Mechanisms for Boron Regeneration1

F. J. PERYEA, F. T. BiNGHAM2, AND J. D. RHOADESS

ABSTRACTThe ability of reclaimed natively high B soils to re-establish el*

evated soluble B concentrations has been termed "boron regenera-tion". Potential mechanisms for this phenomenon were qualitativelyexamined. Soil samples reclaimed in columns contained appreciableconcentrations of residual, potentially reactive B and Cl~. Recla-mation by multiple batch extraction removed 100 percent of the Cl~;however, the soils could not be completely reclaimed of B. Boronregeneration was observed in both column and batch reclamationsystems, while Cl~ regeneration occurred only in the column sys-tems. The data suggest that two mechanisms may account for Bregeneration. Post-reclamation release of B from residual sparinglysoluble or desorbable B-containing sources is confirmed by the batchand column experimental results. The column data suggest that por-tions of the soil were bypassed by the leaching solution during re-clamation. By analogy with Cl~ behavior, post-reclamation diffusionof B from bypassed to teachable pores also contributes to the Bregeneration phenomena.

Additional Index Words: reclamation, salt-affected soils, arid zonesoils.

Peryea, F.J., F.T. Bingham, and J.D. Rhoades. 1985. Mechanismsfor boron regeneration. Soil Sci. Soc. Am. J. 49:840-843.

RHOADES ET AL. (1970) used the B concentrationsof soil column effluents to monitor the status of

high B soils undergoing reclamation by leaching. Sol-uble B concentrations increased in the leachates fol-lowing a period of post-reclamation storage. The phe-nomenon was termed "boron regeneration" and wasattributed to the continued dissolution of sparinglysoluble sources of B. Boron regeneration occurred inboth natively high B and B-amended soils (Peryea etal., 1985).

The work of Kingston et al. (1974) suggests thatpost-reclamation desorption of B cannot be ruled outas an additional mechanism for regeneration. Dis-solved B occurs primarily as uncharged B(OH)§ mol-ecules and B(OH)4 anions. Boron tends to be specif-ically adsorbed to hydrous metal oxide-type surfacesin soils; the B(OH)^ anion appears to be the preferredadsorbed species (Keren and Bingham, 1985). Kings-ton et al. (1974) demonstrated that repeated washingsof such surfaces with a solution of constant pH re-sulted in reduced desorbability of specifically-ad-sorbed anions. Increased surface-anion affinity wasproduced through the loss of net negative surfacecharge or by the development of multiply-bonded co-ordination structures. Analogous behavior may be an-ticipated in soil systems subjected to long-term leach-ing, such as reclaimed high B soils. Changes in surfacecharge characteristics during post-reclamation storage

1 Contribution from the Dep. of Soil and Environmental Sciences,Univ. of California, Riverside, CA 92521. Received 18 May 1984.Approved 22 Jan. 1985.2 Graduate Fellow and Professor of Soil Science, Uniy. of Cali-fornia, respectively. The senior author is currently Postdoctoral Re-search Associate, Dep. of Agronomy and Soils, Washington StateUniv., Pullman WA 99164.3 Research Leader (Soil and Water Chemistry), U. S. Salinity Lab-oratory, Riverside CA 92501. i

could result in the release of residual surface-adsorbedB, causing soluble B levels to increase. The desorptionand the dissolution mechanisms cannot be differen-tiated macroscopically; hence, they are combined anddescribed as the dissolution mechanism in the ensuingdiscussion.

A second major mechanism is provided by consid-ering soil voids to be composed of two behavioralclasses of pores. Percolating solution derived fromsurface ponding will be transported primarily by a sys-tem of convectively dynamic, continuous pores. Asecond class of pores are those which are constrainedfrom transporting water convectively, due to small size,discontinuity, or mobile and immobile water parti-tioning (Kissel et al., 1973; McMahon and Thomas,1974; van Genuchten and Wierenga, 1977a). Nativesoil B will be preferentially leached from the soil vol-ume that is accessible to the leaching solution. Usingsome low threshold concentration of B in the drainagewater to signal the achievement of reclamation mayresult in a soil that is only apparently reclaimed. Con-siderable amounts of B may reside in the bypassedsoil pores and associated surfaces. Subsequent diffu-sion of B from the bypassed to the teachable poreswould result in elevated concentrations in the initialdrainage effluents after the equilibration of reclaimedhigh B soils.

