Humic matter: basis for life—a plea for humics care
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Humic matter (HM) is the leftover from life and at the same time the source for new life. The resulting complex system with many interactions has become a crucial part for the anthropocene and by this for the survival of mankind. Based on the results of the application of advanced analytical tools, the structures, reactions and interactions of HM are discussed.
Materials and methods
HM was investigated from different water samples (ground water, bog lake, waste water effluent). Fulvic acids (FA) and humic acids (HA) were isolated from a bog lake and from waste water effluent according to the XAD-method described by the International Humic Substances Society. Parameters like dissolved organic carbon (DOC), spectral absorption coefficient at λ = 254 nm (SAK), AOX (on activated carbon absorbable organically bound halogen) and THM (trihalomethanes) were determined according to ISO standard methods. For additional characterization, size exclusion chromatography coupled with online DOC detection and solid-state NMR were applied. The degradation of HM was studied by heterogeneous photocatalysis with titanium dioxide. Membrane separation, done as ultra- and nanofiltration, was used to characterize different size fractions of HM.
Results and discussion
The water solubility and hence the omnipresence of HM in aquatic systems opens the door for obtaining well-defined samples for experiments with meaningful results. Information on transport properties and reactivity, derived from the molecular size of HM, was obtained by using membrane filtration at different pore sizes. Photocatalytic degradation of HM was investigated by irradiation of suspensions with TiO2 as catalyst. Small organic acids (e.g. formic acid) were formed before total mineralisation occurred.
It can be concluded that the properties of HM are well derivable from their molecular data. The resulting character of HM with respect of the human environment seems to be obviously ambivalent and asks for a sound understanding and proper management to support life in a sustainable way.
KeywordsAquatic HM Humic matter (HM) Reactivity Refractoriness Structure
Primary production is the basis for the biochemical synthesis of carbohydrates starting from CO2 and water (Eq. (1)). Further on, carbohydrates in nature can be (bio-)degraded, oxidized or combusted, and finally, they can react or be bio-transformed to organic molecules, which have been assigned to be humic matter (HM). Its building blocks reflect their origin and the pathway they went (Schnitzer and Neyroud 1975; Christman and Gjessing 1983; Ertel and Hedges 1985; Ziechmann 1994; Swift and Spark 2001). The environmental relevance of the processes involved and especially the interdependences of the reactions and the climate are obvious (Schnitzer and Khan 1972; Stevenson 1982; Frimmel and Christman 1988; Abbt-Braun and Frimmel 2002).
1.1 Building blocks of HM
Common analytical tools for the detection of building blocks in the structure of HM
Analytical methods and principles
Atomic force microscopy; elemental analysis; isotope ratios; fluorescence spectroscopy (time resolved), FT-IR; mass spectrometry (high resolution); NMR solid state (1H, 13C, 14N); X-ray tomography
Selected building blocks
Allyl-groups; amines; aryl-groups; cellulose; fatty acids; hydrocarbons; lignins; polyaromatics; polysaccharides; proteins
Some of the analytical methods are quite sophisticated (e.g. Hatcher 1983; Hertkorn et al. 2006, 2013; Hsu and Hatcher 2005; Lankes 2001; Reemtsma 2009; Reemtsma and These 2005; Riedel et al. 2016; Stenson et al. 2003), and the determinations are time consuming. Therefore, they are often used for a kind of calibration for more simple and hence faster methods (e.g. Gjessing 1976; Swift 1989; Huber and Frimmel 1994; Frimmel and Abbt-Braun 2011).
The building blocks reflect the genesis of the HM and its fate. In aquatic systems, allochthonous and autochthonous HM can be distinguished according to the outside origin, e.g. from trees or other terrestrial plants and organisms or from inside sediments from algae and hydro fauna, respectively. Conclusions can be drawn on the kind of degradation and by this on the refractory status. Further interest in information can be deduced from the identification of synthetic pollutants (xenobiotics) or from metals, which HM met on their way of humification. Bound residues of pesticides or mercury attached to sulfidic groups are prominent examples for that (Kumke et al. 2005; Perminova et al. 2005; Tercero Espinoza et al. 2011).
2 Materials and methods
2.1 Aquatic humic samples
HM was taken from Lake Hohloh, a typical bog lake in the Black Forest (Germany). The lake is part of an upland moor situated in a preservation area approximately 1000 m above sea level. The original water samples were filtered through 0.45 μm membranes, leading to operationally defined DOC (dissolved organic carbon). HM from waste water was taken from the effluent of the biologically working municipal waste water treatment plant (WWTP) Neureut (Karlsruhe, Germany). For the membrane filtration experiments, ground water was used showing at high total hardness (ρ(CaCO3) = 333 mg/L) (Gorenflo et al. 2002).
