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The consideration of soil boron adsorption and soil solution boron concentration as affected by moisture content
GEODER&A Geoderma66 (1995) 99-111
The consideration of soil boron adsorption and soil solution boron concentration as affected by moisture content P.V. Perkins Department of Geographical and Environmental Sciences, The University of Huddersjield, West Yorkshire, HDl3DH, UK Received 21 February 1994; accepted after revision 21 September 1994
Abstract Concentration-adsorption-moisture equations (CAM equations) were developed that enabled adsorbed boron and the boron concentration at any soil moisture content to be calculated from the measured boron concentration and moisture content of a saturated water extract, the bulk density and the measured coefficients defining the Langmuir and Freundlich boron adsorption isotherms. The
CAM equations were used to calculate the variation in boron concentration as a function of moisture content for three minespoils. Both Freundlich and Langmuir solutions to the CAM equations were tested against experimentally determined boron concentrations of the minespoils for moisture contents between 200 and 1200 l/m3. Both isotherm solutions to the CAM equations explained the experimentally determined variation in boron concentration as a function of moisture content to a high degree of accuracy. For two minespoils the Freundlich isotherm solution to the CAM equations provided the best fit to experimental data, for one minespoil the Langmuir solution to the CAM equations was better. Also the CAM ‘equations were used to predict potential boron phytotoxicity problems associated with seasonal moisture content variations within one of the minespoils and the implications of ameliorating the minespoil with pulverised fuel ash (PFA).
1. Introduction Boron is known to be highly phytotoxic and can cause severe problems to plants grown on some saline soils (Sankary, 1985) and soils irrigated with high-boron water (Gupta et al., 1985), and plants growing on Pulverised Fuel Ash (PFA or “fly ash”) or PFA ameliorated soils (Adrian0 et al., 1980; El-Mogazi et al., 1988). Boron can be extracted from soil using a wide range of extractants but extraction with cold water (Bingham, 1982) or hot 0016-7061/95/$09.50 0 1995 Elsevier Science B.V. All rights reserved .SsoIOO16-7061(94)00065-4
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water (Wolf, 1974; Gupta, 1979) are most widely used. Cold water extraction was used in this present study for two reasons: (i) boron adsorption studies are conventionally undertaken at room temperatures (Prasad, 1978; Evans and Sparks, 1983; Keren et al., 1985), and (ii) the boron contents of saturated extracts have been correlated with yield reductions for a wide range of plant species (U.S. Salinity Laboratory Staff, 1954; Bingham, 1973). A long-standing problem concerns how the boron concentration of a soil solution determined at saturated extraction (or some other suitable extraction ratio) is related to the boron concentration of the soil at different moisture contents within the soil under actual field conditions. It is this problem which this paper attempts to address. Equations using the Langmuir adsorption isotherm have been derived by Jame et al. ( 1982) which allowed the concentration of boron in soil solution under actual field moisture conditions to be estimated from laboratory determinations at saturated extraction. These equations for the Langmuir isotherm have been further developed in this paper to estimate the actual concentration of boron in soil solution at any moisture content. In addition, equations have been developed for the Freundlich isotherm which have been shown to provide a better description of boron adsorption for certain soil types ( Singh, 197 1; Elrashidi and O’Connor, 1982; Nicholaichuk et al., 1988). Both sets of equations utilise easily measurable properties; the boron concentration and water content of the saturated extract, dry bulk density and measured values of the parameters defining the Langmuir and Freundlich isotherms.
