Humic Acid Layer Research Articles

Cation-π Interactions Contribute to Hydrophobic Humic Acid Removal for the Control of Hydraulically Irreversible Membrane Fouling.

Hydraulically irreversible membrane fouling is a major problem encountered during membrane-based water purification. Membrane foulants present large hydrophobic fractions, with humic acid (HA) being a prevalent example of hydrophobic natural organic matter. Furthermore, HA contains numerous aromatic rings (π electrons), and its hydrophobic interactions are a major cause of irreversible membrane fouling. To address this issue, in this study, we used the cation-π interaction, which is a strong noncovalent, competitive interaction present in water. Because the strength of cation-π interactions depends on the combination of cations and π molecules, utilizing the appropriate cations will effectively remove irreversible fouling caused by hydrophobic HA. We performed macroscale experiments to determine the cleaning potential of the test cations, nanomechanically analyzed the changes in HA cohesion caused by the test cations using a surface force apparatus and an atomic force microscope, and used molecular dynamics simulations to elucidate the HA removal mechanism of test studied cations. We found that the addition of 1-ethyl-3-methylimidazolium, an imidazolium cation with an aromatic moiety, effectively removed the HA layer by weakening its cohesion, and the size, hydrophobicity, and polarity of the HA layer synergistically affected the HA removal mechanism based on the cation-π interactions.

Direct Measurements of the Forces between Silver and Mica in Humic Substance-Rich Solutions.

Deposition of engineered nanoparticles onto porous media from flowing suspensions is important for soil and groundwater quality. The deposition mechanism is controlled by interaction forces between particles and collectors. We investigated the origin and magnitude of opposing forces between silver and mica surfaces (representing nanosilver and sand grains) in solutions relevant to agricultural soils with direct measurements using a surface force apparatus. Solutions of variable NaNO3, Ca(NO3)2, and humic acid (HA) concentrations were used to differentiate individual contributing forces and quantify surface properties. The measured Hamaker constant for silver-water-mica was consistent with Lifshitz theory. Our results indicate that HA forms an adsorbed surface layer, but its charge, thicknesses, compressibility, and mass are significantly larger on mica than silver. Ca2+ primarily reduced the differences between the initially adsorbed HA layer properties on each surface, making them more similar. Force-distance profiles indicate that, when silver-mica systems were exposed to HA, osmotic-steric, electrostatic, and van der Waals forces dominate. Soft particle theory was deemed inappropriate for this system. Derjaguin's approximation was utilized to translate force measurements into interaction energy between nanosilver particles and mica collectors. We propose attachment efficiency estimates from measured surface properties, which suggest high particle mobility when nanosilver is applied to HA-rich agricultural soils with modest ionic strength.

Re-collectable and recyclable epichlorohydrin-crosslinked humic acid with spinel cobalt ferrite core for simple magnetic removal of cationic triarylmethane dyes in polluted water

Abstract Spinel cobalt ferrite (CoFe2O4) was successfully functionalized with humic acid (CoFe2O4-HA) via hydrothermal method. In order to prevent detachment, the humic acid layer on the CoFe2O4-HA surface was crosslinked with epichlorohydrin to obtain CoFe2O4-HA-ECH. Synthesized adsorbents were then tested for their ability to remove of basic fuchsin (BF), methyl violet 2B (MV), and malachite green (MG) from aqueous solutions. Particle size analysis, vibrating sample magnetometry, scanning electron microscopy, X-ray diffractometry, Fourier transform infrared spectrophotometry, thermogravimetric analysis, and ζ-potential analysis were conducted to characterize the as-synthesized adsorbents. The effects of adsorption parameters including pH, contact time, initial dye concentration, temperature, and ionic strength were explored. The kinetics data fitted well with a pseudo-second order model with Coefficient of Determination (R2) ≥ 0.998, Chi-squared (χ2) ≤ 0.171, and Average Relative Error (ARE) ≤ 3.443, suggesting that adsorption is the rate-limiting step. The Langmuir isotherm model provided R2 ≥ 0.999, χ2 ≤ 0.025, and ARE ≤ 0.891, indicating that adsorption occurs on a single layer on a homogenous surface. The maximum adsorption capacities of CoFe2O4-HA-ECH were 96.494, 62.627, and 48.74 μmol ⋅ g−1 for BF, MV, and MG, respectively, which were 10 times higher than those of CoFe2O4. Thermodynamic studies suggested that the adsorption processes are spontaneous-endothermic. Furthermore, CoFe2O4-HA-ECH can be re-collected and recycled for up to 10 cycles.

