Extremely High Soil Copper Content, yet Low Phytotoxicity: A Unique Case of Monometallic Soil Pollution at Kargaly, Russia.

Ecotoxicological studies on soil metal toxicity often rely on artificially contaminated soils. A major difficulty in using soils contaminated by anthropogenic activities (e.g., mining and agriculture) is the presence of multiple metals, which can make it impossible to distinguish the toxic effects of a particular metal. Therefore, sites with monometallic pollution have great potential for ecotoxicological research. One such site is an agricultural field in Kargaly, Orenburg region, Russia, where copper mining and smelting activities were carried out during the 18th-20th centuries. Samples of Mollisols (chernozems) were collected in the studied field. At several sampling points there were copper ore rocks on the surface, containing malachite (CuCO3 · Cu(OH)2 ). The soil samples had a high copper content, up to approximately 10 g kg-1 , compared with 75 mg kg-1 in the background soil. Importantly, the content of other elements in all soil samples was similar to that in the background soil, highlighting the uniqueness of the monometallic contamination in the study area. Despite the extremely high total copper content, exchangeable copper was relatively low, with a maximum of approximately 0.5 mg kg-1 . We performed a short-term (21-day) ecotoxicity assessment using perennial ryegrass as an indicator of copper toxicity. Contrary to expectations, plant growth was not affected by the high copper content in the studied soils. The low copper phytotoxicity may be explained by the low solubility of malachite. However, future long-term experiments may be warranted to determine copper toxicity thresholds for plants under field conditions. The site discovered in the present study could potentially acquire the same significance as the Danish Hygum site for the study of monometallic soil contamination. Environ Toxicol Chem 2023;42:707-713. © 2023 SETAC.

Gypsum soil amendment in metal-polluted soils-an added environmental hazard.

Scientists around the world have long been searching for effective strategies to reduce the bioavailability of metals in contaminated soils. In case of metal-spiked soils, some studies have proposed gypsum as a soil amendment to alleviate metal phytotoxicity. However, for real field-collected soils, evidence on the efficacy of gypsum as a metal phytotoxicity amendment is limited. Therefore, the present study was designed to examine the effect of gypsum on plant growth in soils polluted by a copper smelter. We grew perennial ryegrass on untreated and gypsum-treated soils (at a dose of 3% by weight) under laboratory conditions. We found that gypsum had no effect on alleviating metal phytotoxicity in our soils. We also demonstrated - for the first time - that gypsum increased the concentrations of soluble metals in the soil, enhancing metal uptake by plants. The calcium ions from gypsum displace metals in the soil exchangeable complex; however, the metals do not get immobilized in soils because gypsum is a neutral salt. While our results contrast with the Terrestrial Biotic Ligand Model, that Model has never been tested on real industrially polluted soils but only on metal-spiked soils. Our main conclusion is that gypsum is ineffective in alleviating metal phytotoxicity in real industrially polluted soils and, moreover, its use is inappropriate as a soil remediation method, because it increases the environmental hazard rather than reducing it. Our study is the very first attempt to recognize that gypsum is a hazardous material when used to ameliorate soils polluted by metals.