Determining the role of humic substances in the fate of pesticides in the environment

Abstract

The data presented in this paper emphasize that the behavior and fate of pesticides in the environment is influenced by humic substances. Various methods most frequently used for the characterization of humic substances are discussed. Both humic acid and fulvic acid can solubilize in water certain organic compounds and are important carriers of some pesticides in soil. Humic substances have the potential for promoting the nonbiological degradation of many pesticides. Several methods of bleaching humus color from drinking water, including chlorination, ozonation, and UV‐radiation, are described. Finally, the photochemical stability to UV‐radiation of certain pesticides in aqueous fulvic acid solution is discussed.     

Organic compounds adsorption onto activated carbon: the effect of association between dissolved humic substances and pesticides

The quantitative determination of pesticide binding to dissolved humic substances is relevant to both water treatment operation using activated carbon adsorption process and the application of transport models that predict the environmental distribution patterns of a given hydrophobic contaminant. In this study and in a first set of experiments, the extent of binding between (i) three pesticides of environmental concern, aldicarb, lindane and pentachlorophenol, and (ii) dissolved commercial humic acid and soil extracted fulvic acid, was determined using dialysis experiments and water solubility enhancement tests. In a second set of experiments, the influence of dissolved humic substances or pesticide on the retention of the other co-adsorbate onto activated carbon was investigated in binary systems. It was found that association was negligible for aldicarb and that the pesticide sorption onto activated carbon was not affected by humic acid (8.5 mg liter(-1) DOC). The association constants K for lindane and pentachlorophenol were identical in the presence of fulvic acid (logK=4.1) but lower than that observed with humic acid. In the presence of humic acid, binding affinity for pentachlorophenol (logK=4.6) was higher than the one observed for lindane (logK=4.4), despite its much higher water solubility. This observation suggests that the aromatic character of the pentachlorophenol molecule contributes to association interactions with humic acid. From co-adsorption experiments onto activated carbon it was found that fulvic acid (7.7 mg litre(-1) DOC) slightly enhances sorption kinetics of pentachlorophenol. Lindane (1 mg litre(-1)) does not affect sorption kinetics for fulvic acid but markedly enhances both the sorption kinetics and adsorptive capacity for humic acid. Activated carbon retention of dissolved humic substances or pesticide appears to be enhanced by the association potential that exists between these co-adsorbates in some binary systems.     

Removal of organic polyelectrolytes and their metal complexes by adsorption onto xonotlite

Natural organic polyelectrolytes (humic and fulvic acids) and their metal complexes were removed by adsorption onto xonotlite. The removal percentages of humic and fulvic acids by xonotlite were approximately 80% and 30%, respectively. Humic acid removal from solution by adsorption onto xonotlite took place more readily than fulvic acid removal. The molecular weight distributions of the humic substances remaining in solution after adsorption with the xonotlite were measured with size exclusion chromatography. A comparison of molecular weight distributions demonstrated conclusively that large molecular weight components were adsorbed preferentially, indicating that adsorption efficiency depends on the number of functional groups of humic substances. Furthermore, the surface topography of the adsorbent was observed before and after adsorption by scanning electron microscopy. The calculated heat of adsorption was of 330 kJ mol(-1) which was evaluated from the Clapeyron-Clausius equation. Therefore, the adsorption type can be considered chemical. Since xonotlite can be easily synthesized and obtained at low cost, the adsorption method of humic and fulvic acids is superior to their precipitation.     

Dissolved organic matter mediated aquatic transport of chlorinated dioxins

The bioavailability and environmental fate of extremely hydrophobic environmental contaminants such as chlorinated dioxins is linked to their solubility characteristics in water. Solubilities of three chlorinated dioxins, viz., 1,2,3,7-T₄CDD, 1,2,3,4,7-P₅CDD, and 1,2,3,4,7,8-H₆CDD, were determined in pure water using a glass bead generator column technique, and their enhanced solubilities in the presence of several dissolved humic fractions quantified at 20, 30 and 40°C. The strengths of these interactions between chlorodioxins and the dissolved humic substances, viz., a fulvic acid, a humic acid, and Aldrich humic acid, were examined using simple thermodynamic calculations. A new partition/association coefficient, K_oc (mobile) is defined.     