2. Theoretical background The total potentially BT = B& + B’A
available boron ( BT) in a soil can be expressed as: (1)
where B’& is the total boron in the soil solution [ mg B /m3], and Bx is the total adsorbed boron [ mg B/m3]. An equation similar to ( 1) in conjunction with the Langmuir adsorption isotherm was used by Bums and Collier ( 1980) to predict the leaching rate of boron (borate) from soils and PFA disposal sites. The total boron in solution will depend upon the boron concentration of the soil solution and the moisture content: B&,,=B,xO
(2)
where B, is the boron concentration of the soil solution [ mg B/l] , and 0 is the moisture content [l/m31 The total adsorbed boron will depend upon the dry bulk density of the soil p, and the adsorbed boron on the soil moisture content 8: BT,=pXB:
(3)
Assuming that the total potentially available boron does not change between field moisture condition (F) and saturated extraction (Se), Eq. ( 1) can be rewritten as:
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B~x~+~xB~=B~xe~,+pxB~
101
(4)
This assumption implies that any change in the boron concentration results from changes in the moisture content of the soil, either as an increase resulting from rainfall, or as a decrease resulting from evaporation. It does not however allow for changes in the boron concentration resulting from solution movement (i.e., leaching). Rearranging Eq. (4) for B& the concentration of boron in the soil solution under field moisture condition, produces: BE= [BF-p/&,x Letp/&=
Wand
BE= [BP-
(B:-By)]
&,/&=X,
x f&,/f&
(5)
so Eq. (5) becomes:
W(Bx-By)]X
(6)
Eq. (6)) having been solved by Jame et al. ( 1982) for one field moisture content, which was assumed to be 250 l/m3, compared to a moisture content of 500 l/m3 at saturated extraction. For r3r< t?,, the increased concentration of the boron in the soil solution would cause a corresponding increase in the boron adsorbed, so let the difference in the adsorbed boron under the two moisture conditions be Z BFA =BSe+Z A
(7)
Substitution of Eq. (7) into Eq. (6) produces: B;= [BF-WZ]X
(8)
For &> es, the decreased concentration of boron in the soil solution would cause a decrease in adsorbed boron, so the difference in adsorbed boron (Z) can also be written as: BF =BSe-Z A A
(9)
Substitution of Eq. (9) into Eq. (6) produces: Bg= [B,s”+ WZ]X
(10)
2.1. Langmuir adsorption isotherm Applying the following form of the Langmuir isotherm: B; = kbB,Bf( 1 + kB,B)
(11)
where Bi is adsorbed boron at moisture content 8 [mg B/kg], Bg is the equilibrium concentration of boron at moisture content 8 [ mg B/l], b is the maximum boron adsorption capacity [ mg B/kg], k is a constant [ I/mg] . For S, < 19,~Eq. (7) can be written as: B:=B:+Z=kbg‘l(l
+kB;)
For $ > es, Eq. (9) can be written as:
(12)
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B;=BS,“-Z=kbBg/(
1 +kB;)
(13)
An equation similar to ( 13) was utilised by Tanji ( 1970) together with a chromatographic equation to predict the leaching of boron applied to soils in columns. This approach allowed the boron in solution between saturated extraction and a 1: 1 soil:water extraction ratio to be predicted. However, it did not allow the boron in solution below saturated extraction to be predicted. This is of more practical importance because reduced moisture contents are likely to increase the concentration of boron in solution and exacerbate any boron phytotoxicity problems. Substitution of Eq. (8) into Eq. ( 12) gives: Bs;:+Z=kb[BFRearrangement
WZ]X/(l
and expansion
+k[Bp-
WZ])
(14)
of Eq. ( 14) gives:
- kxwzz + Z( 1 + kXBp - kXWBs,” + kbXW) + BF + kXB?BS,’ - kbXBp = 0
(15)
Let: A = - kXW,
B = 1 + kXBp - kXWBp + kbXW;
C = B”A” + kXB?BF
- kbXB;
Eq. ( 15) is a quadratic in Z, the solution to which is given by: Z=(
-B+(B2-4AC)“2)/2A
For &> (!I,, substitution
of Eq. ( 10) into Eq. ( 13) gives:
BF-Z=kb[Bp+WZ]X/( Expansion
(16)
1 +k[Bp+
WZ])
(17)
of Eq. ( 17) gives:
-kxwz2+Z(
- 1 -kXBp+kXWBF-kbXW)
+BF+kXBpBS,“-kbXB$==O (18)
Let A and C take the same values as for Eq. ( 15) but: B= -I-kXBF+kXWBF-kbXW Eq. (18) can now be solved by: Z=(
-B-(B2-4AC)“*)/2A
(19)
3. Freundlich adsorption isotherm solution Using the following form of the Freundlich B;=k(B:)”
adsorption isotherm, (20)
where Bz is adsorbed boron at moisture content 13 [ mg B/kg], Bl is the = equilibrium concentration of boron at moisture content 0 [ mg B/l], k and n are experimentally determined constants.
