Degradation Characteristics Of Dissolved Organic Matter In Karst Rivers Under The Influence Of Acidic Coal Mine Drainage And Its Response Relationship With Metal Ion

Dissolved organic matter（DOM）is an important link in the energy and material cycle of karst aquatic ecosystems,while photochemical reactions and biodegradation are important processes controlling the transformation,degradation and recycling of DOM.The degradation process is easily affected by changes in the water environment（e.g.pH,iron ions,etc.）.Acid mine drainage（AMD）from coal mines is characterized by low pH and high iron content,and its untreated direct discharge will bring serious harm to the water environment around the mines,and the typical metals in AMD will easily complex with DOM to form complex pollutants,which becomes a difficult problem for environmental management.By studying the degradation process of DOM in karst river water（KRW）under the influence of AMD and its interaction with typical metals,and revealing the mechanism of removing or transforming DOM in water by photochemical and microbial effects,we can further grasp the geochemical processes of composite pollutants in karst areas under the influence of AMD,understand the geochemical behavior between DOM and typical metals in karst rivers under the interference of human mining activities,and also the geochemical behavior of DOM under different environmental conditions can be clarified.In this study,the composition and sources of DOM in AMD-affected karst rivers in Guizhou,China,were investigated by fluorescence excitation-emission spectroscopy combined with parallel factor analysis,and the interactions between DOM components,TDC（Total dissolved carbon,TDC）and pH and typical metals were revealed based on structural equation modeling（SEM）.The changes of water chemistry and optical parameter characteristics of DOM in water samples mixed with AMD and KRW in different ratios under visible light irradiation only（VL）,UV irradiation only（UV）,biodegradation only（BD）and ultraviolet irradiation+biodegradation（UV+BD）were also analyzed through 12 days of indoor experiments to explore the effects of AMD and its chemical evolution on the photochemical and microbial degradation of DOM in the receiving river.The main conclusions are as follows.（1）There are obvious differences in the seasonal distribution of TDC and typical metal concentrations in AMD-disturbed karst rivers.The concentrations of DOC,DIC,and typical metals were generally higher in the dry season than in the wet season,which is related to the dilution effect of rainfall during the wet season.Among these typical metals,Fe and Mn pollution was the most pronounced.From a temporal perspective,the fluorescence intensity of DOM was greater in the dry season than in the wet season.On the other hand,DOM in AMD was greater than in karst rivers from a spatial perspective.Along the river,DOM in AMD was composed of protein-like substances mainly from autochthonous inputs,while DOM in AMD-disturbed karst water was composed of protein-like and humic-like substances mainly from autochthonous and allochthonous inputs.（2）The SEM analysis showed that DOM components had a greater influence on the distribution of typical metal concentrations than TDC and pH.Among these,DOM and TDC had a direct positive effect on typical metals,which was due to complexation effects of DOM components and DIC in TDC with metal ions,while humic-like substances had a larger effect on the distribution of typical metal concentrations.Conversely,pH had a direct negative effect on typical metals,which is explained by an increase in pH leading to the precipitation of metal ions.（3）The initial DOC concentration in KRW was lower than that in AMD,and after 12 days,the DOC concentration in KRW and AMD were decreased by 17%,21%,and 20%in VL,BD,and UV,while the efficiency of DOC removal by UV+BD was higher,reaching 27%.With the increase of AMD proportion,the removal rate of DOC in water samples gradually increased,indicating that the addition of AMD may accelerate the conversion and removal process of DOC in water through the adsorption and co-precipitation of iron hydroxide and the generation of·OH by Fe²⁺redox reaction under the effect of light irradiation.