Original Research
Chemical and spectroscopic characterization of humic substances from sediment and riparian soil of a highly polluted urban river (Suquía River, Córdoba, Argentina)

https://doi.org/10.1016/j.ijsrc.2019.10.004Get rights and content

Abstract

The Suquía River, the largest urban river in Córdoba (Argentina), has been severely polluted for decades. Actions must be taken to restore its environmental quality by managing riparian zones for increased water-self purification. The current study aimed to characterize organic matter (OM) dynamics and humic substances (HS) spectrochemical properties along the lower-middle basin of the Suquía River. Riparian soil (0–20 cm) and sediment (0–10 cm) samples were collected from a reference location (S1) and four polluted sites (S2–S5) during a low-flow period. The contents of soil and sedimentary OM and HS fractions were analyzed by wet oxidation, as well as HS Fourier transform infrared (FT-IR) and ultraviolet–visible (UV–Vis) spectrochemical properties. The OM and HS fractions from riparian soil were high upstream of Córdoba City (S1 and S2, 50.2–50.4 g/kg OM) and within a 50 km downstream location (S5, 30.9 g/kg OM) owing to a surplus of fresh plant biomass-carbon (C) inputs. Highly heterogeneous sediment samples did not show any significant differences among sites (P > 0.05). The lowest values of the ratio of absorbances at 465 and 665 nm (E4/E6) (1.78) and the Δ log K (0.15) coefficient (a measure of HS maturity degree) were obtained downstream of Córdoba City, for both riparian soil and sediment, indicating that HS were enriched by more condensed aromatic structures within highly degraded portions of the river. All samples exhibited similar IR spectra, implying overlapping recalcitrant-C structures at the functional group level, but with different absorbance intensity. Data from the current study constitute a baseline for understanding the chemical nature of HS from sediment and riparian soil along the Suquía River and can be used as a reference for future studies tracking OM compositional changes over time.

Introduction

Organic matter (OM) is a natural constituent of soil, sediment, and water originating from complex physicochemical transformations of animal-, plant-, and microbial-derived precursors (Paul, 2007, Wetzel, 2001). There is a wide range of constituents encompassing OM mixtures (Prentice & Webb, 2010). Chemically, OM can be conceptualized in terms of biologically active and stable humified pools. Biologically active compounds are aliphatic constituents (i.e. carbohydrates, polysaccharides, phospholipids) of low molecular weight rapidly metabolized by soil microorganisms (Ghani et al., 2003, Marinari et al., 2010). Contrarily, humic substances (HS) are complex aromatic fractions (Vázquez et al., 2016, Vergnoux et al., 2011) derived from decomposing lignocellulosic polymers (Stevenson, 1994).

Humic substances can be separated into different fractions according to their solubility in either an alkaline or acidic pH (Stevenson, 1994). Fulvic acids (FA) are soluble in both alkaline and acidic conditions. Contrarily, humic acids (HA) are extracted from soil or sediment by dilute alkali [0.1 M (sodium hydroxide, NaOH)] but precipitated at a pH of 2 (Vergnoux et al., 2011). These fractions differ in molecular size, with weights being estimated to range from 1000 to 5000 Da, and from 50,000 to 500,000 Da for FA and HA, respectively (McDonald et al., 2004). Accordingly, HA are more polymerized and aromatized than FA (Stevenson, 1994), with the HA/FA ratio being a surrogate of the soil OM maturity degree (Aranda and Oyonarte, 2006, Guimarães et al., 2013).

The chemical nature of HS is governed by the source and origin of humic-like materials, which are either autochthonous OM synthesized in situ; allochthonous OM derived from terrestrial debris of higher plants (D’Andrilli et al., 2013); or anthropogenic OM occurring in wastewaters and sewage outfalls (Gonsior et al., 2011). Techniques commonly applied to characterize the complexity of HS include solid-state 13C (Carbon - 13) nuclear magnetic resonance (NMR) spectroscopy (de Alcantara et al., 2004, Liu et al., 2016), fluorescence excitation-emission matrices (EEMs) spectroscopy coupled with parallel factor (PARAFAC) analysis (Coble et al., 2014), Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry (D'Andrilli et al., 2013), Fourier transform infrared (FT-IR) spectroscopy, and ultraviolet–visible (UV–Vis) absorbance spectroscopy (Carletti et al., 2010, Gezici et al., 2012, Guimarães et al., 2013, Mao et al., 2011). FT-IR spectroscopy is a widely used analytical approach that provides relevant information regarding the nature, reactivity, and structural arrangement of HS functional groups (Giovanela et al., 2010, Tivet et al., 2013). The ease of obtaining an IR spectrum with increased energy throughput and a higher signal-to-noise ratio has prompted the use of FT-IR spectroscopy over traditional IR techniques (McDonald et al., 2004). Similarly, UV–Vis absorbance spectroscopy is a fast, non-destructive approach utilized to probe physicochemical features of HS at the bulk level (Giovanela et al., 2010, Morán Vieyra et al., 2009). Previous studies have shown that the ratio of absorbances (Absx, where x is the evaluation size) at 465 and 665 nm (E4/E6) and Δ log K coefficient (log Abs465nm - log Abs665nm) are indicative of HS aromaticity and molecular size, with lower values reflecting a higher degree of OM microbial processing and decomposition (Miralles et al., 2012, Vázquez et al., 2016).

