Sorption of atrazine and diuron on powdered activated carbon in the presence of surfactants, calcium and dichromate ions - Modelling trial
Contamination of natural waters by organic micropollutants like pesticides requires the use of powdered activated carbon (PAC) during the flocculation step of water treatment. One of the main factors affecting the efficiency of this process is the presence in natural waters of organic and mineral compounds. The aim of this study was to determine the influence of two surfactants [(i) anionic (sodium dodecyl sulfate : SDS) and (ii) cationic (hexadecyl-trimethyl ammonium bromide : HTAB)], and two mineral ions (calcium and dichromate) upon atrazine and diuron adsorption on powderd activated carbon resulting from Chemviron Filtrasorb-400. The adsorption capacities on PAC of atrazine and diuron alone were determinated according to the Langmuir model for the isothermes (table I). They were estimated to be 1.42 and 1.72 mmol.g-1 respectly at pH 5.5. Coadsorption results pointed out that the herbicide adsorption was inhibited in the presence of surfactants whatever their nature (fig. 1 and 2).This inhibition was most importante with HTAB (45%), according to the zeta surface potential and the streric dimensions of molecules, than with SDS (23%) for atrazine . The same result was noted for diuron with an inhibition respectly of 15% for SDS and 27% for HTBA. Calcium and dichromate ions, each one with one herbicide had no effect. In any case, Langmuir isothermes were drawn and adsortion capacities were calculated with a good correlation (tables II and III). The influence of coadsorbats was always greatest for atrazine than for diuron. All the experiments were performed at pH=5.5, without influence upon the adsorption, even for the dichromate ions. Desorption tests showed that a part of adsorbed pesticides was desorbed with surfactants introduction (fig. 3) : with SDS 12.3% of atrazine and 8% of diuron against respectly 23% and 17% with HTBA. The phenomenon was increased with SDS in presence of calcium ions (fig. 4 and 5) according to the increase of SDS adsorption (table II and III). The use of two simple coadsorption models was reinforced by the calculated values obtained for adsoartion capacities (fig. 6 and 7) which were very closely with the experimental ones. The calculation of constantes given by the models confirmed the existence of specific sites of adsorption for every compound and a non competitive inhibition due to a change in the nature of the interactions between pesticides and activated carbon surface (table III).