The goal of the current study was to qualitativelydemonstrate the existence of one or both of the mech-anisms proposed in the preceding paragraphs. Thiswas accomplished in part by examining the ability ofnatively high B soils to regenerate B after reclamationby a multiple batch extraction procedure. This tech-nique should effectively eliminate the contribution ofthe pore bypass mechanism. The pore bypass mech-anism was tested by comparing the leaching and re-generation behavior of B with that of Cl~ in columnsystems. The latter soil constituent is highly solubleand generally inert with respect to adsorption phe-nomena in soils. The presence of residual Cl~ and theobservation of Cl~ regeneration should indicate porebypass during reclamation and redistribution by dif-fusion during storage.

MATERIALS AND METHODSSoils

Surface horizon samples of a Traver silt loam, Traver loam,and Twisselman clay loam were used in the study. The soilscontained naturally high levels of B and Cl~. They are fur-ther described in Peryea et al. (1985). The Traver soils areboth classified as coarse-loamy, mixed, thermic Natric Hap-loxeralfs. The Twisselman clay loam is a fine, mixed (cal-careous), thermic Typic Torriorthents.

Batch StudiesThe air-dry equivalent of 20.00 g of oven-dry soil per tube

was placed into acid-washed, 40 ml, polypropylene, OakRidge-type centrifuge tubes (18 tubes per soil). Sufficientdeionized water was added to each tube to bring the totalsolution volume to 20.00 ml. The tubes were sealed with

840

Published July, 1985

PERYEA ET AL.: MECHANISMS FOR BORON REGENERATION 841

screw-on caps and paraffin film and were placed on a wrist-action shaker. After shaking for selected time periods up to30 d, two replicates of each soil were centrifuged and thesupernatants collected following filtration. Boron, Cl~, andpH were measured in the supernatants.

The air-dry equivalent of 3.00 g of oven-dry soil per tubewas placed into a second set of centrifuge tubes (3 replica-tions per soil). These were extracted with 10 consecutive,48-h, 1:10 [solid/0.033 M Ca(NO3)2] extractions. Followingeach reaction period, the solids were spun down using ahigh-speed centrifuge fitted with an anglehead rotor(RCF=42850 for 15 min). Ninety percent of the supernatantwas removed by gentle suction and replaced with fresh ex-tractant. Following the 10th extraction, the soils were re-suspended and equilibrated while sealed for 30 d at 298 K.The solids were gently resuspended manually on a daily ba-sis. The supernatants were subsequently recovered by cen-trifugation. Boron, Cl~, and pH were analyzed in all super-natants following filtration. Boron was measured by themethod of John et al. (1975), modified by using a higherazomethine-H concentration to enhance sensitivity, and Cl~was analyzed using a Cl-titrator.

Column StudiesThe air-dry equivalent of 2.50 g of oven-dry soil per col-

umn was uniformily packed in chromatography columns,producing four replications per soil. The soils were contin-uously leached with 0.033 M Ca(NO3)2 under saturated flowconditions until 40 pore volume displacements were col-lected. The saturated columns were sealed and stored for 30days at 298 K. The columns were leached with 10 additionalpore volumes. The effluents were analyzed for B, CI~ andpH. Details of the procedure appear in Peryea et al. (1985).

RESULTS AND DISCUSSIONIn the 1:1 batch reaction experiment, Cl~ achieved

100% of its 30-d supernatant concentration in < 15min for the Traver loam and Twisselman soils and by30 min for the Traver silt loam. The respective c(Cl~)values were 498.4, 28.61, and 11.67 mol m"3. Boron,on the other hand, was continuing to solubilize at ratesof 0.001 to 0.30 mol m-3 d"1 after 30 d of reaction.At this point, supernatant B concentrations for thethree soils were 6.96, 3.50, and 0.88 mol m~3, respec-tively. These data indicate that the sources of reactiveB in the soils were found at lower concentrations andin less readily soluble states than were sources of Cl~.