Influence of dissolved humic substances on the leaching of MCPA in a soil column experiment

The influence of dissolved humic substances on the transport of (4-chloro-2-methylphenoxy) acetic acid (MCPA) in a sandy soil with a low organic carbon content was studied in a column experiment. Soil columns were eluted with aqueous solutions containing different fractions of humic substances. More than 70% of the applied compound was found in the leachate in all sandy soil experiments, but distinct differences were obtained depending on the composition of the eluent. The addition of both humic and fulvic acids to the eluent affected the leaching behaviour of MCPA. While the presence of humic acids increased and accelerated the movement of MCPA in the investigated sandy soil, fulvic acids caused the opposite effect: increased retention was observed relative to the control. We concluded that a possible carrier transport or retention strongly depends on the composition of the dissolved organic matter. Thus, changes in the composition of dissolved organic matter may affect MCPA movement into deeper soil layers.     

Charge characteristics of humic and fulvic acids: comparative analysis by colloid titration and potentiometric titration with continuous pK-distribution function model

Charge characteristics of humic and fulvic acids of a different origin (inshore soils, peat, marine sediments, and soil (lysimetric) waters) were evaluated by means of two alternative methods - colloid titration and potentiometric titration. In order to elucidate possible limitations of the colloid titration as an express method of analysis of low content of humic substances we monitored changes in acid-base properties and charge densities of humic substances with soil depth, fractionation, and origin. We have shown that both factors - strength of acidic groups and molecular weight distribution in humic and fulvic acids - can affect the reliability of colloid titration. Due to deviations from 1:1 stoichiometry in interactions of humic substances with polymeric cationic titrant, the colloid titration can underestimate total acidity (charge density) of humic substances with domination of weak acidic functional groups (pK>6) and high content of the fractions with molecular weight below 1kDa.     

Effects of aquatic humic substances on a hydrophobic ultrafiltration membrane

Natural humic surface water (pH 5.9), ion exchanged samples of the same water (pH 5.5), and aqueous solutions of isolated humic substances at pH 4.5, 5.5 and 6.5, respectively, were ultrafiltered (15°C, 0.5 bar) using hydrophobic polysulfone membranes (GR51) in a cross-flow flat sheet module. The used membrane did not completely retain natural organic matter from the surface water and the addition of complexing metals did not affect the retention any further. The changes which were induced in the membranes during each filtration run were studied by simultaneous streaming potential and flux measurements in 0.01 M KCl solutions. Zeta potentials were calculated based on the streaming potentials and the results showed changes towards more negative values for all the samples due to adsorption of organic matter onto the surface of the membrane pores. Humic acid affected the membrane charges more than fulvic acid. High ionic strength and low pH enhanced flux reduction and fouling. Filtration of natural waters caused more pore plugging and flux reduction than filtration of solutions of the isolated humic substances.     

Concentrations of monosaccharides in humic substances in the early stages of humification.

Deteriorated liquid packaging board (LPB) and biowaste compost are matrices, mainly consisting of cellulose, in the early stages of humification. Degradative studies on these matrices allow an examination of the role of carbohydrates in the synthesis of humic substances. Samples of different age were collected and divided by extraction into hot water extract (HWE), bitumen, humic acid (HA), fulvic acid (FA) and humin or residual fibre fractions. The following monosaccharides were identified in these fractions: L-arabinose, D-ribose, D-xylose, L-fucose, D-mannose, D-fructose, D-galactose, D-glucose, L-rhamnose and xylitol. The main component in all fractions was glucose. The concentrations of monosaccharides in humic acids (HAs) of LPB ranged from 67 to 503 mg/g of organic matter, and the concentrations in HAs of compost from 52 to 101 mg/g. As a general trend, the concentrations of monosaccharides decreased during LPB degradation and composting in all fractions. At the same time the relative amounts of D-xylose, D-mannose and D-galactose increased in HAs of compost samples.     

