Metal cations could enhance the sorption of tetracyclines but sometimes the effects are negligible. It is still not clear how these metals produce different effects. In this study, the sorption of chlortetracycline (CTC), tetracycline (TC), and oxytetracycline (OTC) was performed in the presence of Cd (II) to reveal the unknown mechanisms with two river sediments. It is found that Cd (II) could enhance the sorption of TCs on sediment SS, while it is negligible on sediment SY. For different tetracyclines, the enhancement effect by Cd (II) was more significant for CTC, while it is inferior for OTC and TC. Sorption isotherms of Cd (II) under strong and weak background electrolyte and pH decrease of sorption solutions indicate specific sorption is major on SY and cation exchange is significant on SS. Consequently, specific sorption is unfavorable for the enhanced sorption of TCs in the presence of Cd (II) because it is not favorable for the sorption of Cd-TCs by complexation and cation exchange. By the theoretical calculations, it is found that the significant enhancement of CTC is due to the higher electron affinity of Cd-CTC complex than the others to the surface groups. In conclusion, TCs sorption will not be affected by Cd (II) on sediments or soils with strong specific sorption characters of Cd (II).
The hyporheic zone is a key area for water circulation. However, hyporheic zone pollution is widespread all over the world and remediations of hyporheic zones, when polluted by both heavy metals and organics, are challenging. Thus, the mechanisms and application of Calcium peroxide (CaO 2 ) coupled with Zero-valent iron (ZVI) to simultaneously remove cadmium (Cd), lead (Pb), and acetochlor were investigated in this work. The effects of mass ratio of ZVI and CaO 2 , initial pH, and water matrices were evaluated. At initial pH 3, with the dosage of 1.5 g·L −1 ZVI and 0.5 g·L −1 CaO 2 , the removal efficiencies of Cd, Pb, and acetochlor were 96.7%, 99.8%, and 100%, respectively. Acetochlor was degraded into four main byproducts, and the toxicity evaluation indicated that the ecotoxicity of acetochlor was reduced during the degradation by CaO 2 /ZVI. In column transport experiments, with the mass ratios of ZVI and CaO 2 to quartz sand of only 0.13% and 0.05%, the mass recovery efficiencies of Cd, Pb, and acetochlor from the effluent were 42.5%, 10.4%, and 0%, respectively. Cd was removed mainly by the adsorption of ZVI and iron(hydr)oxides (represented Fe 3 O 4 and lepidocrocite); removal of Pb chiefly resulted from ZVI reduction, and hydroxyl radicals derived from the Fenton reaction accounted for acetochlor degradation. This work provides new insights and a facile strategy for in-situ remediations of hyporheic zones polluted simultaneously by heavy metals and organics.
In rivers, riverbeds are considered to have dual properties as a short-term sink and a source of further mobilization for microplastics. To better understand the sources, storage, and fate of microplastics in river systems, this study quantified the formation of microplastic hotspots in riverbeds and seasonal variations in microplastic inventories in riverbeds, especially for small-sized microplastics (<330 µm), with a fluorescence-based protocol. This study provides first-hand measured evidence for the sequestration of microplastics in the riverbed under low-flow conditions and its export from the riverbed under high-flow conditions. The results show that riverbeds in urban areas are still hotspots for microplastic pollution and that high inputs of urban microplastics control microplastic load in its downstream areas. Seasonal rainfall exported 34.86 % (equivalent to 4.34 × 10 11 items/8.57 t) of microplastic pollution from the riverbed, and its removal capacity may be related to the rainfall intensity. Wider riverbeds are conducive to the formation of microplastic hotspots due to the flow slow down. Most importantly, rainfall-driven scouring of the riverbed can enhance the pollution of small-sized microplastics in the riverbed, especially the smallest-size microplastics (<100 µm). Therefore, this study not only contributes reliable information about the sequestration and export of microplastics in the riverbed, but also provides a possible mechanism to explain the lack of small-sized microplastics (<330 µm) in the ocean.
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