The fulvic acid (FA) and humic acid (HA) fractions of the aquatic HM samples were obtained according to the standard method of the International Humic Substances Society (IHSS) applying the XAD-8 resin (Aiken et al. 1985; Leenheer 1981). A detailed description can be found in Abbt-Braun and Frimmel (2002).
HM samples were analysed or reacted either as solid samples after freeze drying (e.g. elemental composition), as original aqueous samples (0.45 μm membrane filtered) or as diluted samples (with ultrapure Milli-Q water; concentrations adjusted).
2.2 Analytical methods
Chromatographic characterization and spectrometric quantification was done using commercially available instrumentation following the internationally standard methods for water analysis (e.g. DOC, SAK (spectral absorption coefficient at λ = 254 nm), AOX (on activated carbon absorbable organically bound halogen) and THM (trihalomethanes)) (Gordalla 2011).
DOC determination was done with an Organic Carbon Analyzer 820 PMT (Sievers) or a Shimadzu TOC Analyzer TOC-VSCN. The inorganic C was quantified as CO2 after acidification with phosphoric acid (pH < 2). After having purged inorganic CO2, OC was quantified via oxidation (UV irradiation, ammonium persulfate) to CO2 and IR detection.
Size exclusion chromatography coupled with online organic carbon detection (SEC-OC) was done according to Huber and Frimmel (1991). A Toyopearl HW 50S resin (Tosoh Corp, Japan) packed Novogrom column (length: 250 mm, inner diameter: 20 mm; Alltech Grom, Germany), and a phosphate eluent (1.5 g/L Na2HPO4· 2 H2O + 2.5 g/L KH2PO4) at a flow rate of 1 mL/min was used. DOC was quantified as CO2 in the acidified (H3PO4) samples after inorganic carbon was purged (oxidation by UV) by an IR detector. Potassium hydrogen phthalate was used for the calibration of DOC.
Distribution of functional groups (in %) in isolated HM from a typical bog lake and from a municipal waste water treatment plant (WWTP) effluent
2.3 Reactions and experiments
The degradation of HM by advanced oxidation (AOPs) was done in a photochemical reactor (solar UV simulator with an atmospheric attenuation filter; Oriel Corp., Stratford, CT, USA) using TiO2 particles (P25, Degussa, Germany) as photocatalyst. The reactions conditions were as follows: V = 40 mL sample, m (TiO2) = 20 mg, irradiation path length d = 3.5 cm, photon flux (290 to 400 nm) = 1 × 10−7 Einstein/s; gentle stirring, open to atmosphere (Tercero Espinoza 2010).
Membrane separation (Fig. 2) was done as ultra- and nanofiltration (UF: BKMF10, polyethersulfone; NF: NF270, polypiperazine). The membrane module was immersed in the tank filled with the investigated aquatic sample. A pump was used to apply a negative pressure on the membrane (e.g. hydrophilic polyethersulfone, nominal cut-off 0.1 μm).
A selection of powerful tools was applied to characterize and quantify HM from nature and from technological settings which reflect the anthropogenically influenced environment.
As a result of biological waste water treatment, humic-like matter also leaves the technical plants (WWTP) in their effluents, which reach the rivers and lakes. Elevated organically bound nitrogen levels are typical for these effluents. Table 2 shows the comparison of the composition of HM from a bog lake with the composition of HM from a WWTP effluent. The high molecular weight fraction (> 30 kDa) in the DOC showed 64% for the brown water and 18% for the WWTP effluent.
This example shows clearly that HM interconnects nature, mankind, and technology. Some of these emerging interdependencies will be further elucidated.
3.1 Water technology, showcase for the genesis and fate of HM
Water and organic carbon (OC) are the dominating components in the biosphere. Their reactions and manifold interactions can be investigated by well-defined lab experiments as well as by watching and analysing large-scale systems. Consequently, the highly relevant field of water management, water analysis and water technology offers a fine basis for studying the biogeochemistry of dissolved and/or suspended HM in detail (Suffet and MacCarthy 1989; Ødegaard 1999; Ødegaard et al. 2010; Yang et al. 2015).
HM related natural and technological processes and their major drivers in aquatic systems
Addition of Al- or Fe-salts
Settling by gravity
Settling pond, centrifuge
Filter bed, membrane filter
Activated carbon, synthetic resins
Oxygen, ozone, UV, OH-radicals (advanced processes); for disinfection: chlorine, ozone, UV, > temperature
3.2 Separation by membranes
Micro- and nano-size membrane processes are unreplaceable for separation of materials on the molecular level. Their functioning serves the living cells in the biosphere according to the same principles as in the technical application for water cleaning. According to the nominal cut-off size of the water constituents to be eliminated, (micro- to nanometer) different membranes are in use. In general, there is an increasing variety of matter in surface water via seepage water to seawater and waste water. HM normally is accompanied by other matter (e.g. organic matter (OM), micro- and nano-particles (mP, nP), microorganisms (μorg), cations and anions (n+/n−)).