Development of antifouling poly(vinylidene fluoride) ultrafiltration membrane with the addition of polyethylene glycol as additive

Poly(vinylidene fluoride) (PVDF) is commonly used to fabricate ultrafiltration (UF) membrane due to its excellent thermal stability and chemical resistance. However, it is hydrophobic and hence has higher fouling tendency due to hydrophobic interaction with foulants. In this study, the antifouling properties of PVDF UF membranes was enhanced through the addition/blending of hydrophilic polyethylene glycol (PEG) of varying molecular weight. The addition of low molecular weight (200 Da, 4000 Da) PEG enhances the membrane’s pure water permeability (from 22.049 L/m2 hr bar to 24.791 L/m2 hr bar) which could be explained by the improved surface wettability. However, the addition of large PEG (35k Da) on other hand reduces the membrane’s pure water permeability (from 20.408 L/m2 hr bar to 9.181 L/m2 hr bar), with increasing rejection on humic acid (82.6 % to 98.5 %). This is due to the formation of a denser membrane with narrower surface pores. The membrane synthesized from 20% (w/v) PVDF with the addition of 4 g PEG 4000 Da showed better antifouling characteristic compared to pure PVDF membrane which could be explained by the enhancement of surface hydrophilicity. Nonetheless, the PVDF/PEG composite UF membrane suffered from rejection loss due to the leaching of PEG molecules from the membrane matrix. Lastly, a regeneration test is performed by flushing the used membrane (after the filtration on humic acid) with distilled water for one-hour duration. SEM images on the membrane surface reveals that the deposited humic acid layer was completely removed and the membrane was successfully recovered to its initial state. The results signified that the addition of PEG could enhances the hydrophilicity of the membrane and hence improving the membrane’s antifouling characteristic.

Studies of Humic Acid Removal from Aqueous Systems by Using Polymeric Membrane Ultrafiltration Process

This paper proposes to investigate in detail the behavior of polymeric membranes ultrafiltration process uses in the retention of humic acid from aqueous systems. Nowadays, the aqueous systems (contaminated water and/or wastewater) resulting from various domestic and industrial activities contain many contaminants, some of them increasingly dangerous to the environment. Laboratory testing of membranes in the ultrafiltration process as well as their experimentation with different chemical compounds is essential because it is important to know how they can be eliminated before they cause a negative impact. Testing of polymeric membranes consists in carrying out the process of ultrafiltration of aqueous systems containing humic acid over a set time, following the variation in time of the volume of permeate obtained as well as the influence of process parameters. Following the investigation of the ultrafiltration process of the water through the polymeric membrane, it was found that the formation of the humic acid layer on the surface of the membrane that favors fouling is delayed, after many hours of operation, due to both the internal fiber structure which offers advantages to this type of membranes as well as high efficiency of humic acid retention. Experimental results showed that the humic acid retention rate increased by up to 37% as a result of the double decrease of the conductivity value in the permeate samples from about 600 mS/cm to 373 mS/cm and the temperature being continuously increasing, from 22.5 at 25oC over 5 h influenced the entire ultrafiltration process. The full process approach as well as the expression and interpretation of the results will be detailed in this paper.

Effect of Acidithiobacillus ferrooxidans on Humic-Acid Passivation Layer on Pyrite Surface

The effect of Acidithiobacillus ferrooxidans on the humic-acid passivation layer on pyrite surfaces was studied by atomic-force microscopy, leaching experiments, and adsorption experiments. Atomic-force-microscopy results showed that humic-acid was adsorbed onto the pyrite surface. The bacteria grew and reproduced on the humic-acid layer. Leaching experiments showed that the humic-acid passivation layer prevented the oxidation of pyrite by Fe3+ under aseptic conditions. Bacteria destroyed the humic-acid layer, promoted pyrite oxidation, and increased the oxidation of pyrite from 1.64% to 67.9%. Bacterial adsorption experiments showed that the humic-acid passivation layer decreased the speed of bacterial adsorption on the pyrite surface but had no effect on the number of bacteria adsorbed on the pyrite surface. The maximum number of bacteria adsorbed by pyrite with and without the humic-acid layer was 4.17 × 1010 cells∙mL−1 and 4.4 × 1010 cells∙mL−1, respectively. Extracellular polymeric stratum layer of bacteria cultured at different concentrations of humic-acid was extracted and analyzed. This layer could destroy the humic-acid layer and promote pyrite oxidation.

