Use of Humic Substances to Remediate Polluted Environments: From Theory to Practice

An overview is given of the interactions encountered between humic substances (HS), ecotoxicants, and living organisms in the context of environmental remediation. The most important interactions identified include: binding interactions affecting chemical speciation and bioavailability of contaminants; interfacial interactions altering physical speciation or interphase partitioning of ecotoxicants; abiotic-biotic redox interactions that influence metabolic pathways coupled to pollutants; and finally direct and indirect interactions coupled to various physiological functions of living organisms. Because humics are polyfunctional, they can operate as binding agents and detoxicants, sorbents and flushing agents, redox mediators of abiotic and biotic reactions, nutrient carriers, bioadaptogens, and growth-stimulators. It is shown that these functions possess significant utility in the remediation of contaminated environments and as such humic-based reactions pertinent to permeable reactive barriers, in situ flushing, bioremediation, and phytoremediation are examined in detail. Finally, this chapter introduces the novel concept of “designer humics” which are a special class of customized humics of the reduced structural heterogeneity and of the controlled size. They are developed and deployed to carry out one or more of the above in situ functions in an optimum manner and for the purpose of enhancing the efficacy of one or more remediation technologies. Designer humics possess specified reactive properties obtained by chemical modification and cross-linking of the humic backbone. This new class of reactive agents portend new opportunities for achieving enhanced remediation and for quantifying remediation performance. The latter is described in the context of the passive flux meter technology developed for direct measuring fluxes of contaminants and biomass.

Soil organic matter (SOM) is the most important source of nutrients and regulates the main physical, chemical, and biological properties of different soils. A fundamental set biogeochemical nature of the humification process, the effects of the thermodynamics and kinetics directing the synthesis and degradation of humic substances (HS), the variability of the properties of SOM and humic substances. In this respect, important features are the physico-chemical diagnostics and identification of humic acids, the special role of mineral components in the humification process, the fundamental questions regarding the authenticity humic acids, and the knowledge derived from the study of altered and denatured products (e.g. industrially manufactured humic-like substances. Five important functions of humic substances provide the opportunity to evaluate the ecological role of humus: 1) accumulative, 2) transport, 3) regulatory, 4) physiological, and 5) protective. The latter involves the ability of humic substances to bind different pollutants: heavy metals, pesticides, radionuclides, and the surplus of fertilizers. Various detoxification mechanisms operate in the soil-plant system in highly diverse natural environments. The relative stability of soil as a whole mainly depends on the stability of humic substances and its fractions, of which humic acids and humin seem to be the most significant.

The chemistry of humic materials has been under world wide investigation for about a 100 years. The pace of the research continues to increase. During at least the last 30 years, evidence has been accumulating in the literature that the principles of classical chemistry as they are understood for monomeric reagents can to some extent be adapted to humic polyelectrolyte mixtures. Published demonstrations of predictive chemical calculations for humic materials have resulted from this. There is a need for the predictive capability to be improved, and applied to environmental remediation and regulatory practice.

Humic substances (HS) are a quantitatively and qualitatively important component of soil organic matter (SOM). Due to their diverse associations with trace elements, they have a special relevance for many geochemical processes, the mobility and bioavailability of pollutants and micronutrients in the soil environment and can find special application as immobilisers of contaminants for remediation purposes.

In the current paper the role of HS in the soil environment is shortly addressed, followed by a review of the binding types and strengths of humic-trace metal complexes found in the literature. The most commonly employed methods for examining these interactions are described. Finally, evidence for stimulatory effects of HS on plant growth is presented, as the result of a better provision with the essential micronutrients Fe and Zn.

The interaction of metals and natural organic matter (NOM) is of influence on the metal transport in aquatic systems and soil. In the presented study, NOM of a brown water lake was oxidized with UV irradiation to elucidate the influence of oxidative degradation on the metal complexing properties of NOM. UV/Vis and fluorescence spectroscopy, as well as size-exclusion chromatography (SEC) with online detection of UV absorption, fluorescence and metal concentration were used to investigate the property changes of NOM caused by oxidation. After oxidation, the fluorescence intensity of NOM increased considerably despite a decrease in the UV absorption. The SEC experiments showed a shift towards smaller molecular sizes in the oxidized NOM samples and a decrease in the stability constants of corresponding Al-NOM complexes. For Pb no such effect could be determined. The studies on the dissociation kinetics of metal-NOM complexes revealed a slower dissociation of Al and Pb complexes with original NOM compared to oxidized NOM. The determined dissociation rate constants were used to predict the migration of metal ion-NOM complexes in column experiments. The experimental data and model predictions were in good agreement for the divalent metals Pb and Zn, but differed substantially for Al.

