The significance of electrokinetic potential in the adsorption process of humic substances

Humic substances (HS) commonly found in ground waters give the ground waters colour and until recently, have been treated as being totally harmless and having no ill effects on human and animal health. However, research carried out in recent years has shown that those substances when undergoing the chlorination process are precursors of halogen-organic pollutants, including mutagenic and cancerogenous compounds such as trihalomethans (a derivative of short chained aliphatic carbohydrates like chloroform) and 3-chloro-4-(dichloromethyl)-5-hydroxy-2(5H)-furanone (MX). Besides, humic substances, because of their structure which gives them hydrophilic character [1] are good sorbents for both organic and inorganic substances including toxic compounds too, such as pesticides, phthalates and polychlorinated biphenyls. On account of these properties, it is necessary to undertake the total removal of humic substances from water intended for public supply. A commonly used process to get rid of water colour is coagulation together with less effective processes such as sorption or ion-exchange [2]. Coagulation may be supported by substances showing sorption features. Those substances are usually added to water being treated. That potential characterizes moving particles and occurs in the place where shear plane forms. To make it clearer, it is assumed that the electrokinetic potential dominates on the border of the adsorptive layer and the diffuse layer. When we know the value of the electrokinetic potential (called potential) we can explain the phenomena taking place between the phases in the physico-chemical processes [3]. That is why the electrokinetic potential was used to examine and explain the mechanism of humic substances removal in the adsorption process and a hypothetical model of that process was developed for defining the optimum process conditions. Activated bentonite was obtained by the exchange of interpack cations against protons. 100 g of natural bentonite was mixed with 500 mL of 6 n HCl and heated for 2 hours to a temperature of 100°C. After two hours the supernatant was separated from the residue by decanting and then the bentonite was purified. This process was repeated twice. Then, the bentonite was dried at 105°C until its weight stabilised. In order to give the water turbidity and colour, silica (10 mg/L) and HS were added. The silica was prepared in the following way: 25 g of technical water-glass was dissolved in 500 mL of distilled water and the solution was adjusted to achieve pH 7.0 by using H2SO4. This solution was stirred very slowly for 2 hours until a colloidal solution was reached. The solution was diluted until it reached a volume of 1 litre. The final concentration of silica was 10g/L. Before using the silica was homogenised. The water reaction was corrected with NaOH or HCl solution to maintain pH of 5.5 or 7.5. The electrokinetic potential of hydrophobic activated carbon Norit CN 1 has negative or positive values depending on the pH of the solution. The electrokinetic potential of fulvic and humic acids and activated silica is negative. Humic acids adsorption takes place as a result of the electrostatic attraction of functional groups or intermolecular interaction forces between hydrophobic functional groups (aliphatic chains and aromatic rings) and adsorbent - depending on the character of the adsorbent surface and its electrokinetic potential. In the case of the adsorbate and the adsorbent surfaces having the same charge, adsorption takes place by bridging with multivalent cations. Silica influences the adsorption capacity of all the compounds participating in the adsorption. Humic acids adsorption on adsorbents depends on the affinity series (according to sorption ability): CN>Sup>Ba>B, which changes in the presence of silica, with a humic substances concentration < 138 mg Pt/L: Sup>CN>Ba>B. The phenomena occurring on the phase boundary during the process of coagulation and the adsorption of humic substances in the presence and absence of adsorbents can be described with chemical and physicochemical reaction equations.

W pracy przebadano proces adsorpcji substancji humusowych na wybranych sorbentach-obciążnikach stosowanych w procesie koagulacji wód powierzchniowych. Określono szereg powinowactwa danych adsorbentów do substancji humusowych. W oparciu o pomiary potencjału elektrokinetycznego cząstek adsorbentu-obciążnika przedstawiono hipotetyczny proces adsorpcji substancji humusowych na powierzchni ciała stałego. Zgodnie z tym modelem adsorpcja substancji humusowych zachodzi w wyniku przyciągania elektrostatycznego grup funkcyjnych lub oddziaływania sił między-cząsteczkowych między hydrofobowymi fragmentami (alifatycznymi łańcuchami i aromatycznymi pierścieniami) i adsorbentem - w zależności od charakteru powierzchni adsorbentu i jego potencjału elektrokinetycznego. W przypadku, gdy powierzchnie adsorbatu i adsorbentu mają ładunki jednoimienne adsorpcja zachodzi poprzez mostkowanie wielowartościowymi kationami. Inne wnioski, które można wyciągnąć na podstawie uzyskanych wyników badań, są następujące: potencjał elektrokinetyczny hydrofobowego węgla aktywnego Norit CN 1 posiada wartości dodatnie lub ujemne w zależności od pH roztworu, potencjał elektrokinetyczny kwasów fulwowych i huminowych oraz krzemionki jest ujemny, krzemionka posiada wpływ na pojemność sorpcyjną wszystkich związków biorących udział w procesie adsorpcji, adsorpcja kwasów huminowych na adsorbentach zależy od szeregu powinowactwa (na podstawie zdolności sorpcyjnej): CN>Sup>Ba>B, który zmienia się w obecności krzemionki, przy stężeniu substancji humusowych < 138 mg Pt/L na: Sup>CN>Ba>B, zjawiska zachodzące na granicy faz w czasie procesu koagulacji i adsorpcji substancji humusowych w obecności i braku adsorbentów można opisać równaniami rekcji chemicznych i fizykochemicznych.