Effect of crystallization of settled aluminum hydroxide precipitate on “dissolved Al”

When aluminum salts are added to water at around neutral pH, a precipitate of Al hydroxide is formed very rapidly. Initially the precipitate is in the form of nano-scale primary particles, which then aggregate to form flocs. The nature of the flocs depends greatly on the solution composition, for instance on the presence of humic acid (HA), which not only increases the size of the primary nanoparticles, but also decreases the connection points between them. The nanoparticles become smaller with aging, both with and without HA, as a result of crystallization. The aggregated amorphous nanoparticles (settled flocs) undergo a room temperature structural modification best characterized as a disorder-to-order transition, following elimination of water. During this process, the apparent Al concentration in the supernatant of water increases with age. The “dissolved Al” concentration in the supernatant becomes higher with increasing pH and, to some extent, in the presence of HA. However, it can be shown that the “dissolved Al” in the supernatant exists in the form of crystalline nano-particles or larger clusters, which are detached from the settled flocs. TEM results confirmed that HA only adsorbed on the surface of nano-particles during the coagulation process, which shows precipitate nanoparticles formed firstly during sweep coagulation before the adsorption of HA or complexed Al3+-HA. However, the adsorbed outer layer of HA does not change the crystallization process for the inner part of nano-particles.This laboratory study may have implications for the release of Al from sediments into lake water, following addition of coagulants to lower phosphorus concentrations.

Deposition of powdered activated carbon (PAC) on ultrafiltration (UF) membrane surface: influencing factors and mechanisms

The deposition of powdered activated carbon (PAC) on an ultrafiltration (UF) membrane surface, a common phenomenon in the PAC/UF hybrid process, may have a pronounced influence on UF operation. In this study, the influencing factors and mechanisms of the deposition of PAC were systematically investigated. The influencing factors included the organic species, membrane material, PAC size, filtration volume, and cleaning water. The experimental results suggested that PAC itself was rarely deposited on the membrane surface, forming a monolayer. However, in the cases when organic matter was heavily deposited on the membrane surface (e.g., humic acid (HA) and sodium alginate (SA) in the presence of 0.5mmol/L calcium ions), multilayers of PAC and the organic matter formed; thus, the mass of the deposited PAC significantly increased. The deposition of PAC was barely affected by the membrane material, and decreased as the PAC size increased. Physical cleaning with pure water could effectively detach the deposited PAC. The HA adsorbed by the PAC and the membrane hindered the deposition of PAC because this part of the HA decreased the attractive force between the PAC and the membrane. However, the HA rejected by the membrane promoted the deposition of PAC, as evidenced by the fact that the mass of the deposited PAC increased as the mass of the deposited HA increased. The PAC could be “trapped” in the layer of HA when it formed a cake layer with the HA rejected by the membrane.

Evaluating the effects of sodium and magnesium on the interaction processes of humic acid and ultrafiltration membrane surfaces

In this study, the adsorption behaviors of humic acid (HA) on ultrafiltration membrane surfaces and the HA adsorbed-layer structure were determined in the presence of Na+ and Mg2+, and the interaction forces of membrane–HA and HA–HA were measured. HA fouling experiments were performed to unravel the influence mechanisms of Na+ and Mg2+ on HA fouling. Our results show that the cations mostly affect HA fouling by controlling the electrostatic forces and hydration forces of membrane–HA and HA–HA. For Na+, HA fouling was mainly dominated by electrostatic forces. With increasing Na+ concentrations, the membrane–HA and HA–HA interaction forces increased because of the shielding effect, resulting in a higher deposition rate of HA onto the membrane surface and a more compact HA layer, accompanied by more serious membrane fouling. Mg2+ affected HA fouling in two stages. Lower Mg2+ concentrations acted similarly to Na+, with membrane fouling dominated by the electrostatic interaction forces. However, when the Mg2+ concentration exceeded a critical value, hydration repulsion forces caused a decrease in the deposition rate and the extent of HA on the membrane surface, and a less rigid HA layer was formed; membrane fouling was therefore mitigated.

