Potassium Alkyl Carboxylates Adsorption
E.A. Streltsova, V.F. Sazonova, E.V. Moyseenko
Odessa I.I. Mechnikov National University,
Pastera st. 27, 65026 Odessa, Ukraine
Lead isolation from its salt diluted solutions can be carried out by lead precipitation particles surface as slightly soluble hydroxide, hydrophobization of by appropriate surfactant (collector) and following hydrophobizated particles flotation. The information about the mechanism of surfactant interaction with lead hydroxide is necessary for efficient prognosis of surfactants action as flotation collectors of lead in lead hydroxide form.
The regularities of potassium alkyl carboxylates adsorption by fresh-precipitated lead hydroxide have been studied and are stated in this work.
It has been stated that isotherms of potassium decyl- (CH3(CH2)8COOK), dodecyl- (CH3(CH2)10COOK), and tetradecyl- (CH3(CH2)12COOK) carboxylates adsorption by lead hydroxide have a form close to one of S-type isotherms by Jiles classification, i.e. correspond to polymolecular adsorption.
The temperature increase from 25 to 45° C slightly increases the adsorption (it shows mainly chemical character of adsorption), however practically doesn‘t change the form of adsorption isotherms.
For small equilibrium concentrations the isotherms are characterized by linear dependence of adsorption from surfactant equilibrium concentration. It allowed to use Langmir equation to describe the initial part of the isotherms.
Cp/A = 1/(A ¥ b)+ Cp/ A ¥.
According to the obtained data the free energy of Gibbs DGo = -RTlnK, enthalpy DHo = RT2(dlnK/dT) and entropy DSo = (DHo - DGo)/T have been calculated.
It has been shown that potassium alkyl carboxylates adsorption was accompanied by considerable decrease of system free energy, whereas the DGo value increases with temperature increase and decreases with adsorbate
hydrocarbon radical length increase. The DHo values are positive and small, that indicates the chemical character of adsorption; DSo values are positive and sufficiently great, that is associated with water molecules displacement by surfactant molecules from adsorption layer and desolvation of adsorbate polar groups.
The adsorption monolayer capacities found from adsorption isotherms decrease with potassium alkyl carboxylate hydrocarbon radical length increase. It allows to suppose that the adsorption mechanism is connected with lead hydroxide surface structure and with the association of adsorbate molecules the adsorption of which in adsorbent pores is difficult because of steric causes.
It can be supposed that adsorbates - potassium alkyl carboxylates – form coordination bonds with lead ion on the one hand and hydrogen bonds with hydroxy groups of adsorbent (lead hydroxide) on the other hand. The first adsorption layer is, in our opinion, formed owing to the formation of the hydrogen bond between adsorbate carboxy groups and adsorbent surface groups.
The second adsorption layer is formed owing to the hydrophobic interactions. The lead hydroxide colloid particles micelle structure in the presence of sufficiently great (necessary for their flotation) amounts of potassium alkyl carboxylates evidently corresponds to formula:
ím[Pb(OH)2]nRCOO-(n-x)K+ýx-xK+
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