The stability of the metal complexes with original NOM decreased in the order Al > Pb > Zn. After UV-oxidation, the complexing ability of NOM towards Al- ions was decreased. In NOM equilibrated quartz columns, Pb and Zn were immobilized in the presence of all types of NOM, whereas Al was readily eluted. The original NOM was more effective for complexing Al than the oxidized form. Colloid formation however can stimulate the Al migration.

Results of long-term investigations of the heavy metal content and speciation in the Dnieper water bodies (Ukraine) are presented. The particular importance was given to the study of dissolved organic matter (DOM) composition, to binding affinity of metals such as Fe, Mn, Cu, Zn, Pb, Cr, and Cd for DOM, and to molecular weight distribution and chemical nature of metal-DOM complexes. The chemiluminescence and anodic stripping voltammetry in combination with membrane filtration, gel permeation chromatography, and ion-exchange chromatography were used for this purpose. It is shown that complexation with natural organic ligands plays the major role in the fate of HM in fresh water systems. The prevailing fraction of dissolved metals is found to present as complexes with humic substances, mainly, with fulvic acids. For the evaluation of the stability of aquatic ecosystems to the toxic action of HM, the potential complexing ability of DOM was investigated. The results on copper toxicity are presented obtained in long-term experiments with distilled and tap waters as well as with the natural water from the Kanev reservoir containing additives of copper ions and humic acids (HA). The toxic effect of free Cu²⁺ ions is determined using biotests with Daphnia magna. It was established that a decrease in integral water toxicity correlated with a reduction in free Cu²⁺ concentration in water medium. Hence, Cu²⁺ ions present in aquatic media in the form of non-toxic complexes with organic ligands. The maximal decrease in toxicity was observed in natural water where the complexation occurred with participation of both DOM and added HA.

Humic substances are ubiquitous compounds presenting specific properties with respect to complexation, sorption and transport. Because they may enhance metal solubilities due to their strong complexing properties, as well as increase or decrease metal retention on mineral surfaces due to their affinity for mineral substrates and their scavenging properties, humic substances may impact metal speciation in the environment, and in particular radionuclides speciation. Various examples based on actinides, lanthanides or iodine will be presented to illustrate the complexation properties as well as speciation calculations with emphasis on the thermodynamic constants used and the techniques used to obtain them.

Humic acids (HAs) and fulvic acids (FAs) are the main components of natural waters that can influence the migration ability of cations. They form stable complex compounds with actinide elements that can change their solubility. In addition, HAs could be responsible for migration of colloidal species. The role of HAs in formation of geochemical barriers in actinide migration in the geosphere is discussed on the basis of results of laboratory experiments and speciation of plutonium and neptunium in field samples collected from the regions contaminated by human-made radionuclides.

Effective remediation technologies for heavy metal polluted soils have not been developed until now. The currently used techniques, such as flushing or immobilizing washing, binding heavy metals to non-toxic forms, and others, have substantial limitations: they are very costly and are applicable to only limited soil types.

The presented research was devoted to a use of brown coal to remediate soils polluted with heavy metals. Brown coal contains 46% (mass) of humic acids that can bind heavy metals into complexes. In addition, brown coal can bind heavy metals by physical absorption.

Sorption-desorption behaviour of heavy metals onto brown coal was studied in several model experiments. The sample taken from the upper horizon of zonal soil was mixed first with different forms of metals. After a month of incubation in vessels, the sample was mixed with various amounts of brown coal. In another experiment, the samples taken from the upper layers of polluted urban soils were mixed with various amounts of brown coal. These soils were put into glass vessels for long-term incubation under optimum temperature and humidity conditions. The soil samples were taken out after a week, 1 month, or 6 months. The amounts of heavy metals in mobile forms dissolved in ammonium-acetate extract with pH 4.8 were measured using atomic absorption spectrometry and the polarographic method. The results were checked under field conditions, and the results showed that brown coal is an effective ameliorant of soils polluted by heavy metals.

HOM enriched areas within aquifers retain or bind organic pollutants by sorptive or specific interactions. This contribution briefly reviews some established concepts on in situ remediation for removal of dissolved organic contaminants from groundwater using reactive and sorptive barriers. In particular, we present a new concept for use of HOM humicc organic matter) as sorptive barrier. According to this concept HOM is sorbed to metal salt surface precipitates on aquifer minerals. Induced by sorptive interactions HOC (hydrophobic organic carbon) groundwater pollutants are retarded while passing the HOM barrier. An extended residence time within the aquifer subjects the pollutant plume to extensive microbial degradation and diminished spreading. We give information on pertinent laboratory experiments and field considerations describing design and possible ways to construct HOM barriers in situ in the subsurface by infiltration techniques. With regard to sorptive pollutant removal we discuss the mechanisms that drive the binding of HOM to mineral surfaces and those that govern the interactions between HOC and HOM. Operational approaches to determine sorption coefficients of HOC on HOM are conferred. The sorption of HOC on HOM in flocculated, surface-bound, and the freely dissolved state is compared.

