multi-media total hazard index, MTHI >1; multi-media total carcinogenic risk, MTCR >10⁻⁴) in the study area. The primary sources of these risks were atmospheric particulate matter and wheat grains. Among the heavy metals, Cr, Cu, and Zn mainly posed non-carcinogenic risks, while Cr and Cd primarily posed carcinogenic risks. These findings highlight the need for focused attention in controlling heavy metal pollution in atmospheric particulate matter and wheat grains. This study provides a theoretical basis for managing and preventing heavy metal pollution in various environmental media to ensure food production safety and protect public health in the study area.

Heterogeneous (gas-solid) chemistry of atmospheric Cr: A case study of Astana, Kazakhstan

Hexavalent chromium (Cr(VI)) is a known carcinogen derived from both anthropogenic and natural sources. This work reports the size-segregated concentrations of total Cr(VI) in particulate matter (PM) in Astana, the capital of Kazakhstan, and provides new insights into the gas-solid reactions of atmospheric Cr. A study of total Cr(VI) in the particulate matter, via a microwave-assisted digestion technique, was conducted using a 5-stage Sioutas Cascade impactor that captures airborne particles in size ranges: >2.5 μm, 1.0-2.5 μm, 0.50-1.0 μm, 0.25-0.50 μm, and <0.25 μm. The total Cr(VI) concentration in the size fraction <0.25 μm was the highest with a maximum value of 9.7 ng/m3. This high concentration may pose a greater risk because smaller airborne particles can penetrate deeper into the lower respiratory tract of the lungs. Total suspended particles Cr(VI) exceeded the 8.0 ng/m3 Reference Concentration (RfC) by 22 times. The overall total Cr(VI) concentration in summer was significantly higher than in fall (p < 0.05), which could be due to factors, including higher temperatures, ozone, and NO2 concentrations in summer and a higher VOC concentration in fall. The results indicate that the interaction between Cr(III) and Cr(VI) through gas-solid reaction can control the speciation of atmospheric Cr.

Ultrasensitive fluorescent carbon dot sensor for quantification of soluble and insoluble Cr(VI) in particulate matter

This study investigates advanced functional materials to address the need for practical and affordable analytical techniques for monitoring large amounts of insoluble Cr(VI). N,S-doped fluorescent carbon dots (f-CDs) were fabricated through microwave synthesis, with an average diameter of 10 nm. These f-CDs were explored as potential sensors for detecting Cr(VI) in ambient particulate matter (PM). Laboratory experiments yielded positive results, showing average recoveries of 106.0%, 102.3%, 96.4%, and 101.7% for PbCrO4, BaCrO4, CaCrO4, and (NH4)2CrO4, respectively. Applying the fluorescence method to field PM samples, a method detection limit (MDL) of 0.32 ng/m3 for total Cr(VI) quantification was achieved. The fluorescence decay of carbon dots remained stable over time, suggesting that Cr(VI) quenching primarily occurred due to the internal filter effect through a static quenching mechanism. These f-CDs exhibited advantageous properties, including affordability, solubility, luminescence, and sensitivity, positioning them as a promising alternative for Cr(VI) detection in ambient air particulates. This study contributes to further developing carbon-based functional materials for detecting metallic air pollutants.