Organic matter is a critical factor modifying the biotic functioning of terrestrial-C, regulating both energy flow and nutrient transfer pathways at the ecosystem level (Williamson et al., 2008). Organic matter may affect food webs either directly by uptake from soil microorganisms, or indirectly by mechanisms such as turbidity, pH, metal chelation, and transport of contaminants (Bolan et al., 2011, Wetzel, 2001). In this regard, HS are important components of polluted environments since they strongly affect the mobility, bioavailability, and fate of different contaminants by a myriad of complexing reactions with heavy metals and organic molecules (Bahemmat et al., 2016, Kulikowska et al., 2015, Parolo et al., 2017).

The Suquía River (Córdoba, Argentina), an endorheic system occupying ∼7700 km2, is currently subjected to a strong anthropogenic impact because it receives an excess of contaminants from various sources, including sewage from the municipal waste water treatment plant (WWTP) located downstream of Córdoba City, as well as agricultural, and industrial effluents from point and non-point pollution sources. Previous studies have reported the negative effect of these activities on the environmental quality of the Suquía River by the combined use of chemical and biological techniques. An extensive monitoring study by Merlo et al. (2011) revealed a strong pollution impact on several reaches and components of the Suquía River, including air, water, riparian soil, sediment, and biota. It was concluded that pollution from Córdoba City, untreated sewage discharges from the WWTP, and pollution from nearby agricultural fields (i) increased the abundance of fecal bacteria (i.e. coliforms, Escherichia coli) in surface water and sediment layers, (ii) depleted dissolved oxygen (DO) concentrations, decreasing local fish total abundance and diversity (iii) exacerbated the accumulation of heavy metals [i.e. copper (Cu), chromium (Cr), nickel (Ni), and lead (Pb)] and a wide range of pesticides (i.e. atrazine, chlorpyrifos and endosulfan) in surface water and sediment layers, and (iv) degraded urban air quality by increasing polyaromatic hydrocarbons (PAHs) (Bonansea et al., 2013, Merlo et al., 2011). Similarly, Monferrán et al. (2011) documented a pollution-dependent response of biomarkers (i.e. detoxification and antioxidant enzymes) measured in Jenynsia multidentata collected close to sewage discharges.

To date, however, no regional studies have addressed changes in the distribution of riverine OM or characterized the chemical nature of associated humic fractions. An adequate knowledge of the structural features that define the reactivity of HS is critical to infer potential interactions with heavy metals, as well as organic and inorganic pollutants. The occurrence of OM along the Suquía River may be variable due to different inputs from anthropogenic-C sources and riparian vegetation. Within this context, some differences in OM composition were expected. The objective of the current study was to characterize the chemical nature of OM following a continuum from minimally disturbed sites, with high plant biomass-C inputs, to more strongly degraded portions of the river, with little or no vegetation coverage. OM and HS from sediment (0–10 cm) and riparian soil (0–20 cm) samples were extracted and further analyzed by means of wet oxidation with potassium dichromate (K2Cr2O7) and bulk optical techniques such as UV–Vis absorbance and FT-IR spectroscopy. It was hypothesized that (i) additional biomass-C inputs at minimally disturbed sites would increase plant-derived OM constituents when compared to more degraded riparian soil and sediment samples (Merlo & Abril, 2014); (ii) such changes would be associated with an increasing maturity degree or processing of HS; and (iii) the influence of riverbank conditions on OM properties would be attenuated from riparian soil to sediment layers (Merlo et al., 2017).

Section snippets

Study area

The study was done in the Suquía River of Córdoba Province (Argentina). This river begins at the San Roque Dam and flows mainly from west to east for about 200 km until reaching the depression of Mar Chiquita Lake (Fig. 1). The watershed is located in a semi-arid region. The mean annual rainfall is in the range of 700–900 mm, with a dry season (from May to November) and a wet season (from December to April) with most of the rainfall occurring in January and February. The mean temperature ranges

Total organic matter and humic substances in riparian soil and sediment

Riparian soil TOM (4.15–50.40 g/kg range) was significantly affected (P < 0.05) by the study sites. Riparian soil TOM was higher at S1 and S2, followed by S5, S3, and S4 (Table 2). Similarly, S1 (9.41 g/kg) and S2 (7.38 g/kg) contained more riparian soil HS than S4 (2.36 g/kg) and S3 (1.70 g/kg) (Table 2). Humic substances accounted for approximately 14–20% of riparian soil TOM at S1, S2, S3, and S5. In contrast, HS accounted for approximately 57% of riparian soil TOM at S4. Humic substances,

Conclusions

The results of the current study supported the hypothesis that changes in OM accretion, as a result of contrasting riverbank conditions, altered the chemical nature of riparian soil TOM. Sites with increased vegetation coverage enhanced the HS pool concentration and compositional diversity. Contrarily, riparian soil HS isolated from highly degraded portions of the river were more humified, reflecting the more decomposed, recalcitrant pool of native OM. Highly heterogeneous sediment samples were

Acknowledgments

This work was supported by the Secretaria de Ciencia y Tecnología - Universidad Nacional de Córdoba, Argentina (SECyT-UNC) and the National Research Council of Argentina (CONICET). The authors acknowledge two anonymous reviewers for providing constructive comments and helpful suggestions that improved the overall quality of this manuscript.