Practical recommendations on use of AHS as flushing agents are given. The use of AHSs at levels > 1 g/L would most effectively flush hydrophobic contaminants (e.g., PAHs). Inexpensive, naturally-derived, non-toxic commercial humic products may offer significant advantages compared to other chemical flushing agents (e.g., surfactants). It may be possible to use AHSs for a combination of flushing, enhanced bioremediation and/or sequestration of organic contaminants in aquifers.

For many centuries soil has become contaminated by human activities, but only recently has soil pollution was considered a threat for man and environment based on increasing knowledge about the effects pollutants may have and on ways to detect them. Now, society is faced with an increasing number of polluted sites that are discovered and considered to need treatment. Although excavation and treatment of the excavated soil seems the most definitive measure, costs of such an operation and damage to buildings and infra structure prevents such an approach. For that reason (biological) in situ techniques have been developed and finally accepted. When carried out properly, in situ remediation leads to environmentally sound solutions in accordance with (the Netherlands) regulations. This does not mean that problems are not encountered. The principles of in situ treatment are discussed in this paper as well as the problems which are associated with certain types of contaminants and possible solutions for these problems. The paper is based on personal experience in the field of the development of in situ techniques, the performance of a large number of in situ projects and the problems to be solved in carrying out this task.

Current wastewater treatment processes generally improve the quality of effluent water by enhancing the efficiency of the pollutant reduction process or by increasing the retention time of the wastewater in the treatment facility. The latter approach, however, requires a larger aeration basin and higher operation cost.

One of the most promising approaches to improve the efficiency and increase the capacity of wastewater treatment plants without increasing size is based upon application of natural zeolites in the aeration basin. Zeolite particles are good carriers of bacteria, which adsorb on the zeolite surface resulting in increased sludge activity. There is a significant drawback to the application of the zeolite additive. Formation of the bacteria layer on the zeolite surface is a slow process and becomes effective only after approximately a week.

A new zeolite modification method accelerates the interaction between zeolites and activated sludge, which further increases the sludge activity. The effects of modified zeolites on organic degradation rates have been tested in laboratory and full-scale experiments. Based on the experiments, new industrial technologies for zeolite modification and wastewater treatment have been developed.

Mitigating activity of HS can be defined as a phenomenon of lowering the adverse effects of contaminants toxicity and of those of abiotic stress factors such as unfavourable temperature, pH, salinity, etc. As a rule, it is related to the detoxifying properties of HS or to their beneficial effects on biota. This review focuses on the latter effects and considers the possible mechanisms of mitigating activity of humic materials in terms of biological activity of HS. The beneficial effects of HS on biota are segregated into the four categories according to the underlying mechanism of their action: an influence on the organism development; an enhancement in nutrient supply; catalysis of the biochemical reactions; and an antioxidant activity. The published data on the mentioned essential biological functions of HS are reviewed.

The given review shows that in spite of the numerous reports published recently to elucidate the mechanisms underlying biological effects of HS, the data reported are quite contradictory. The main reason is the complexity of HS structure. A wide variety of the effects observed can be explained by the polyfunctionality of HS. The general conclusion can be made — all biological effects of HS have been more pronounced under stress conditions independent of what stress factors are involved. Thus, HS can be considered as an environmental modulator mitigating the harmful consequences of stress factors.

Mechanisms of humic substances (HS) physiological activity in relation to different organisms at a molecular—cellular level were studied. Adaptation mechanisms of HS action under radiation and chemical injuries were examined. Antimutagenic and detoxifying properties of HS were revealed and a usage of physiologically active HS for remediation of both biotic and abiotic components of the environments and human's health recovery was proved. By the example of industrial area nearby the Dnepr River the HS applicability for remediation of disturbed soil was demonstrated. Experimental evidences of reduction of genetic damage caused by chemical and physical stress factors in the presence of HS were obtained using various bioassay techniques. A theoretical basis for HS application to retard uptake of contaminants by plants from soil and to improve biota resistance towards anthropogenic factors was developed. Recommendations on improving environmental conditions, recovering human health, and remediation of polluted areas were elaborated based on application of HS.

In the present work, we attempted to elucidate the mechanisms of soil humic acids associations with some proteases. The stability of humic-enzyme complexes and the ways humic acids influence the activity of enzymes were determined. It was shown that humic acids are non-specific inhibitors of proteases. Enzyme-inhibitor complexes are formed by electrostatic, hydrophobic and other types of intermolecular interactions. The protease inhibiting ability of humic acids is influenced by their interaction with metal ions. The stability of enzyme-inhibitor complexes that form by association of proteases with humic-metal complexes depends on the nature of the metals and on the number of coordination centres on the polymeric chains of humic acids.

