al., 1985; Marcano-Martinez and McBride, 1989). Nordstrom (1982) contends that rates of amorphous basic Al sulfate precipitation are sufficiently rapid to.
Solution Sulfate Chemistry in Three Sulfur-Retentive Hydrandepts J. D. Wolt,* N. V. Hue, and R. L. Fox ABSTRACT
pK,,p = 117.6, Adams and Rawajfih, 1977] quite closely (pIAP = 117.5 ± 1.4). Marcano-Martinez and McBride (1989) observed equilibrium batch extracts of acid Oxisols to be undersaturated with respect to basaluminite, while alunite [KA13(OH)6(SO4)2] precipitation was possible at higher rates of S addition. Alunite precipitation has been advanced as a mechanism for S retention by Ultisols subject to high acidsulfate inputs (Wolt, 1981). Recent S fertilization of soils may favor precipitation of basaluminite (Hue et al., 1985) and subsequent release of basaluminite-retained S in plant-available form (Wolt and Adams, 1979). Sulfate sorption by highly weathered, S-retentive soils is biphasic in nature, indicative of two adsorption maxima (Aylmore et al., 1967; Barrow, 1967; Barrow et al., 1969; Chao et al., 1962; Hasan et al., 1970). Biphasic-sorption phenomena have been interpreted to indicate differing sites of ligand exchange (Aylmore et al., 1967), but are equally interpretable as a precipitation reaction that is kinetically controlled in its early stages by surface-catalyzed reactions (Nordstrom, 1982). Tropical soils derived from volcanic ash and rich in amorphous Al and Fe oxyhydroxides exhibit widely varying capacities for S retention dependent on degree of weathering (Barrow et al., 1969; Gebhardt and Coleman, 1974; Hasan et al., 1970). Hasan et al. (1970) noted decreasing soil S along a gradient of decreasing rainfall for Inceptisols derived from volcanic ash on the island of Hawaii. They associated varied content of indigenous S and S-retention capacities, as determined from sorption-desorption isotherms, with the combined effects of soil organic-matter content, degree of soil weathering, and S accretion from rainwater. Highly S-retentive Hydrandepts from Hawaii have been reported to frequently contain < 150 (jumol SO4-S L~ l in soil solution while containing > 200 mmol kg~ l of P-extractable S (Hue et al., 1990). Our research was undertaken to more fully characterize solutionsolid phase S relationships for these S-retentive Hydrandepts in terms of the nature of sorption isotherms observed and the possible role of basic Al sulfate precipitation.
Highly S-retentive Hydrandepts frequently contain large quantities (>200 mmol kg-') of sorbed S while exhibiting low S intensities in soil solution. Solution-solid phase S relations for three representative Hydrandepts from Hawaii were investigated in terms of the nature of sorption isotherms and the role of basic Al hydroxysulfate precipitation as a mechanism for S retention. Sorption data were fit to linear, Langmuir, and multisite Langmuir isotherms using a modeling routine that performed parameter estimates and model optimization using weighted nonlinear fitting. Best-fit models indicated sorption data were adequately described by Langmuir isotherms. Total P-extractable SO45 cm and tended to decline at depths > 120 cm. Extractable SO4-S was strongly regressed on gravimetric moisture content, a relative indicator of reactive surface for these thixotropic soils. Soil solution SO4-S concentrations were consistent with alunite or basaluminite solubility when gibbsite controlled solution A13+.