The results of the multiple, 1:10, batch extractionseries are presented in Table 1. The term "new" con-stituent refers to the supernatant concentration of thatconstituent corrected for the previous extraction, i.e.,

St = \CWAf,) - C,-, [(J/_, - ^-,)/M-,]},[l]where, for extraction i, S is the freshly solubilized gra-vimetric concentration of the constituent (mmol kg~')>Cis the supernatant concentration (mol m~3), Fis thevolume of the supernatant (m3), P is the volume ofthe supernatant removed following centrifugation (m3),and M represents the oven-dry mass of soil (kg). Thecumulative gravimetric concentrations of B (Sfa) andCl~ (So) extracted by the 1:10 batch series were usedto estimate the reactive fraction of each constituentoriginally present in the soils, where

S1 = [2]

The reactive class of compounds are those subject torelatively rapid transformations and translocations.They would roughly correspond to the total diffusible(Sulaiman and Kay, 1972) or the leachable (Rhoadeset al., 1970) components found in the soils.

Table 1 indicates that freshly dissolved B was pres-ent in the supernatant of each extraction. The elevatedB concentrations found after the 30-d regenerationphase demonstrate that equilibrium was not attainedin the 48-h reaction period used in the extraction pro-cedure. The supernatant pH also increased during theregeneration phase. This may have resulted in a re-generated B concentration that under-reflects the Brelease rate, due to possible readsorption by pH-de-pendent sites (Keren and Bingham, 1985).

Each extraction of the 1:10 batch procedure can beconsidered a separate, 48-h regeneration opportunity,with the sum of the previous extractions representingthe effects of reclamation. The average rate of post-reclamation B dissolution is inversely related to thecumulative amount of B removed by preceding ex-tractions. This may be due to exhaustion of the re-active B content and subsequent changes in the ratesof approach to the equilibrium concentration value,alteration of surface and solution properties which

Table 1. Newly released B and Cl~ and supernatant pH in successive extractions of three natively high B soils.New constituent extracted

Extraction

It23456789

10REGEN

B

1.260.270.170.090.060.050.040.040.030.030.11

Traver ail

ci-

11.890.410.000.000.000.000.000.000.000.000.00

Traver 1

PH B ci- PH B

7.667.357.237.107.026.936.846.766.746.677.23

Et 2.16±0.01 12.29±0.12

10.552.041.030.610.390.250.180.170.120.110.35

15.81 ±0.14

503.560.150.000.050.000.000.000.000.000.000.00

8.347.987.487.337.016.916.886.836.826.737.14

503.74 ±3.58t Extraction 1 to 10—48 h at 1:10 (soil:0.033 MCafNO.),). Regeneration extraction: 720 h at 1:

5.521.050.600.350.230.160.130.100.080.060.23

Twisselman cl

ci-

26.530.000.000.000.000.000.000.000.000.000.00

pH

7.817.487.267.227.036.956.906.936.836.727.40

8.50±0.10 26.57 ±0.21:10 (aoil:0.033 M Ca(NO,),).

842 SOIL SCI. SOC. AM. J., VOL. 49, 1985

TRAVER LOAM

(i) Constant Heed

(ii) Driving Force = 9.8J/kg-m

(iii) 0.033M Co(N03)2

(iv) 30 day Equilibration

Table 2. Selected properties of key pore volume displacements(PVD) of soils leached and equilibrated in columns.

-•- BORON

-o- CHLORIDE

0 10 20 30 40 50

PORE VOLUME DISPLACEMENTFig. 1. Boron and Cl concentrations in successive pore volume dis-

placements (PVD) of the Traver loam soil. The solid vertical lineat PVD = 40 indicates an intervening 30-d, saturated storageperiod.

would enhance the energy of association of B withsolid phase materials (Kingston et al., 1974), or re-moval of soluble B-containing solids with the kineticsof solubility controlled by more recalcitrant forms(Griffin and Burau, 1974). The data in Table 1 implythat the capability of a soil to regenerate B will be afunction of the extraction history of the soil and thetime scale over which the phenomenon is observed.

