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Adsorption-Desorption and Hysteresis ot Sultonateson Kaolinite: pH Effects
PAUL A. SIRACUSA AND P. SOMASUNDARAN1Henry Krumb School of Mines. Columbia University. New York. New York 10027
Received May 13, 1985; revised January 22, 1986
Adsorption and desorption behavior of dodecylbenzenesulfonate on kaolinite was investigated undercarefully controlled conditions of pH, ionic strength, and dissolved mineral species. The nature of theadsorption maximum and the hysteresis obtained is found to strongly depend on the pH change thatoccurs during equilibration and dilution. Interestingly, hysteresis can be positive or negative dependingmainly on the pH perturbations that take place during the experiment. The results are examined on thebasis of available data for the pH-dependent dissolution equilibria for kaolinite. Kaolinite dissolutionand subsequent sulfonate precipitation and redissolution are identified to be the major factors in deter-mining abstraction in this system. Many complex interactions are involved in determining surfactantadsorption; studies of desorption behavior along with that of adsorption have proved to be a powerfultool for studying the mechanisms involved. 0 1986 Academic ~ I~
Adsorption isotherms of sulfonates onminerals such as kaolinite have been shownto exhibit special features such as a maximumin the critical micelle concentration region (I-S). While a number of interactions have beenconsidered for the existence of a ~aximum(2-4, 6-11), none can adequately explain theexistence of hysteresis that these systems havebeen found to exhibit upon surfactant dilution.An approach involving the study of adsorptionsimultaneously with that of desorption isadopted in this work since the adsorption pro-cess in these systems has been clearly shownto exhibit many nonequilibrium effects (1,3).
Surfactant adsorption on oxides and claysis dependent on such solution properties aspH and ionic strength (12-15), and a precisecontrol of these variables is necessary for theisolation and identification of various mech-anistic features of these systems. In this study,adsorption and desorption experiments wereconducted for the dodecylbenzenesulfonate-
I To whom all correspondence should be addressed.
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kaolinite system as a function of the sulfonateconcentration with emphasis on the identifi-cation of the role of pH changes that invariablytake place in these systems. The measuredsurfactant depletion can include in additionto the actual adsorption, precipitation and en-trapment, and all these processes will have sig-nificant dependence on mineral dissolutionwhich in turn is pH dependent. Adsorption isexamined here in light of the mineral-solutionequilibria and its dependence on pH.
MATERIALS AND METHODS
Kaolinite. A well crystallized sample ofGeorgia kaolinite purchased from the clay re-pository at the University of Missouri wassubjected to an ion-exchange treatment (16)to produce the monoionic sodium kaolinite(Na-kaolinite) used in this study. BET surfacearea of the treated sample was determinedby N2 adsorption (using a Quantasorb) to be9.8 m2/g.
Surfactant and chemicals. Sodium dodec-ylbenzenesulfonate purchased from LachatChemicals, Inc., was purified by deoiling withpetroleum ether. desalting with hot isopro-
18'SULFONA TES ON KAOUNrrE: pH EffECI'S
panol. and recrystallization from cold acetone(17). High-pressure liquid chromatographicanalysis revealed the presence of not only Cl2isomers, but also CIa isomers and other lesshydrophobic components as shown in Fig. I.
Inorganic salts used to adjust the ionicstrength and pH were of A.R. grade. Tripledistilled water was used for all tests.
Adsorpt;on-desorpt;on procedure. The ad-sorption-desorption tests involved equilibra-tion of the mineral in the desired surfactantsolution and detennination of the initial sur-factant depletion (adsorption density) by re-moving a specific volume of supernatant andanalyzing for residual concentration. Desorp-tion tests were conducted by adding a diluentadjusted to the System ionic strength and pHat the same volume as supernatant removed.The total volume and thus solid to liquid ratioremain constant as the dilution stage is re-peated for many cycles with the surfactant
tdsorption in mollsnitial sulfonate concentration;esidual sulfonate concentration:otal solution test volumemass of adsorbent solids.
[Cj- C.]oV/W]. . . initial adsorption.
~i - C,J = ~C.
,= [C 1)a+ ~C_1)- C.",][V/W]
[(CrA-l)a + dC"-I] = CiJIdilution factor [I - volume removedl V]
- initial adsorptionr ~ ~ adsorption after n dilutions.
Sulfonate concentrations were determinedeither by two-phase titration using dimidiumbromide/disulphine blue mixed indicator with10-) kmol/m) hexadecyltributyl ammoniumbromide (18, 19), or by UV absorbance at 223nm. Titrations were conducted for samples ofsulfonate concentration greater than 10-.kmol/m) with optical methods employed fordilute solutions.
The adsorption-desorption procedural flowdiagram is given in Fig. 2.