Comparison of photochemical behavior of various humic substances in water: III. Spectroscopic properties of humic substances

Spectral absorption coefficients and fluorescence quantum efficiencies were determined for humic substances from a variety of sources. Specific absorption coefficients k_h, for humic substances at wavelengths λ from 300 to 500 nm can be closely described by the relation Ae^B(450-λ), where A and B are constants. When the k_h values are in units of liter (mg organic carbon)^?1meter^?1 and wavelength λ is in nanometers, mean values of A and B for aquatic humus in the 12 water bodies studied were 0.6±0.3 and 0.014±0.001, respectively. Spectral absorption coefficients of dissolved organic matter in blackwater rivers, of the “yellow substance” in the sea, and of fulvic acids extracted from soils are very similar. Fluorescence quantum yields of humic substances were low and more variable than the absorption coefficients, ranging from 0.0005 to 0.012 with excitation at 350 nm (average of 0.0045±0.0038 for 6 waters). Fluorescence spectra for the humic substances were remarkably similar with maximum emission at 430 to 470 nm. Results of this study can be used to compute photolysis rates of pollutants as a function of depth in natural water bodies.     

Influence of dissolved humic substances on the mass transfer of organic compounds across the air-water interface

The effect of dissolved humic substances (DHS) on the rate of water-gas exchange of two volatile organic compounds was studied under various conditions of agitation intensity, solution pH and ionic strength. Mass-transfer coefficients were determined from the rate of depletion of model compounds from an apparatus containing a stirred aqueous solution with continuous purging of the headspace above the solution (dynamic system). Under these conditions, the overall transfer rate is controlled by the mass-transfer resistance on the water side of the water-gas interface. The experimental results show that the presence of DHS hinders the transport of the organic molecules from the water into the gas phase under all investigated conditions. Mass-transfer coefficients were significantly reduced even by low, environmentally relevant concentrations of DHS. The retardation effect increased with increasing DHS concentration. The magnitude of the retardation effect on water-gas exchange was compared for Suwannee River fulvic and humic acids, a commercially available leonardite humic acid and two synthetic surfactants. The observed results are in accordance with the concept of hydrodynamic effects. Surface pressure forces due to surface film formation change the hydrodynamic characteristics of water motion at the water-air interface and thus impede surface renewal.     

Formation and release of residues of the 14C-labelled herbicide isoproturon and its metabolites bound in model polymers and in soil

The humic monomer catechol was reacted with (14)C-isoproturon and some of its metabolites, including (14)C-4-isopropylaniline, in aqueous solution under a stream of oxygen. Only in the case of (14)C-4-isopropylaniline, incorporation in oligomers, in fulvic acid-like polymers, and in humic acid-like polymers by covalent bonds was observed. The main oligomer was identified by mass spectrometry as a trimer, 4,5-bis-(4-isopropylphenylamino)-3,5-cyclohexadiene-1,2-dione. Biomineralization of (14)C-compounds to (14)CO(2) in a loamy soil and release of (14)C from soil columns into percolate water decreased in the order: free isoproturon >free 4-isopropylaniline>fulvic acid-like polymers>trimer>humic acid-like polymers. In soil columns, a small but measurable migration of (14)C from polymers from upper to deeper soil layers could be detected; most of this (14)C was bound again in a non-extractable form. It is concluded that aniline-derived pesticides bound in soil by covalent binding may not be fully undegradable, nor fully immobile.     

Competition between alga (Pseudokirchneriella subcapitata), humic substances and EDTA for Cd and Zn control in the algal assay procedure (AAP) medium

The chemical speciation of trace metals in natural waters has important implications for their biogeochemical behavior. Trace metals are present in natural waters as dissolved species and associated with colloids and particles. The complexation of one trace metal (Cd and Zn at 200 and 390 microg/l respectively) with a green alga Pseudokirchneriella subcapitata in colloid-free algal culture medium and in presence of colloidal humic substances (HS) is presented. The influence of the nature of colloids was also addressed using three "standard" HS: fulvic acid (FA) and, soil (SHA) and peat humic acids (PHA). The chemical speciation model, MINTEQA2, was used to simulate the influence of pH and standardized culture medium on metal association with humic substances. The model was successfully modified to consider the differences in the metal complexation with fulvic (FA) and humic acids (HA). The deviations of concentrations of metals associated with HS between experimental results and model predictions were within a factor of approximately 2. The results of speciation model highlight the influence of the experimental conditions (pH, EDTA) used for alga bioassay on the behavior of Cd and Zn. The computed speciation suggests working with a pH buffered/EDTA-free mixture to avoid undesirable competition effects. The behavior of Cd and Zn in solution is more strongly influenced by HS than by alga. Metal-HS associations depend on metal and humic substance nature and concentration. Cd is complexed to a higher extent than Zn, in particular at larger HS concentration, and the complexation strength is in the order FA相似文献   