Table 1 also indicates that essentially 100% of theCl~ content of each soil was dissolved in the first 48-h extraction. No Cl~ was found to come into solutionduring the 30-d regeneration phase. These observa-tions point out that the Cl~ sources in the soils arereadily soluble and that these highly saline soils canbe completely reclaimed of Cl", but not B, if the re-clamation solution is not restricted from interactionwith all of the solid phase components. This conclu-sion provides strong qualitative support for the post-reclamation dissolution mechanism for B regenera-tion.

Figure 1 shows the column leachate data for theTraver loam soil. As in the batch extraction, even inthe presence of considerable native constituent, theeffluent concentrations of Cl" rapidly dropped to non-detectible levels. The values for key points along theleaching curves for all three soils are in Table 2. Ineach case, the behavior of Cl" and B in the drainagewater was qualitatively similar to the 1:10 batch ex-traction results. The important exception is that bothCl" and B regenerated in the column systems. Al-though no Cl" was apparent in the drainage water ofany of the soils by the 40th pore volume displacement,from one to 10% of the original Cl~ content was cal-culated to still be present. The observation of Cl~ re-

Soil

Traver ail

Traver 1

Twisselman cl

Condi-tiont

NatRecReg§NatRecReg§NatRecReg§

AssociatecPVD

140411

40411

4041

, PVD properties

ci-

28.92 (90.3)0.00(90.5)0.14(91.0)

1149. (99.6)0.00 (99.7)0.41 (99.7)

62.64 (95.4)0.00(95.4)0.15(95.6)

B

(%)t ———1.26(23.3)0.01 (76.2)0.12(78.4)

14.29 (36.2)0.06 (90.3)0.32 (91.3)6.42 (30.4)0.03 (87.8)0.18 (88.6)

pH

8.357.607.858.357.907.957.957.707.95

t Nat = native; Rec = reclaimed; Reg = regenerated.j Percent of reactive constituent (S4) leached by indicated PVD.§ First PVD after 30-d of saturated storage.

generation qualitatively supports the existence of thepore bypass mechanism in soils that are reclaimed byleaching.

The simultaneous presence of potentially regener-able sources of B in the leached soil volumes (i.e.,dissolution mechanism) is also suggested by the col-umn effluent data. Chloride located in the bypassedsoil regions appears to be effectively isolated from theleaching solution. It is reasonable, therefore, to pre-sume that B in the bypassed regions is similarly con-strained from interaction. Since B was always presentin the column effluent, the contribution of a source ofregenerable B located external to the bypassed soil vol-ume is indicated.

The 1:10 batch extraction and the column leachatedata indicate the existence of appreciable quantitiesof potentially reactive B residing in natively high Bsoils after reclamation. The phenomenon of B regen-eration appears to be generated by at least two pro-cesses: dissolution of residual sparingly soluble/de-sorbable B sources and post-reclamation redistributionof B by diffusion from bypassed to leachable soil poreregions. Although the experiment was not designed toquantify the relative contributions of the two mech-anisms or to recreate field behavior, it does show thatthe causes of B regeneration are more complex thanoriginally proposed and that quantitative mechanisticmodels of B reclamation should incorporate compo-nents describing pore bypass and slow dissolution ef-fects. The relative contribution of each mechanismwill be a complex function of the nature and rates ofsurface exchange and solid solubilization, inter- andintra-aggregate diffusion rates, pore water velocity, andsoil and pore water characteristics (Tanji, 1970; vanGenuchten et al., 1977b; Rao et al., 1980).

ACKNOWLEDGMENTSThis research was supported in part by a grant from The

Kearney Foundation of Soil Science, University of Califor-nia.

MCBRIDE: SORPTION OF COPPER(II) ON ALUMINUM HYDROXIDE AS AFFECTED BY PHOSPHATE 843

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