Characteristics Of Dissolved Organic Matter In Black And Odourous River And Its Relationship With Dissolved Heavy Metals

In recent decades,China has experienced rapid urbanization.However,severe human activities and disturbances have caused the water quality of the aquatic environment to decline.Up to now,more than 2,000 black and stinky rivers have been formed in urban areas in China.Dissolved organic matter(DOM)plays an important role in various biogeochemical processes in aquatic environments and has important environmental effects.However,urban rivers have received a large amount of exogenous DOM impacted by human activities.The nature,origin,and chemical composition of the DOM have become more complicated.Using the Maozhou River in Shenzhen as an example,we comprehensively applied optical spectroscopy,Fourier transform ion cyclotron mass spectrometry(FT-ICR MS),and stable carbon isotopes to analyze the source,chemical composition,and molecular-level characteristics of urban rivers.In addition,to understand the environmental effects of urban river DOM,the correlations between DOM characteristics and dissolved metal concentrations were explored.A series of results were obtained as follows.1)In the four aquatic environments,the dissolved organic carbon(DOC)and total dissolved nitrogen(TDN)of the disturbed water bodies(mainstream,tributaries,and ponds)were significantly higher than those of the reservoirs;the DOC concentration of Maozhou River increased from 2.8 mg/L to 5.9 mg/L and TDN increased from 13.6 mg / L to 18.5 mg / L from the upper to lower mainstream.Based on the spectral fingerprint parameters,the disturbed water DOM has lower specific ultraviolet absorbance at 254 nm(SUVA254)and humification index(HIX),higher E2/E3 and biological index(BIX),indicating that the DOM has lower aromaticity and humification degree and more low-molecular-weight DOM.2)In the parallel factor analysis(PARAFAC)of all samples,four effective fluorescent components were obtained.The input of tyrosine and tryptophan greatly affected and changed the composition of DOM,and protein-like components were found dominated along the disturbed water body and the mainstream of the Maozhou River.3)Based on FT-ICR MS analysis,the disturbed water DOM had lower aromaticity and average molecular weight,and higher biolability.Although the DOM had terrestrial (e.g.,lignin-like)sources of plant and soil origin,the microbial sources of proteinlike substances and anthropogenic input of sulfur-containing compounds(e.g.,surfactants)were more important sources.Therefore,in highly urbanized areas,river DOM should not be considered solely as natural organic matter.Its nature and environmental functions,such as greenhouse gas emissions and pollutant transport,have been altered significantly.4)Under the influence of rapid urbanization,compared with ponds and reservoirs,the mainstream and tributaries of the Maozhou River are more disturbed by humans,showing soluble Cu and Zn metal pollution.The point source input in urban areas was the main cause of rising mainstream DOM concentrations and heavy metal pollution.The point source caused increases in protein components(C3,C4)and microbial contribution and decreases in aromaticity,humification,and molecular weight for DOM.More attention should be paid to the point source pollution to improve water quality of urban rivers in future environmental management.5)The rapid urbanization has resulted in a weakened correlation between the concentration of dissolved heavy metals and DOM.Specifically,the dissolved metal concentrations had no correlation with the total amount of DOM(measured by DOC)or CDOM(measured by UV254),but is significantly associated with the protein-like component of DOM.At the same time,the significant correlation between Zn/DOC,Cu/DOC,and SUVA254 found in previous studies has not been found in this study.However,the Zn/DOC,Cu/DOC,and DOM fluorescence spectral parameters(such as BIX and Fmax / DOC Ratio)has a more significant correlation.To sum up,it can be considered that the impact of urbanization may weaken or enhance the correlation between the properties and composition of heavy metals and DOM,and the main ligands of dissolved metals in the mainstream may be shifted from natural sources(such as humic-like FDOM)into anthropogenic sources(such as wastewater treatment plant’s protein-like FDOM).It highlights that the DOM in the urban black and odourous river alters the behavior of environmental pollutants.