RESUl1'5 AND DISCUSSION
Adsorption-desorption results obtained forthe purified dodecylbenzenesulfonate/Na-ka-olinite system at pH 3.8 and an ionic strengthof 10-1 kmol/mJ Naa are given in Fig. 3. Inagreement with the results obtained earlier forthis type of system (1.3). an abstraction~ max-imum is exhibited. with the maximum locatedin the region of CMC as determined by thedye solubilization technique (20). Also. the
z A~ ~ts all ~bIe means of SUrtt1anl
depietion-adSO(1)tion. precipitation. and/or entrapment.
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concentration monitored at each stage. Thusthe adsorption density at each step can be cal-culated from the foUowing:
r. = ~Ci =iC. = IV=1
SIRACUSA AND SOMASUNDARAN186
desorption pathways are found to depend onthe location of the initial (prior to dilution)abstraction point with respect to the maxi-
mum; further, the point at which dilution iscommenced from the position of the maxi-mum, larger is the hysteresis.
. .. , , , , , . , . . , . . . . . .'. . . jJ ,HoDGeS (V-RC-41/No-KAOL.INITE (CH-I) T. 3Otl.C
pH . 3..~0.3S/L . 0.2.
I . 10-1 W_I/",3 NoCI
. INITIAL ABSTRACTION: }~O'" VOLUME DILUTION WITI1.a 10-1 kMOI/",3 HoCl AT pH. 3.B
. . . . . . . . . . . . . . . . ... . ...0 2 4 6 8 10 12 14 16 18 30 32 "40
RESIOUAt. SUt.FONATE CONCENTRATIOH, 11101,..3
FIG. 3. Adsorption-dcsorption iSOtherms for DOBS-Na-kaoIinite at pH 3.8 :t 0.3 in 10-1 kmol/mJ NaCl
~cfC~_I-t-Sc"-. Vol. "4. No. I. - 1986
187SULFONA TES ON KAOUNITE: pH EFFECTS
For adsorption of ionic surfactants on min-erals which undergo hydroxylation and ion-ization, pH is an important variable since itcan affect the electrostatic interactions gov-erning the adsorption process (12-15). It isnecessary, therefore, for systems made up ofminerals such as kaolinite to take into accountpossible effects of pH change during equili-bration while interpreting adsorption data. Inthe present case, while the pH of the solutionswas adjusted to 3.0 prior to mixing with ka-olinite, the pH drifted to higher values duringthe equilibration. The pH 3.8 reported in Fig.3 is the average of the final pH values obtainedfor all tests. An examination of the pH changesfor each test along with the corresponding ab-straction values (Fig. 4) shows a systematicdecrease in the equilibrium pH values abovethe concentrations corresponding to the ab-straction maximum. It is to be noted that theabstraction will increase with any decrease inpH and therefore the abstraction isotherm inFig. 4 can be expected to exhibit an evensharper maximum if the pH was maintainedat constant values during the equilibration.
( . . .. I . . . . I ... . .. ,. I ... . . . .. f10-4 10-' 10-2 KS"1
RESIDUAL SULFONATE CONCENTRATION. ~I/""
FK8. 4. Abstraction density and pH :IS a function of ~dua1 DDBS concentration for Na-kaolinite in10-1 kmol/mJ NaCl at JOoC. Initial pH - 3.0.
J I..C ..,..~ Sci~. Val. 114. No. I. - 1-
Concentration dependence of abstractionunder constant pH conditions was detenninedby producing a family of abstraction versuspH curves and estimating, by interpolation,abstraction as a function of surfactant con-centration at the desired constant pH values.The resultant data given in Fig. 5 show all iso-thenns in the pH range 3 to 7 to exhibit asharp maximum, with the maximum appear-ing in the region 3 X 104 to 10-3 kmol/m:3sulfonate. The CMC of the system was deter-mined by dye solubilization to be at 2.6 X 10-4kmol/m3 sulfonate; therefore, in all cases, thedecrease in abstraction can be stated to ac-tually occur in miceUar solutions.
A comparison of the experimental isothermin Fig. 4, pH = 3.8 :t 0.3, with abstractionisotherms interpolated for constant pH con-ditions is made in Fig. 6. The experimentalisotherm has abstraction levels between theisotherms at constant pH 3.0 and 3.5 ratherthan between the isothenns at pH 3.5 and 4.0,even though the equilibrium pH was betweenthe latter values for all the experimental tests.This result suggests that most of the abstraction
188 SIRACUSA AND SOMASUNDARAN
the desorption isotherms were generated fromdilutions with solutions adjusted to the equi-librium or final pH of 3.8. Since the pH canalso be expected to playa major role in theprocess of desorption, the effect of such dif-ferences in the pH of the diluent was nextinvestigated. Toward