ABSTRACT
Dissolved organic matter (DOM) is widely present in surface water environments and plays a critical role in the biogeochemical cycling of metal ions. Metal ions in acid mine drainage (AMD) have seriously polluted karst surface water environments, but few studies have explored interactions between DOM and metal ions in AMD-disturbed karst rivers. Here, the composition and sources of DOM in AMD-disturbed karst rivers were investigated by fluorescence excitation-emission spectroscopy combined with parallel factor analysis. In addition, correlations between metal ions and other factors (DOM components, total dissolved carbon (TDC) and pH) were determined using structural equation modeling (SEM). Results showed that there were evident differences in the seasonal distribution of TDC and metal ion concentrations in AMD-disturbed karst rivers. The concentrations of DOC, dissolved inorganic carbon (DIC), and metal ions were generally higher in the dry season than in the wet season, with Fe and Mn pollution being the most pronounced. The DOM in AMD contained two types of protein-like substances that were mainly from autochthonous inputs, while DOM in AMD-disturbed karst rivers contained two additional types of humic-like substances from both autochthonous and allochthonous inputs. The SEM results showed that the influence of DOM components on the distribution of metal ions was greater than that of TDC and pH. Among the DOM components, the influence of humic-like substances was greater than that of protein-like substances. Additionally, DOM and TDC had direct positive effects on metal ions, while pH had a direct negative effect on these. These results further elucidated the geochemical interactions between DOM and metal ions in AMD-disturbed karst rivers, which will assist in the pollution prevention of metal ions in AMD.
ABSTRACT
Acid mine drainage (AMD) threatens the water quality and safety of karst river water (KRW), and the formation of secondary iron or aluminum-bearing minerals during the mixing of AMD with KRW plays a crucial role in the migration of elements. However, the variations in the mineralogical, morphological and elemental compositions of secondary minerals and their influences on the migration of elements during AMD-KRW mixing have not been systematically studied. In this study, we mixed different proportions of AMD and KRW in a laboratory experiment to simulate seasonal hydrological conditions in a river to understand the major and trace elemental distributions in the mixed water and in precipitates and we discuss the formation process for the secondary minerals. The results showed that AMD can lead to a decrease in pH and DO and an increase in heavy metals and rare earth elements (REEs) in KRW. With the biological or chemical oxidation of Fe2+, Fe3+ combines with SO42- to form schwertmannite or hydrolyzes to form Fe(OH)3(s) and FeOOH(s), accompanied by the formation of amorphous Al hydroxide, resulting in a decrease in pH and an increase in Eh. Schwertmannite had strong adsorption and coprecipitation effects on Mn, Cr, Cu and As, so the adsorption and coprecipitation effects of schwertmannite on REEs were inhibited, while the migration of REEs were mainly affected by Al hydroxides. Therefore, after the AMD mixes with KRW, it not only causes severe water and sediment pollution but also adsorbs and enriches high concentrations of heavy metals in the secondary minerals formed during the mixing process, creating a major ecological hazard that requires further attention.