NC60 deposition kinetics: the complex contribution of humic acid, ion concentration, and valence

The demonstrated toxicity coupled with inevitable environmental release of nC60 raise serious concerns about its environmental fate and transport, therefore it is crucial to understand how nC60 will interact with subsurface materials including attached phase soil and sediment organic matter (AP-SOM). This study investigated the attachment of nC60 onto a Harpeth humic acid (HHA) coated silica surface under various solution conditions using a quartz crystal microbalance with dissipation monitoring. The HHA coating greatly enhanced nC60 attachment at low ion concentrations while hindering attachment at high ion concentrations in the presence of both mono and divalent cations. At low ion concentrations, the HHA greatly reduced the surface potential of the silica, enhancing nC60 deposition through reduction in the electrostatic repulsion. At high ion concentrations however, the reduced surface potential became less important due to the near zero energy barrier to deposition and therefore non-DLVO forces dominated, induced by compaction of the HHA layer, and leading to hindered attachment. In this manner, observed contributions from the HHA layer were more complex than previously reported and by monitoring surface charge and calculated DLVO interaction energy alongside attachment experiments, this study advances the mechanistic understanding of the variable attachment contributions from the humic acid layer.

Determination of the protonation enthalpy of humic acid by calorimetric titration technique

In this study, the calorimetric titration technique was used to determine the protonation enthalpy of two reference humic acids and polyacrylic acid. First, we obtained the apparent protonation constant of two kinds of humic acid purchased from IHSS (International Humic Substances Society) and polyacrylic acid by potentiometric titration. Second, we obtained the protonation enthalpy of them by calorimetric titration. The protonation enthalpy of humic acid was affected by pH and the ionic strength of bulk solution. From the comparison of ΔH between humic acid and polyacrylic acid, it was concluded that the pH dependence of ΔH is attributed to the heterogeneity of humic acid. And ΔH of phenolic hydroxyl group in humic acid is strongly influenced by the electric double layer of humic acid’s surface. This is considered to be a reason of the ionic strength dependence of ΔH. On the other hand, Gibbs free energy of the protonation of humic acid is affected by the electrostatic attraction with the progress of dissociation of functional groups such as carboxyl group and phenolic hydroxyl group. Consequently, the thermodynamics of the protonation of humic acid is affected by the polyelectrolyte effect and the heterogeneity.

Effect of the chemical structure of anion exchange resin on the adsorption of humic acid: Behavior and mechanism

Polystyrenic (PS) anion-exchange resin and polyacrylic (PA) anion-exchange resin were used to investigate the effect of resin chemical structure on the adsorption of humic acid (HA). Due to the rearrangement of HA to form layers that function as barricades to further HA diffusion, PS resin exhibited 12.4 times slower kinetics for the initial adsorption rate and 8.4 times for the diffusion constant in comparison to that of the PA resin. An HA layer and a spherical cluster of HA can be observed on the surface of the PS and PA resins after adsorption, respectively. The considerable difference in HA adsorption between the PS and PA resins was due to the difference in molecule shape for interaction with different resin structures, which can essentially be explained by the hydrophobicity and various interactions of the PS resin. A given amount of HA occupies more positively charged sites and hydrophobic sites on the PS resin than were occupied by the same amount of HA on the PA resin. Increased pH resulted in an increase of HA adsorption onto the PA resin but a decrease in adsorption onto PS resin, as the non-electrostatic adsorption led to electrostatic repulsion between the HA attached to the resin and the HA dissolved in solution. These results suggest higher rates of adsorption and higher regeneration efficiency for interaction of HA with more hydrophilic anion exchange materials.

Characterization of Adsorption of Humic Acid onto Alumina using Quartz Crystal Microbalance with Dissipation

In this paper, a quartz crystal microbalance with dissipation monitoring (QCM-D) is used to investigate humic acid (HA) adsorption onto alumina (Al(2)O(3)). The amount of adsorption and layer structures of HA were determined by the real-time monitoring of resonance frequency and energy dissipation changes (Δf and ΔD). The effect of HA concentration, HA molecular characteristics (molecular weight and polarity), and pH on HA adsorption onto Al(2)O(3) were investigated. The mass of HA adsorption increases as the concentration of HA increases. The masses are about 24, 60, and 87 ng cm(-2) as the concentration of DOC is 1.0, 4.85, and 92.0 mg L(-1), respectively. The adsorbed layer of HA is more nonrigid, and the mass of HA adsorption is higher at weakly acidic pH values. It was 20, 80, 65, and 45 ng cm(-2) at pH values of 4.5, 5.5, 6.5, and 8.0, respectively. This reveals that efficient HA removal by coagulation at weakly acidic pH values is not just due to the hydrolysis of Al ions as previously presumed. The adsorbed layer of hydrophobic HA is more nonrigid than hydrophobic HA (fractionated by Amberlite XAD-8 resin), and the mass adsorption for the hydrophobic fraction is about four times higher than the hydrophilic fraction (120 ng cm(-2) and 30 ng cm(-2)). The method is of value in the research to establish a quantified calculation model for the coagulation process.

