Molecular Characteristics Of Organic Matter And Their Roles In Arsenic Mobility In Typical Yellow River Basins

Organic matter plays an important role in groundwater arsenic enrichment processes.How organic matter promotes arsenic enrichment in groundwater at the molecular level is an important scientific problem that remains to be further explored.In this paper,three typical arsenic-hosting basins within the Yellow River catchment,namely the Guide Basin,the Yinchuan Basin,and the Hetao Basin,are selected as the study area.By utilizing FT-ICR MS technique,this paper explores the differences of organic matter molecular compositions in groundwater under different hydrogeological conditions,reveals spatial evolution of molecular signatures of organic matter in groundwater,discerns relevant sources of biodegradable organic matter in groundwater system,and evaluates the significance of organic molecules on arsenic enrichment under different hydrogeological backgrounds at the watershed scale.The outcomes are of great significance in both theoretical and practical values.The main findings are listed as follows.(1)In a deep confined aquifer(CG)tapped from Pliocene strata,groundwater p H,temperature,and arsenic concentrations as well as the numbers of the unique recalcitrant molecules(polycyclic aromatics and polyphenols)remarkably increased along the path,but relatively labile molecules(highly unsaturated and phenolic compounds and aliphatic compounds)tended to be enriched in the arsenic-poor(<10?g/L)unconfined shallow groundwater(UG).High groundwater p H and temperature in the CG aquifers may be responsible for the selective mobilization of the unique polycyclic aromatics and polyphenols,and also facilitate arsenic desorption.Additionally,the mobilized recalcitrant organic molecules may enhance arsenic release via electron shuttling,complexation,and competition.(2)The low-arsenic hydrogeological profile from a sand gravel aquifer is dominated by oxidizing environment with arsenic concentrations<10?g/L.However,the high-arsenic hydrogeological profile from a sand aquifer is dominated by reducing environment,where dissolved arsenic concentrations were significantly positively correlated to DOC and NH₄⁺concentrations.The spatial evolution trends of arsenic concentrations were analogous to the labile molecules(e.g.,highly unsaturated and phenolic compounds and aliphatic compounds).This suggests that labile organic matter degradation is the primary driver for the dissimilatory reduction of Fe(III)oxides and thereby release of arsenic.In addition,groundwater arsenic concentrations were positively correlated with the number of polycyclic aromatics and polyphenols,which suggests that these recalcitrant organic compounds can also promote arsenic enrichment.(3)Both the number and the intensity of biodegradable organic molecules(including aliphatic/proteins and carbohydrates)were higher in water soluable organic matter(WSOM)in sediment than those of dissolved organic matter(DOM)in depth-matched high-arsenic groundwater,while they were lower in sedimentary WSOM than those in depth-matched low-arsenic groundwater DOM.The number of the unique(relative to groundwater DOM)biodegradable organic molecules in sediment WSOM is positively correlated with dissolved arsenic concentration of depth-matched groundwater.Anoxic incubations of sediments and deionized water show that more arsenic and Fe(II)were released from aquifer sediments with the greater number and the greater intensity of consumed biodegradable compounds in WSOM,accompanied by the higher amount of Fe(III)oxides being reduced.These observations indicate that the biodegradable organic matter in sedimentary WSOM is an important source of biodegradable groundwater DOM,which is closely related to arsenic enrichment in groundwater.(4)Relative abundances of H/C?1.0 molecules among the unique organic compounds in high-arsenic groundwater relative to low-arsenic groundwater increased from 24%in the Guide Basin,through 40%in the Yinchuan Basin,to 50%in the Hetao Basin.This suggests that,along the flow of the Yellow River,roles of recalcitrant organic molecules on arsenic enrichment gradually decreased,while roles of biodegradable organic compounds increased in groundwater from the basins.In the Guide Basin,groundwater arsenic enrichment is closely related to high abundances of recalcitrant organic molecules.In the Yinchuan Basin,both the biodegradable and recalcitrant organic molecules are involved in arsenic release.In the Hetao Basin,degradation of biodegradable organic molecules is the primary driver of high groundwater arsenic concentrations.