Environ. Sci. Technol. 1989, 23, 187-191
Competitive Adsorption of Phosphate on Goethite in Marine Electrolytes David Hawke, Peter D. Carpenter, and Keith A. Hunter" Department of Chemistry, University of Otago, Box 56, Dunedin, New Zealand
Phosphate adsorption by the Fe(II1) oxide geothite in seawater and in electrolyte mixtures containing major ions and humic acid has been studied. Humic acid, Mg2+,SO:-, and F- decreased phosphate adsorption at low pH, while Ca2+ increased adsorption at high pH. In seawater, phosphate adsorption followed a Langmuir isotherm at pH 8.2. Experimental data for phosphate adsorption and the effects of the major ions were modeled with a surface complexation model. Introduction
The availability of the micronutrient orthophosphate to marine organisms in the water column is strongly influenced by interaction with specific mineral components of bottom sediments (1-4). Laboratory studies in simple electrolytes have shown that metal oxides and clay minerals are strong adsorbers of phosphate ion in solution (5-8). However, little is known of the adsorptive equilibria involving orthophosphate and solid phases in seawater electrolytes or of possible competitive interactions involving the major ions of seawater. In estuaries, phosphate cycling appears to be partly associated with the behavior of Fe(1II) oxides and humic acids that undergo complex mutual interactions as colloidal phases (S1.l). Here we report a study of the adsorption of orthophosphate by the Fe(II1) oxide geothite (a-FeOOH) in marine electrolytes. The effects of the major ions of seawater, fluoride ion, and sedimentary humic acid were investigated. The effect of fluoride ion was investigated because it is important in many phosphate-containing industrial effluents and is involved with phosphate in fluorapatite formation. Experimental Section
Materials. Goethite was prepared by the method of Atkinson et al. (12). It was characterised by X-ray diffraction (reflections at 2.45, 2.69, and 4.17 A) and by determination of a pH at the point of zero charge pH(pzc) of 7.6. Electrolytes were prepared from AR grade reagents and contained negligible phosphate impurity. Adjustment of electrolyte pH was by addition of HC1 or NaOH. Phosphate solutions were prepared from AR grade Na3P04. 12Hz0. Sedimentary humic acid was extracted with 2 M NaOH from surface sediment collected from the Chatham Rise to the east of New Zealand, at a depth of 400 m. Purification was by repeated dissolution and reprecipitation and then dialysis. Microanalysis of the product gave the results (expressed as weight percent) 49.1% C, 6.5% H, and 5.6% N. These are very similar to those obtained for marine humic acids (13).Solutions in NaOH were prepared from the dry solid and were found to have negligible phosphate content. Procedures for Adsorption Measurements. The degree of phosphate adsorption by geothite was measured in solutions of different pH in the range 3-10. The method involved equilibrating geothite slurry, electrolyte, phosphate, and deionized distilled water at 20 f 2 "C in open borosilicate glass centrifuge tubes for 24 h in the dark. 0013-936X/89/0923-0187$01.50/0
Most experiments used a goethite concentration of 0.25 g/L. Phosphate adsorption was determined by colorimetric analysis (14) of the supernatant liquid after centrifugation (10 min at 2OOg). Precision was estimated at f l %at low percent adsorption and f2-3% at high percent adsorption, although experimental scatter exceeded these estimates on occasion. Electrolyte pH was measured (h0.05)with a glass electrode calibrated with pH 4.00 phthalate buffer. Modeling of Adsorption Processes. To aid in the interpretation of the results, adsorption of phosphate in different electrolytes was also calculated by using a surface equilibrium complexation model. Our model was derived from the triple layer model used in earlier reported studies (15,16) in which adsorption of ions is considered to take place through