Copper binding by peat fulvic and humic acids extracted from two horizons of an ombrotrophic peat bog

We studied the binding of Cu(II) to humic acids and fulvic acids extracted from two horizons of an ombrotrophic peat bog by metal titration experiments at pH 4.5, 5.0, 5.5, and 6.0 and 0.1 M KNO3 ionic strength. Free metal ion concentrations in solution were measured using an ion selective electrode. The amounts of base required to maintain constant pH conditions were recorded and used to calculate H+/Cu2+ exchange ratios. The amount of Cu(II) bound to the humic fractions was greater than the amount bound to the fulvic fractions and only at the highest concentrations of metal ion the amount of Cu(II) sorbed by both fractions became equal. The proton to metal ion exchange ratios are similar for all humic substances, with values ranging from 1.0 to 2.0, and decreasing with increased pH. The amount of Cu(II) bound is practically independent of the horizon from which the sample was extracted. The results indicate that the humic substances show similar cation binding behaviour, despite the differences in chemical composition. The copper binding data are quantitatively described with the NICA-Donnan model, which allows to characterize only the carboxylic type binding sites. The values of the binding constants are higher for the humic acids than for the fulvic acids.     

A comparative characterization of dissolved organic matter by means of original aqueous samples and isolated humic substances

The aim of our study is to test the use of less time-consuming spectroscopic methods applied on original water samples in order to obtain information about DOM composition without any sample preparation. These results were directly compared with results from a conventional isolation and characterization procedure of dissolved humic substances (fulvic acids – FA) isolated from the same water sample. FAs were characterized by UV-, fluorescence-, FTIR spectroscopy and elemental composition. UV absorbance and fluorescence behavior of FAs and original water samples follow the same pattern. A lower UV absorbance and a lower humification index (derived from the synchronous fluorescence spectra) of about 15% is typical for water samples compared to the FAs. We computed linear relationships between properties of the original water sample (UV-, synchronous fluorescence spectra) and the isolated FA (IR absorption, C/N ratio). The application of synchronous fluorescence and UV spectroscopy of aqueous samples has been proved to result in similar information about DOM composition as the characterization of isolated humic substances concerning the content of aromatic structures and the degree of humification.     

Effects of humic substances on the decomposition of 2,4-dichlorophenol by ozone after extraction from water into acetic acid through activated carbon

The objectives of this study are to clarify the behavior of humic substances throughout the processes of 2,4-dichlorophenol (2,4-DCP) adsorption on granular activated carbon (GAC) from water and extraction into acetic acid, and the influence of the extracted humic substances on the decomposition of 2,4-DCP by ozone in the acetic acid. The adsorption capacity of GAC for 2,4-DCP was not influenced by the humic substances preloaded to have equilibrium concentration of 24.9mg Cl(-1) (14.5mg Cg(-1)). The adsorption capacity of GAC for 2,4-DCP decreased to one tenth of new GAC after the first adsorption-extraction step because of only 16% desorption in the first step. However, 2,4-DCP adsorbed on GAC was completely extracted after the second step suggesting that GAC can be used as adsorbent to transfer 2,4-DCP from water to acetic acid. The concentration ratio of 2,4-DCP from water into acetic acid was around 2x10(5), whereas the concentration ratio of humic substances was about 3.5, indicating that 2,4-DCP was selectively adsorbed and extracted by this system. The first order degradation rate constant for 2,4-DCP by ozone in acetic acid increased with the addition of humic substances. The rate constant with 16mg Cl(-1) of humic substances was 2.6 times as high as that without humic substances. Humic substances behaved as a promoter for the degradation of 2,4-DCP by ozone.     