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For $ < es, substitution of Eq. (20) into Eq. (7) gives: Bx=p,“+Z=k(B;)”
(21)
and substitution of Eq. (21) into Eq. (8) gives: Bp+Z=k(
[BP-WZ]X)”
(22)
Taking logs of both sides produces: log(BT+Z)
=nxlog(XBp-WXZ)
+log k
(23)
For $ > 13,~substitution of Eq. (20) into Eq. (9) gives: B;=psA”-Z=k(B;)’
(24)
and substitution of Eq. (24) into Eq. ( 10) gives: BF-Z=k(
[Bp+WZ]X)”
(25)
Taking logs of both sides produces: log( BF -Z) = n x log( XBF + WXZ) + log k
(26)
Eqs. (23) and (26) can be solved to calculate Z by simultaneous computer iteration. The right-hand sides of Eqs. (23) and (26) becoming:
G, (r) = n X log(XBp - wXZ( t) ) + log k G2(f) =nXlog(XBp+WXZ(t))
+log k
and the left-hand sides becoming: H,(t) =log(B:+Z(t)) Z&(t) =log(B:-Z(t)) Computer based iterations allows the absolute value of the difference between the two sides to be less than a small specified value. A suitable condition for the iteration is IGr (t) -H,(t) I < 0.0005 for r increasing in increments of 0.00025 and similarly for G2( t) and H*(f).
4. Experimental method Three acidic minespoils were selected as part of an extensive study of potential physicochemical amelioration with pulverised fuel ash (PFA) . The potential benefits of PFA application to badly compacted minespoil include the neutralisation of acidity (Warren, 1992), improved moisture holding properties (Plass and Capp, 1974) and a reduction of bulk density (Vann et al., 1988). However, the phytotoxicity of boron released from the PFA has for a long time been recognised as a major problem to the establishment of vegetation (Townsend and Hodgson, 1973; Gutenmann et al., 1979; Shaw and Moffat, 1993).
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Table 1 Selected properties of minespoils Source of minespoil
Woolley Edge Hoyland Common Crowedge
pH
Boron concentration extract
(1:l)
Bulk density Moisture content of saturated extract (kg/m’) (l/m3)
4.8 4.7 3.2
1695 1580 1660
1.6 0.9 2.7
536 490 560
of saturated
(mg B/l)
British minespoils tend to suffer the twin extremes of summer drought and water-logging during winter (Rimmer, 1982). This wide variation in moisture content needs to be taken into account in order to relate the boron content of a minespoil (or PFA ameliorated minespoil) determined under laboratory conditions to changes in the actual field moisture content. Three minespoils from the Yorkshire area were selected for study. All three are compacted and consist of shales and clay interspersed with coal fragments and sandstone. These “soils’ ’ are azonal and pedogenetically very immature (Table 1) . Crowedge minespoil is overburden resulting from opencast mining of fireclay (Ordnance Survey 110; Grid Reference 185042). Woolley Edge minespoil (O.S. 110; 305115) and Hoyland Common minespoil (O.S. 110; 355013) result from the deep mining of coal. To allow some consistency between field determinations of bulk density and moisture content on unsieved minespoils, all experiments were conducted on 10 mm sieved samples oven-dried at 110°C for 24 hours. 4.1. Experiment
1
Preliminary investigations had shown that for all three minespoils, shaking for 24 hours with boron solutions between 0 and 50 mg B/l was sufficient to ensure “near” equilibrium conditions at room temperatures. Indeed, no statistically significant difference in the boron concentration of the equilibrium solution was apparent between 18 hours and 14 days. However, between 18 hours and 42 days a reduction in the boron concentration did occur, but this was only significant for solutions in excess of 30 mg B/l for Hoyland Common and Crowedge minespoils and above 40 mg B/l for Woolley Edge minespoil. For adsorption studies, 100 g samples of the three minespoils and sufficient boron solution (containing 0, 5, 10, 20, 30, 40 and 50 mg B/l as boric acid) to produce saturated extracts (Rhoades, 1982) were continuously shaken for 24 hours at room temperature, after which time the boron concentration of the filtered equilibrium solution was determined using the azomethine-H spectrophotometric technique (Bingham, 1982). Using standard graphical techniques (Evans and Sparks, 1983) adsorption constants were determined for the Langmuir isotherm (linear plot of C/x/m vs. C) and the Freundlich isotherm (linear plot of log x/m vs. log C) , where x/m and C represent the amount of boron adsorbed by unit mass of minespoil and the equilibrium concentration in the solution, respectively. A statistical analysis of the 95% confidence limits of the true gradient of the regression lines for the linear plots of both isotherms was undertaken. This demonstrated