Surface-Induced Patterns from Evaporating Droplets of Aqueous Carbon Nanotube Dispersions

Evaporation of aqueous droplets of carbon nanotubes (CNTs) coated with a physisorbed layer of humic acid (HA) on a partially hydrophilic substrate induces the formation of a film of CNTs. Here, we investigate the role that the global geometry of the substrate surfaces has on the structure of the CNT film. On a flat mica or silica surface, the evaporation of a convex droplet of the CNT dispersion induces the well-known "coffee ring", while evaporation of a concave droplet (capillary meniscus) of the CNT dispersion in a wedge of two planar mica sheets or between two crossed-cylinder sheets induces a large area (>mm(2)) of textured or patterned films characterized by different short- and long-range orientational and positional ordering of the CNTs. The resulting patterns appear to be determined by two competing or cooperative sedimentation mechanisms: (1) capillary forces between CNTs giving micrometer-sized filaments parallel to the boundary line of the evaporating droplet and (2) fingering instability at the boundary line of the evaporating droplet and subsequent pinning of CNTs on the surface giving micrometer-sized filaments of CNTs perpendicular to this boundary line. The interplay between substrate surface geometry and sedimentation mechanisms gives an extra control parameter for manipulating patterns of self-assembling nanoparticles at substrate surfaces.

In situ study of the adsorption of humic acid on the surface of aluminium oxide by QCM-D reveals novel features

Novel features of the adsorption of humic acid on the surface of aluminium oxide were revealed by means of an in situ study by quartz crystal microbalance with dissipation (QCM-D). The adsorption of humic acid on the surface of aluminium oxide shows a special case at pH 3 where the adsorption takes place in two steps. Each step has characteristic thickness, kinetics and viscoelasticity. While the first step shows low thickness, fast kinetics and almost no viscoelastic character, indicating a rigidly adsorbed humic acid monolayer, the second step is of much higher thickness, slower kinetics and with significant viscoelastic character, indicating a large diffuse layer with significant bulk contributions. In contrast, adsorption at pH 5 and 6.8 was found to be limited to a monolayer which is more rigid at pH 5 than at pH 6.8. Ellipsometry and X-ray photoelectron spectroscopy (XPS) were used to prove and quantify the adsorption in the dry state. The monolayer thickness measured by ellipsometry is close to the Sauerbrey thickness obtained by QCM-D. Adsorption takes place at a dilute concentration of 1 mg L−1, followed by saturation at a concentration of 10–20 mg L−1. XPS spectra of the carbon 1s peak indicated almost no desorption of the adsorbed humic acid layer after immersion in water for 48 hours, implying irreversible adsorption based on a strong binding between humic acid and aluminium oxide.

Natural organic matter fouling of microfiltration membranes: Prediction of constant flux behavior from constant pressure materials properties determination

The characteristics of calcium-aggregated humic acid assemblages that deposit on microfiltration membranes are determined in this study using a novel approach involving the analysis of filtration and deliquoring behaviour as a function of time over a range of constant applied pressures. The filtration-deliquoring approach initially provides the cake thickness and porosity at the conclusion of the filtration phase which, when combined with the steady state fluxes that result over a range of particular applied pressures, enables determination of the local permeability and porosity through the cake at any applied pressure. Within the range of the applied pressures examined (12.5–150 kPa), the humic acid layer has been shown to be very porous with a moisture content of more than 96%. However, despite this high porosity, the fouling layer has very low permeability with the permeability decreasing with increase in pressure. With knowledge of these material properties, the filtration behavior of the humic acid fouling layer under constant flux operating condition can be predicted.