References (69)

  • C.J.W. Feral et al.

    Carbon and nitrogen in the soil-plant system along rainfall and land-use gradient in southern Africa

    Journal of Arid Environments

    (2003)
  • A.P. Fernández-Getino et al.

    Assessment of the effects of environmental factors on humification processes by derivative infrared spectroscopy and discriminates analysis

    Geoderma

    (2010)
  • L.N. Galanti et al.

    Accumulation and detoxification dynamic of cyanotoxins in the freshwater shrimp Palaemonetes argentinus

    Harmful Algae

    (2013)
  • O. Gezici et al.

    Subtractive FTIR spectroscopy to characterize organic matter in lignite samples from different depths

    Spectrochimica Acta

    (2012)
  • A. Ghani et al.

    Hot-water extractable carbón in soils: A sensitive measurement for determining impacts of fertilization, grazing and cultivation

    Soil Biology and Biochemistry

    (2003)
  • M. Giovanela et al.

    Chemical and spectroscopic characterization of humic acids extracted from the bottom sediments of a Brazilian subtropical microbasin

    Journal of Molecular Structure

    (2010)
  • M. Gonsior et al.

    Molecular characterization of effluent organic matter identified by ultrahigh resolution mass spectrometry

    Water Research

    (2011)
  • D.V. Guimarães et al.

    Soil organic matter pools and carbon fractions in soil under different land uses

    Soil and Tillage Research

    (2013)
  • M. He et al.

    Characterization of humic acids extracted from the sediments of the various rivers and lakes in China

    Journal of Environmental Sciences

    (2008)
  • K. Kaiser et al.

    The role of DOM sorption to mineral surfaces in the preservation of organic matter in soils

    Organic Geochemistry

    (2000)
  • D. Kulikowska et al.

    Humic substances from sewage sludge compost as washing agent effectively remove Cu and Cd from soil

    Chemosphere

    (2015)
  • S. Liu et al.

    Cation-induced coagulation of aquatic plant-derived dissolved organic matter: Investigation by EEM-PARAFAC and FT-IR spectroscopy

    Environmental Pollution

    (2018)
  • S. Liu et al.

    Characteristics and degradation of carbon and phosphorus from aquatic macrophytes in lakes: Insights from solid-state 13C NMR and solution 31P NMR spectroscopy

    The Science of the Total Environment

    (2016)
  • J. Mao et al.

    Characterization of humic substances by advanced solid state NMR spectroscopy: Demonstration of a systematic approach

    Organic Geochemistry

    (2011)
  • S. Marinari et al.

    Differences of stabilized organic carbon fraction and microbiological activity along Mediterranean vertisols and alfisols profiles

    Geoderma

    (2010)
  • S. McDonald et al.

    Analytical chemistry of freshwater humic substances

    Analytica Chimica Acta

    (2004)
  • R.E. McIntyre et al.

    Rewetting and litter addition influence mineralisation and microbial communities in soils from a semi-arid intermittent stream

    Soil Biology and Biochemistry

    (2009)
  • M. Mecozzi et al.

    An ultrasound assisted extraction of the available humic substance from marine sediments

    Ultrasonics Sonochemistry

    (2002)
  • C. Merlo et al.

    Integral assessment of pollution in the Suquía River (Córdoba, Argentina) as a contribution to lotic ecosystem restoration programs

    The Science of the Total Environment

    (2011)
  • C. Merlo et al.

    Environmental factors associated with heterotrophic nitrogen-fixing bacteria in water, sediment, and riparian soil of Suquía River

    Limnologica

    (2014)
  • I. Miralles et al.

    Assessment of biogeochemical trends in soil organic matter sequestration in Mediterranean calcimorphic mountain soils (Almería, southern Spain)

    Soil Biology and Biochemistry

    (2007)
  • I. Miralles et al.

    Surrogate descriptors of C-storage processes on crusted semi-arid ecosystems

    Geoderma

    (2012)
  • F.E. Morán Vieyra et al.

    Combined UV–Vis absorbance and fluorescence properties of extracted humic substances-like for characterization of composting evolution of domestic solid wastes

    Geoderma

    (2009)
  • N. Nunan et al.

    Metabolising old soil carbon: Simply a matter of simple organic matter?

    Soil Biology and Biochemistry

    (2015)
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