The rejection (R) of a membrane module for a specific constituent can be determined according to a mass balance (Eq. 2) for the concentrations in the feed (cF), in the obtained permeate (cp) and in the discarded concentrate, neglecting the amount remaining in the fouling layer on or in the membrane.
3.3 How refractory is HM?
In addition to several poorly defined terms of HM, there are not quantifiable expressions for their general properties. One of them is “refractory”. There is no doubt about the resistance of HM against rapid biodegradation, against fast weathering or chemical reactivity. However, there are still the questions on comparativity or age of the sample or on the power of the experimental and analytical tools applied. Aquatic HM and the application of advanced spectrometric methods may help to gain a realistic multidimensional picture of the humics scene. Size exclusion chromatography (SEC), e.g. with a selection of powerful detection systems for organic carbon and nitrogen detection, or other element and functional group specific methods combined with bioeffect quantification have opened the door to learn and understand the life-specific role of HM (Huber et al. 2011; Lankes et al. 2009; Müller and Frimmel 2002; Perminova et al. 2003)—even beyond the borders of our Earth.
The power of nanofiltration (NF) in cleaning the investigated ground water is quite obvious from the rejection of the typical HM-parameters DROM and SAK (Fig. 2). It is interesting to note that also in the parameters for the integrated organically bound halogens and the tri-halomethanes, all products from disinfection with chlorine (Johnson et al. 1982; Barrett et al. 2000; Zwiener 2006) follow the same tendency, which is in favour for nanofiltration as method of elimination. By far, the strongest effect can be seen for Ca2+. The strong decrease of hardness can be explained by the formation of rather stable HM-complexes, which can be best removed by nanofiltration. This also fits to the observation that natural HM-rich waters are mostly soft (Frimmel et al. 2002). Their use, even for drinking, needs a rethinking of the classical treatment strategy. Since disinfection with chlorine is only necessary in case of microbially polluted water, it can be neglected for raw water, which is free of germs. This can be achieved best by thorough water protection or sterile filtration (< 0.45 μm). The refractory character of HM hinders regermination in addition.
The final low molecular size fractions (tirr > 150 min) of the photocatalyticaly degraded HM contained several small organic acids (formic acid, oxalic acid, succinic acid, glutaric acid) were identified by ion chromatography (IC) (Fig. 3). They suggest the increasing biodegradability of HM by increasing photocatalytic degradation. This fits to the increasing biodegradability of HM after OH-radical attack in advanced oxidation processes (Morel and Hering 1993). In addition, it was found that the presence of metal ions (μmol/L range of Cu2+, Mn2+, Zn2+, Fe3+) may have an inhibiting effect on the yield of photocatalytic HM degradation (Tercero Espinoza et al. 2011).
There are acid/base reactions, (photo)redox reactions, complex formation, dissolution and precipitation, absorption and adsorption and a range of biologically driven degradation and growth processes (e.g. Thurman 1985; Buffle 1988; Specht and Frimmel 2000; Frimmel and Abbt-Braun 2006, 2012; Kamjunke et al. 2017).
Of special relevance in the anthropocene is the complexation of xenobiotics and toxic (heavy-) metals (Perminova et al. 2005). The HM-derived toxic—partly mutagenic—halogenated disinfection by-products in drinking water supply are a worldwide problem (Rook 1974; Suffet and MacCarthy 1989; Barrett et al. 2000).
A close look on the organic matter, which we call humics (HM, humic substances (HS)), reveals a highly complex system. Value and clearness of results from research in the field may often suffer from randomness and from operationally defined data. However, it is beyond doubt that this matter does exist and that it is—together with water—a fundamental part of life and mankind on earth.
Most abundant is HM in soil and in the biosphere. There it is responsible for fertility. The capability to trap water and to detoxify polluted seepage water gives HM a key function for food production and foresting and by this an eminent impact on the climate. A more problematic part of the ambiguous character of HM is reflected in its dust and dirt appearance forcified by the vehicle function for toxic metals and xenobiotics and also in the potential for the formation of mutagenic disinfection by-products in case of advanced water treatment (Suffet and MacCarthy 1989; Barrett et al. 2000). In general, however, HM acts as major stabilizer in the environment. Most of the easily biodegradable part of the former living matter has been taken away. The “dead” rest can be called refractory, including the remains of the starved and degraded organisms (at the end of their cannibalism). All that leftover material is a storage for living matter to grow with moderate speed, an example for recycling and sustainability of the best.
By this, HM is not only an interesting and valuable subject for research and education, but it is also one of the most precious resource we have. The responsible management of the humic world (Swift and Spark 2001), based on its profound understanding, must become one of the essentials of our survival strategy on Earth.
Financial support by the German Research Foundation (DFG), the State of Baden-Württemberg, and the German Association for Gas and Water (DVGW) is gratefully acknowledged.
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