surface complex formation (15) in the /3 plane of the interface. This mechanism is considered appropriate for adsorption of seawater major ions on goethite (17). However, available evidence (18) suggests that orthophosphate adsorbs on Fe(II1) oxides by ligand exchange with surface hydroxyl groups. This is equivalent, in triple layer model terms, to surface complexation in the surface plane rather than the 0 plane. Therefore the ability to calculate surface ligand displacement equilibria was added for the phosphate modeling. The model calculations were performed on a modified version of the solution equilibrium program HALTAFALL(19) developed in our laboratory. Model-adjustable parameters for the computations include the number of surface.sites per kilogram of oxide (determined by potentiometric titration as 0.13 mol/kg), the specific surface area of the oxide [determined by the BET N2 adsorption method as 5.18 X lo4m2/kg (In], and the inner and outer layer capacitances [140and 20 MF/cm2, respectively (15, 17)]. Equilibrium constants for solution equilibria involving the major ions and phosphate species were taken from the literature (20-22). Table I shows the surface adsorption equilibria and constants included in the model. Adsorption constants for the major ions from a previous study (17) were used. Adsorption constants for phosphate and fluoride ions were varied systematically until the best agreement of the calculated adsorption with the experimental results was achieved. Such constants are model dependent (23) and may not be compared with results from other surface complexation models, or results generated from isotherm-type models. Results and Discussion
Phosphate Uptake in NaCl Electrolyte. The degree of phosphate uptake by goethite as a function of pH in 0.422 mol/kg NaCl solution is shown in Figure la. Uptake decreases quite rapidly with increasing pH, from near 100%at pH 3 to ~ 2 0 % at pH 9. This behavior is expected for anions in a simple electrolyte where specific adsorption of electrolyte ions is absent (8). Separate experiments with NaCl electrolytes in the concentration range 0.042-0.70 mol/ kg NaCl gave results identical with those in 0.422 mol/kg, indicating a negligible ionic strength effect. This finding made it possible to investigate the effects of other seawater major ions by adding them to the base 0.422 mol/ kg NaCl electrolyte. The electrolyte mixtures used are listed in Table 11. Each
0 1989 American Chemical Society
Environ. Sci. Technol., Vol. 23, No. 2, 1989
187
Table I.Summary of Equilibrium Surface Reactions and Intrinsic Equilibrium Constants Used To Describe Phosphate Adsorption on Goethite
adsorpn log (intrin plane equil const)
equilibrium reaction SOHZ' SOH + Ht SOH I;) SO- Ht SOH + Nat I;) SO-Nat + Ht SOH + Mg2+e SO-MgZt Ht SOH + CaZt S o f a 2 ++ Ht SOH + MgZt + HzO SO-MgOH+ -k 2Ht SOH + CaZt + HzO SO-CaOH+ + 2Ht SOH + Ht C1- I;)SOHztClQ
surface surface
+
- -
+
-5.57" -9.52' -8.400 -5.45" -5.00" -14.25" -14.50" 7.00"
P P P P P P
+
Q
adsorpn log (intrin plane equil const)
equilibrium reaction SOH + Ht + S042- SOHztSOf SOH + 2Ht + S042- SOHZtHSO4SOH + Ht + PO,'S-P0d2- + HzO SOH + 2Ht + P04'S-HPOI- + HzO SOH + 3Ht + PO4" S-HzP04 + HzO SOH -k Ht + F- e SOHz+FSOH + H+ + Ca2++ SO4" I;)SOHZtCaSO4 Q
Q
P P surface surface surface P P
9.10" 15.40* 24.00 29.70 33.40 11.85 11.70
"From ref 19. *Literature value, 14.40 (19);see text. Table 11. Composition of Electrolyte Mixtures Used for Phosphate Adsorption Experiments
electrolyte
[NaCll
base electrolyte Mg electrolyte Ca electrolyte (U0.422 (2)0.422 sulfate electrolytes (1)0.422 (2)0.422 major ion seawater
0.422 0.422
Table 111. Major I o n Composition of Seawater"
concn, mol/kg [MgCbl [CaClZ1 [NaZSO41 0.054
0.054
concn, mol/kg
Nat M92' CaZt
0.478 0.054 0.010
concn, mol/kg
ion
c1-
0.550 0.028
SO4"
"After ref 39.
0.010 0.050
0.422
ion
0.028 0.0014 0.028
0.010
sma o m 0
I b I
I
1
I
I
I - r
a
60
q 0
a 0 W
i-
80
z
U ti
m 0 0
a
4o
W
a m 60
E
20
0 W
80
0