Characterization of Humic Acid Fouled Reverse Osmosis and Nanofiltration Membranes by Transmission Electron Microscopy and Streaming Potential Measurements

Reverse osmosis and nanofiltration membranes fouled by humic acid were systematically characterized by transmission electron microscopy. All fouled membranes, except those with very low initial flux, were completely covered by a layer of humic acid whose thickness and density were greatly affected by the feedwater composition ([H+] and [Ca2+]) and initial flux. A low-density humic layer (about 0.1 g of purified Aldrich humic acid (PAHA)/cm3) was formed at low initial flux (2 m/day or less) at pH 7 without calcium. It was several times denser at a higher initial flux, pH 4.5, or 1 mM Ca2+. Corresponding to the denser foulant layers under these conditions, PAHA accumulation was greatly increased. The denser foulant layers together with the greater PAHA accumulations were responsible for the severe flux reductions. Both virgin and fouled membranes were characterized by streaming potential measurements. While considerable differences existed for virgin membranes, humic acid fouled membranes exhibited identical surface charge properties. The zeta potential of the fouled membranes was controlled by the humic acid layer due to its complete coverage of the membrane surfaces.

Influence of Humic Acids on the Growth of the Microorganisms Utilizing Toxic Compounds (Comparison between Yeast and Bacteria)

Humic acids, a heterogenic group of natural organic macromolecules with a complex polymeric structure, form a significant part of the soil and water environment. They have a high surface activity, thus they are able to interact with other components in the environment (mineral and microbial cell surfaces, organic and inorganic compounds). This work is focused on humic acid effects on the growth of microorganisms utilizing phenolic compounds as the sole carbon and energy source. For this purpose a bacterial population Rhodococcus erythropolis and a yeast strain Candida maltosa were used. Both microorganisms had been adapted to the degradation of phenol and phenolic compounds for many years. However, a toxic effect of these compounds still occurs. The addition of humic acids into the cultivation medium extended toleration of the bacteria to higher concentrations of phenolic compounds and increased the growth of this microbial population as well. In the case of the yeast, humic acids inhibited the growth. Humic acids easily form an additive layer on the surface of the investigated microorganisms. The generated humic acid layer can probably serve as a transport barrier and thus influence the growth of microbial populations.

Interfacial and rheological properties of humic acid/hematite suspensions

This work deals with the effect of humic acid (HA) adsorption on the interfacial properties, the stability, and the rheology of aqueous iron oxide (hematite) suspensions. It is first of all demonstrated that HA effectively adsorbs onto hematite, mainly at acid pH. Since the charge of the HA chains is negative, it will be electrostatically attracted to the hematite surface below the point of zero charge of the particles, when they are positively charged. Electrophoresis measurements of hematite suspensions as a function of pH in the presence and absence of HA clearly demonstrate the adsorption of negatively charged entities on the oxide. Since the HA-covered particles can be thought of as “soft” colloids, Ohshima's theory was used to gain information on the surface potential and the charge density of the HA layer (H. Ohshima, in: A.V. Delgado (Ed.), Interfacial Electrokinetics and Electrophoresis, Dekker, New York, 2002, p. 123). A different procedure was also used to ascertain the degree of modification experienced by the hematite surface when placed in contact with HA solutions. The contact angles of selected liquids on pretreated hematite layers lead to the conclusion that the humic acid molecules impart to the particles a significant electron-donor character, in turn increasing their hydrophilicity. All this amount of information is used in the work for the interpretation of the rheological properties of hematite suspensions; the results are consistent with a stabilizing effect of HA adsorption on the suspensions, mainly as a consequence of the increased electrostatic repulsion between particles.

Electroanalytical Techniques Applied for Studying the Interaction of Organic Matter and Particles with Metal Ions in Natural Waters

Electroanalytical techniques were applied and proved suitable for studying the adsorption of metal ions, metal organic complexes and organic ligands on different mineral particles (γ-Al2O3, SiO2, northupite and chrysotile). The complexing capacity of the model solutions with added adsorbents were determined. The complexing capacity and apparent stability constants for a mixture of organic ligands present in a seawater sample for different metal ions (Cu ions, Pb ions and Cd ions) were determined by voltammetry as well. Mercury electrode surface was used as a model of hydrophobic surface and the oxido-reduction processes of the cadmium ions on the mercury surface covered with the layer of humic acid were studied by phase selective a.c. voltammetry. By simple voltammetric measurements it was possible to determine the extent of adsorption of organic matter and its influence upon the speciation scheme of trace metals and organics.