ULFUR RETENTION by acid, highly weathered soils has been described variously as an anion-specific S ligand exchange involving sulfate-hydroxyl exchange on the surface of amorphous Al and Fe oxyhydroxides (Chao et al., 1964; Gebhardt and Coleman, 1974; Parfitt and Smart, 1978; Rajan, 1979), as electrostatic attraction to a plane adjacent to positively charged surfaces (Marsh et al., 1987), and as a precipitationdissolution process involving Al and Fe hydroxysulfate minerals (Adams and Rawajfih, 1977; Nordstrom, 1982). These processes have been advanced to explain increased S retention with increasing clay content and decreasing pH of acid, S-retentive soils. Investigations of soil SO4-S sorption-retention phenomena have frequently considered the role of precipitation in controlling solution-^solid phase S distribution (Adams and Rawajfih, 1977; Gebhardt and Coleman, 1974; Hodges and Johnson, 1987; Marcano-Martinez and McBride, 1974; Rajan, 1979; Wolt, 1981). Rajan (1979) discounted basic Al sulfate precipitation as an operative mechanism controlling SO4-S sorption in his studies, principally because of the short reaction times involved in the sorption process. Others have cited under- or oversaturation of their experimental systems with regard to basic Al sulfates (Gebhardt and Coleman, 1974; Hodges and Johnson, 1987; Hue et al., 1985; Marcano-Martinez and McBride, 1989). Nordstrom (1982) contends that rates of amorphous basic Al sulfate precipitation are sufficiently rapid to account for observed sorption phenomena. Ion activity products for highly acid (spH 4.1) batch extracts of allophanic tropical soils (Gebhardt and Coleman, 1974) bracketed the solubility of basaluminite [A14(OH)10SO4;
MATERIALS AND METHODS Soils of the Hilo (Site 150; thixotropic, isohyperthermic Typic Hydrandept) and Kaiwiki (Sites 300 and 450; thixotropic, isothermic Typic Hydrandepts) series were sampled from fields in long-term sugarcane (Saccharum officinarum L.) culture along an elevational gradient from 150 to 450 m above sea level on the island of Hawaii (Table 1). These soils are well-drained silty clay loams derived from volcanic ash of relatively uniform age; they are slightly
J.D. Wolt, DowElanco North American Environmental Chemistry Lab., Midland, MI 48641; and N.V. Hue and R.L. Fox, Dep. of Agronomy and Soil Science, College of Tropical Agriculture and Human Resources, Univ. of Hawaii, Honolulu, HI 96822. Contribution from the Dep. of Agronomy and Soil Science, Univ. of Hawaii and DowElanco. Received 27 Feb. 1991. 'Corresponding author.
Abbreviations: pK,p, negative logarithm of the solubility product; pIAP, negative logarithm of the ion activity product; AAS, atomic absorption spectrophotometry; IAP, ion activity product; s, sorbedphase concentrations; c, solution-phase concentration; /, ionic strength; SD, standard deviation; p/Cw, negative logarithm of the dissociation constant for water.
Published in Soil Sci. Soc. Am. J. 56:89-95 (1992).
89
90
SOIL SCI. SOC. AM. J., VOL. 56, JANUARY-FEBRUARY 1992
Table 1. Selected characteristics of the three Hawaiian Hydrandepts used in this study. Organic Soil solution Salt Gravimetric Depth moisture increment Soil Site c* PH pHt Hilo
Kaiwiki
150
300
cm 0-15 15-30
30-45 45-60 60-75 75-90 90-105 105-120 120-135 135-150 0-15 15-30
30-45
45-#)
Kaiwiki
450
60-75 75-90 90-105 105-120 120-135 135-150 0-15 15-30
30-45 45-60 60-75 75-90 90-105 105-120 120-135 135-150
gkg850 739 858 2159
2455 2289 2431
2939
2189 2178 1074 1512
2247 2267 2220 2635 2800 2766 2764 2919 1597 1417 1369