Humic substances are natural organic compounds recalcitrant to biodegradation. Their structure is rich in quinone moieties that provides redox activity of humics. The evidence provided in this review indicates that quinonoid substructures of humics can play a number of significant roles in the anaerobic (bio)transformation of a wide variety of organic and inorganic compounds, including priority pollutants. Quinones can support the anaerobic microbial oxidation of different organic compounds by serving as terminal electron acceptors. Quinones, once being reduced, can also channel electrons to several distinct contaminants by acting as redox mediators. Reduced quinones can also serve as electron-donating substrates to bacteria supporting their growth.

The explosive 2,4,6-trinitrotoluene is a substance of environmental concern because of its toxicity. In reducing environments TNT is reduced to amino-metabolites that are immobilized into soil organic matter. The humification of TNT was tested with good results for the cleanup of contaminated soil and water. Molasses proved to be a suitable source of organic carbon for the biological reduction.

Properties of three samples of coal-derived commercial humates (CH), produced as organic fertilizers by Lomonosov Moscow State University the same company, were examined. Despite the same origin, the samples possessed different properties even with respect to such basic parameters as content of C, N, and humic acids (HA). Two of the CH samples (CH-1 and CH-2) studied were very similar, but CH-3 differed significantly in having a lower content of C and HA; a higher content of N and FA-fraction; and in the character of molecular-weight distribution. The CH samples with similar properties (CH-1 and CH-2) exhibited different growth-stimulating effects; CH-2 was less effective. The least effective was CH-3 in spite of the highest N content. This brought us to the conclusion that either CH-bound N is unavailable for plants, or the amount and quality of HA are more important for growth-stimulating effects of CH than the total amount of nutrients. High application rates of CH inhibited plant development in spite of the higher nutritional value, which indirectly proves their physiological activity. Other possible reasons of inhibiting action of CH could be toxicity of their high doses, or inactivation of bound to HA nutrients.

There is a great variety of soil types in Ukraine. Recently, some industrial regions of the country have encountered the problem of soil degradation due to technogenic pollution. Technogenic soil contamination with heavy metals has been found in the mining and metallurgical regions in the southeast part of Ukraine. Within Ukraine, 79% of all atmospheric emissions occur in the Donetsk-Pridneprovsky region. In the Dniepropetrovsk region, the concentration of mining and metallurgical production exceeds average indexes 7 to 10 times.

The zone of radioactive anomaly caused by the Chernobyl disaster is characterized by complex pollution of the environment. The objective of our work was to elaborate methods of soil detoxification with humates and to reduce the risk of plant contamination with heavy metals, radionuclides, and herbicides. Studies aimed to decrease the negative impact from industrial pollution of soils by heavy metals and radionuclides were conducted in the areas of industrial air and soil pollution in various areas of Ukraine. At the same time the risk of increased total toxicity from the combination of heavy metals and some herbicides presented in the local soils was also taken into account.

In the laboratory experiment a humic preparation “huminate” promoted benzoxazinones formation in the barley. Benzoxazinones formation further led to a 20% decrease of atrazine content in the seedlings. Introduction of humate preparation into the soil reduced also atrazine concentration to the trace concentrations. The protective action of peat, coal-derived humates and biohumus in relation to toxicity of heavy metals to plants was also studied.

A combination of two kinds of trials (introduction in soil and treatment of seeds before planting) for humic substance application led, as a rule, to increase of the barley productivity. The introduction of the vermicompost product (biohumus) into the soil reduced the negative heavy metals impact and the atrazine after-effect on barley growth. In particular, the introduction of biohumus and preliminarily processed poultry dung into the soil increased the barley yield by 12–27%.

NMR spectroscopy offers uniquely versatile options to obtain molecular level structural characterization of natural organic matter (NOM) and humic substances (HS). By defining the relative amounts and some structural detail of fundamental building blocks, multinuclear quantitative one dimensional NMR spectroscopy provides the key margin of any structural model of NOM/HS. A large array of higher dimensional NMR spectra serves to enhance the reliability of NMR resonance assignments and allows to define rather extended substructures of NOM/HS.

Owing to the nature of NOM/HS as a mixture of small and large molecules, related by a continuous range of weak and strong interactions, any exhaustive analysis of the NMR spectra of NOM/HS requires understanding of the physical processes initiated by the pulse sequences. The NMR spectral analysis has to be corroborated at all levels by comparison with NMR spectra of model compounds, data base search and adjusted schemes of calculations of NMR parameters.