Significance of analytical parameters for the understanding of natural organic matter in relation to photocatalytic oxidation

In this review, special interest was devoted to provide information on the surrogate parameters expressing both quality and quantity of organic matter for the understanding of the photocatalytic oxidation of humic substances. Detailed investigation was directed to the application of photocatalysis with reference to source, origin and modeling of organic matter. Evaluation of the literature findings emphasizes that organic matter taken from natural waters are site specific and should be characterized in detail to be comparable to other studies. Taking into account the photocatalytic degradation studies of natural organic matter, humic substances, humic acids and fulvic acids in slurry systems, a procedure could be deduced that depends on the selection of a standard model sample with a representative concentration, selection of a standard photocatalyst and dose (e.g., TiO₂ Degussa P-25, 0.25 mg mL⁻¹), application of standardized reaction conditions such as light intensity, pH, and temperature. Furthermore, standardized filtration step avoiding organic leaching and selection of the most suitable analytical parameter are the crucial points to be considered. The use of such a protocol could form a basis for the determination of “relative degradation efficiency” of any sample containing natural organic matter, humic substances, humic acids and fulvic acids regardless of dependency on source and origin.     

城市污水二级出水中溶解性有机物特性分析

分别采用凝胶色谱、亲疏水性组分分离、荧光色谱等方法,研究了城市污水处理厂二级出水中溶解性有机物的分子量分布、亲疏水组分含量以及荧光光谱特性。结果表明,二级出水中疏水性组分较亲水性组分多,疏水性组分约占总有机物的64.3%,而亲水性组分占35.7%左右;二级出水及其不同亲/疏水组分中溶解性有机物分子量分布基本集中在4.5 kDa以下,其中弱疏水性组分和亲水性组分中主要为分子量小于1.5 kDa的有机物;二级出水溶解性有机物中含有腐殖酸类、富里酸类以及蛋白质类物质,其中含量以腐殖酸类为主。     

Influence of humic substances on the photolysis of aqueous pesticide residues

The present work investigated the direct and indirect photolysis of pesticide residues (atrazine, imazaquin, iprodione), in aqueous solutions and under UV-visible radiation (280-480nm). Different kinds of humic substances (HS) were added to samples in order to evaluate their behaviour as possible photocatalysts and their effect on the photolysis of pesticides. The fulvic acids were purchased from the International Humic Substances Society, and they were added to samples in concentrations ranging from 1 to 150 mgl(-1). Titanium dioxide was used as the photocatalyst, in concentration ranging from 10 to 150 mgl(-1). Pesticides photolysis were measured by UV-visible absorption spectroscopy and differential pulse polarography with all used pesticides, reaching total degradation after 2h of irradiation, thus indicating a fast direct photolysis. Photocatalysis by TiO(2) could increase the pesticides photolysis rate up to 40%. This effect, however, was not observed for imazaquin photolysis. Again, except for imazaquin, HS presence showed a positive effect in increasing pesticide degradation, but only within specific concentration ranges (below 10mg l(-1) for iprodione and about 30mgl(-1) for atrazine). Above these ranges HS induce a decrease in the pesticides photolysis rate. Spin-trapping measurements by electronic paramagnetic resonance spectroscopy, using the spin-trap DMPO, showed that HS are able to photogenerate hydroxyl radicals, increasing the pesticides molecule degradation. However, the HS also react with the photogenerated hydroxyl radical, influencing the pesticide photolysis, leading to a decrease in the photolysis rate and causing it to be strongly dependent on the nature and concentration of residues in the water to be treated.     

Fulvic and humic acids isolated from groundwater: Compositional characteristics and cation binding

Samples of humic substances were obtained from a waterworks at Fuhrberg, Germany. The material had a bimodal molecular size distribution with 40% of the total carbon in the 50,000–100,000-D (nominal molecular weight, NMW, in daltons) size fraction and 50% of the carbon in the <10,000-D (NMW) size fraction. The fulvic and humic acids isolated from the bulk humic substances were low in nitrogen content and had low H/C atomic ratios. Furthermore, the fulvic and humic acids had very similar elemental, spectral and copper binding characteristics. Over 70% of the carbon in both the fulvic and humic acids was present in aromatic or aliphatic groups, with ¹³C NMR analyses indicating approximately even distribution among the two types. Competitive elemental binding studies indicated that Ca²⁺, Mg²⁺, Al³⁺ and Fe³⁺ do not effectively compete for copper binding sites on these compounds. In humic acids, these cations are predominantly bond by carboxylic groups.