2899 2654 3057 3299 2770 3203 1896
1
4.5 4.6 4.9 4.9 4.9 5.0 5.1 5.7 5.4 5.6 4.7 4.7 4.8 4.8 5.0 5.3 5.6 5.5 5.4 5.3 5.3 5.3 5.6 5.6 5.6 5.6 5.6 5.7 5.6 5.4
5.0 5.1 5.6 5.1 4.8 5.1 4.8 4.9 5.7 5.4 5.3 5.1 5.3 4.9 5.1 5.2 5.5 5.4 5.5 5.7 5.7 5.5 6.6 5.7 5.6 5.7 5.9 5.7 5.8 5.7
gkg' 68 61 66 61 58 38 49 53 41 39 68 64 64 58 51 56 52 48 43 41 70 73 79 62 54 58 68 63 53 36
1
P-e (tractable !i(VS§ minolkg- 1
KCl-extractable
42
29 52 166 176 163 165 207 153 147 67 82 117 114 137 182 181 203 165 183 92 62 69 183 222 218 212 216 187 121
All
cmolc kg"1 0.76 0.64 0.48 0.24 0.05 0.10 0.16 0.18 0.15 0.15 0.77 0.45 0.65 0.49 0.55 0.33 0.23 0.41 0.41 0.43 0.20 0.19 0.22 0.18 0.28 0.45 0.47 0.41 0.39 0.22
11:20 (w/v) soil/0.12 MKC1. | Combustion and infrared detection. § 1:10 soil/0.04 MCa(H2PO4)2, pH 4 (Fox et al., 1987). 1 1:10 soil/1 MKCI (Barnhisel and Bertsch, 1982). acid throughout the profile, and allophane and gibbsite are the dominant minerals. Soil Ap (0—45 cm) and B (45-150 cm) horizons were sampled in 15-cm increments with a hand auger. Soil samples were placed in knotted plastic bags and refrigerated field moist at 4 °C until analyzed. Due to the thixotropic nature of these soils, samples were not screened or air dried prior to analysis. Soil moisture content was determined immediately prior to sorption measurements. Soil solutions were obtained by centrifugation as described by Adams et al. (1980). Soil solution pH was determined immediately after centrifugation. Soil solution SO4, NO3, and Cl were determined by ion chromatography with a Perkin-Elmer Series 4 LC (Perkin-Elmer, Norwalk, CT) with LC-21 conductivity detector using a 25-cm-long Vydac 302IC column (Separation Group, Hesperia, CA) and mobile phase of 2 mM phthalic acid (1,2-benzenedicarboxylic acid) adjusted to pH 5.0 with saturated Na2B4O7-10 H2O solution. Soil solution Na, K, Ca, and Mg were determined by AAS. Ion activity and IAP for soil solution components were computed using an3 ion speciation model (Wolt, 1987) modified to estimate Al * activities, assuming cryptocrystalline gibbsite solubility (pAl = 3pH — 9.2; calculated from Helgeson, 1969). Indigenous SO4-S was removed using four sequential extractions with 0.04 M Ca(H2PO4)2, pH 4, with a 1:10 soil/solution ratio (oven-dry basis) as described by Fox et al. (1987). Sulfate was determined by ion chromatography. The sum of SO4-S extracted is reported as P-extractable S. Sulfate sorption was determined by batch equilibration of soil with pH 4 K2SO4-KC1 solutions in which SO 4-S concentrations were varied from 0 to 40 mmol L,-1 at a constant / of 0.12 M. A 1:20 soil/solution ratio (oven-dry basis) and 16-h equilibration time were used. The equilibration solutions obtained after centrifugation of the batch
extracts were analyzed for pH and SO4—S using ion chromatography. Sorption data, adjusted for P-extractable indigenous SO4S, were fit to linear, Langmuir, and multisite Langmuir isotherms of the following forms, respectively:
[1]
5 =
s = s =
(1 + kf) k3c)
[2]
k4b4c 1 + k4c)
[3]
where 5 (mmol kg-1) and c (mmol L,-1) are sorbed- and solution-phase concentrations, respectively; k{ is the sorption1 coefficient; and b-t is the maximum quantity (mmol kg- ) sorbed. Parameter extimates, isotherm optimization, and model discrimination were performed using SimuSolv (Dow Chemical Co., Midland, MI) modeling and simulation software (Steiner et al., 1986). The best-fit isotherm for each site and depth-increment combination was selected by optimization of weighted nonlinear models by the method of maximum likelihood (Reilly and Blau, 1974). The maximum-likelihood function, percentage of variation explained, and standard deviation of parameter estimates were used as criteria for model selection. RESULTS AND DISCUSSION
Soil Solution Composition Soil solutions were acid (pH 4.80-6.62) and had low 7 (0.35-2.19 mmol L- *) (Table 1). Total soil solution SO4-S concentrations ranged from 0.014 to
WOLT ET AL.: SOLUTION SULFATE CHEMISTRY
91
Table 2. Soil solution composition, ion activities, and ion activity products for Hydrandept profiles sampled from Hawaii. Sitef
Depth increment
PH
Ca
Mg
300
450
0-15 15-30 30-45 45-60 60-75 75-90 90-105 105-120 120-135 135-150 0-15 15-30 30-45 45-60 60-75 75-90 90-105 105-120 120-135 135-150 0-15 15-30 30-45 45-60 60-75 75-90 90-105 105-120 120-135 135-150
NO3
+3
SO4
fl
(Al )
0.059 0.056 0.291 0.063 0.022 0.020 0.014 0.022 0.018 0.017 0.072 0.072 0.063 0.088 0.050 0.059 0.028 0.026 0.027 0.021 0.056 0.041 0.347 0.194 0.125 0.122 0.116 0.109 0.109 0.103
1.00 1.90 1.91 1.00 0.84 0.73 0.60 0.49 0.45 0.44 1.04 0.86 0.70 0.83 0.53 0.47 0.37 0.38 0.37 0.35 0.59 2.19 1.75 1.37 0.85 0.69 0.62 0.59 0.55 0.61
5.77 5.98 7.48 6.19 5.23 5.98 5.20 5.35 7.90 7.12 6.55 6.19 6.64 5.41 6.07 6.28 7.30 7.00 7.24 7.93 7.93 7.39 10.66 7.99 7.51 8.02 8.71 7.78 8.08 7.93
„„„! i _ |
cm 150
K
4.99 5.06 5.56 5.13 4.81 5.06 4.80 4.85 5.70 5.44 5.25
5.13 5.28 4.87 5.09 5.16 5.50 5.40 5.48 5.71 5.71 5.53 6.62 5.73 5.57 5.74 5.94 5.66 5.76 5.71
0.225 0.465 0.383 0.160 0.133 0.108 0.063 0.040 0.043 0.043 0.111 0.085 0.085 0.095 0.073 0.053 0.038 0.038 0.040 0.028 0.015 0.550 0.258 0.171 0.075 0.070 0.060 0.060 0.073 0.095
0.066 0.054 0.058 0.045 0.037 0.029 0.017 0.012 0.017 0.017 0.049 0.033 0.029 0.033 0.025 0.016 0.012 0.012 0.012 0.008 0.029 0.045 0.049 0.037 0.017 0.012 0.012 0.012 0.012 0.012
0.045 0.052 0.141 0.067 0.092 0.045 0.047 0.024 0.024 0.027 0.143 0.150 0.119 0.132 0.070 0.065 0.052 0.056 0.051 0.052 0.084 0.075 0.278 0.283 0.231 0.107 0.065 0.046 0.033 0.037
0.514 0.850 0.457 0.114 0.066 0.064 0.068 0.079 0.068 0.066 0.471 0.307 0.221 0.179 0.107 0.061 0.044 0.039 0.050 0.052 0.321 0.314 0.007 0.027 0.101 0.036 0.013 0.016 0.016 0.042
Ion activity product§
Ion activity
Soil solution composition
+
(K )
2
(S04- )
pJurb
pBasa
19.2 19.2 19.5 19.2 18.9 19.5 19.1 19.2 20.9 20.4 19.5 19.2 19.6 18.6 19.3 19.4 20.3 20.1 20.3 20.9 20.6 19.9 21.5 19.8 19.8 20.2 20.7 20.1 20.3 20.2 19.8 0.7
117.6 117.6 117.9 117.6 117.3 117.9 117.5 117.6 119.3 118.8 117.9 117.6 118.0 117.0 117.7 117.8 118.7 118.5 118.7 119.3 119.0 118.3 119.9 118.2 118.2 118.6 119.1 118.5 118.7 118.6 118.2 0.7 117.6*
- log activity ———— 4.38 4.36 4.29 4.30 3.61 3.87 4.12 4.19 4.49 4.05 4.56 4.36 4.75 4.36 4.69 4.63 4.66 4.63 4.69 4.58 4.20 3.86 4.17 3.84 4.21 3.94 3.89 4.03 4.16 4.33 4.27 4.20 4.53 4.29 4.52 4.26 4.54 4.30 4.69 4.29 4.40 4.09 4.05 4.15 3.48 3.58 3.58 3.56 3.83 3.65 3.91 3.98 3.94 4.20 3.94 4.35 3.97 4.49 4.44 3.98 Mean SD
17.211
Reference
pAlun
84.5 84.5 84.2 84.2 83.9 85.1 84.7 85.0 87.4 86.7 84.4 84.0 84.6 83.0 84.5 84.6 86.2 85.9 86.2 87.2 86.4 85.2 86.8 84.3 84.4 85.4 86.4 85.6 86.1 85.9 85.2 1.1 85.4#
t Described in Table 1. i Ionic strength. § pJurb = p(Al)(OH)(SO4); pBasa = p(Al)4(OH)10(SO4); pAlun = p(K)(AI)3(OH)6(SO4)2, I van Breeman, 1973. # Adams and Rawajfih, 1977.
0.347 mraol L-1. These SO4 intensities were, for the most part, below the critical limit (=0.16-0.47 mmol L-1) for plant growth on Andosols (Fox, 1974). Soil solution SO4-S was present dominantly as free SO42~; SO4 complexes made up