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that a correction of the O-50 mg B/l solutions, achieved by taking into account the boron found naturally in the minespoils, did not significantly alter the calculated gradient of either plot, compared to uncorrected plots (p > 0.05). Consequently, no correction was made for naturally occurring boron. Adsorbed boron was therefore calculated from the difference in the concentration of the added solution (O-50 mg B/l) and the boron concentration of the equilibrium solution. For moisture content-boron concentration studies, sufficient distilled water was added to 2 kg of each minespoil to produce a saturated extract, mixed several times over 24 hours and stored in a plastic container. A 100 g sample of the saturated extract was removed, filtered and boron determined using azomethine-H. To produce samples with reduced moisture contents, 100 g samples of the saturated extracts were weighed into plastic specimen tubes and allowed to air-dry for between 1 and 10 days. After which time a sub-sample was oven dried to determine moisture content and the solution from the remaining sample extracted for boron analysis. Solution less than 0.2 ml/g minespoil, was extracted by ceramic pressure plate apparatus set at - 1.5 MPa (equivalent to permanent wilting point), and solution greater than 0.2 ml/g was extracted by vacuum filtration. To produce samples with increased moisture contents, distilled water was added to 100 g samples of the saturated extracts, stirred several times and stored in sealed plastic containers for three days. After which time samples were weighed to determine moisture content and the solution extracted by vacuum filtration used to determine boron concentration. 4.2. Experiment 2 British PFA samples have been shown to contain between 3 and 150 mg B/kg of coldwater soluble boron (Townsend and Hodgson, 1973). Taking a possible application rate of 10% by weight as an example, this would introduce between 0.3 and 15 mg B/kg into a minespoil. For the Crowedge minespoil, which has been studied extensively between 1990 and 1994, the following experiment was designed to study the consequences of boron additions over this possible range. For moisture content-boron concentration studies 2 kg of Crowedge minespoil were mixed with boron solutions containing either 14.8,29.6 or 44.4 mg B/l to produce saturated extracts (equivalent to additions of 5, 10 and 15 mg B/kg, respectively). The boron concentrations of 100 g samples of these “doped” saturated extracts were then determined using the procedures outlined for experiment 1 over a range of moisture contents.
5. Results A computer programrne was developed to calculate adsorbed boron and the boron concentration in the minespoil solutions for different moisture contents. Input data were measured and consisted of the boron concentration of the saturated extract (@) , moisture content at saturated extraction ( &.), dry bulk density (p), and the Langmuir and Freundlich adsorption constants (listed in Tables 1 and 2). Output data consisted of adsorbed boron at saturated extraction (By), adsorbed boron at field moisture condition (Bz), the boron
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Table 2 Adsorption constants of minespoils Source of minespoil
Adsorption constants Freundlich
Langmuir
Woolley Edge Hoyland Common Crowedge
b (mg B/kg)
k Nmg)
”
k
1.748 7.057 0.770
0.898 0.053 1.070
0.694 0.615 0.510
0.392 0.541 0.193
concentration of the minespoil solution at field moisture condition (BEj and the total potentially available boron ( BT). The concentration-adsorption-moisture equations (CAM equations) were solved for both adsorption isotherms in the following order: Freundlich isotherm equations (26) and (24) for & > es,, (23) and (21) for & < I!&. Langmuir isotherm equations ( 19), ( 17) and ( 10) for &> es,, ( 16), ( 14) and (8) for 10
1
9
1
6
1-r
0
I
200
400
600
800
1000
1200
Fig. 1. Calculated and experimentally determined variation in the boron concentration of three minespoils as affected by moisture content. L = Langmuir solution to the CAM equations. F= Freundlich solution to the CAM
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107
Experimental data and predicted curves calculated from the CAM equations are shown in Fig. 1 for the three “undoped” minespoils. For moisture contents less than at saturated extraction, the Freundlich isotherm solution to the CAM equations predicts boron concentrations slightly lower than the Langmuir solution. For moisture contents greater than at saturated extraction, the Freundlich solution predicts boron concentrations which are slightly higher than the Langmuir solution. The boron adsorption characteristics of the three minespoils exert a considerable influence upon the boron concentration of the solution as the moisture content changes. If no such influence occurred a halving of the moisture content would result in a corresponding doubling of the boron concentration. This is not the case. The minespoils tend to buffer the solution against decreases in the boron concentration by a decrease of adsorbed boron, so tending to attenuate any changes. Both isotherm solutions to the CAM equations fit the experimental variation in boron concentration with moisture content. For Woolley Edge minespoil R2 = 0.915 (p < 0.001) for the Freundlich solution and R2 = 0.833 (p < 0.01) for the Langmuir solution. For Crowedge minespoil R2 = 0.889 (p
125-
loo7 H g75-
so-
2s
05
0
100
200
300 400 0 (l/m31
500
600
Fig. 2. Experimental boron concentration of Crowedge minespoil solution as a function of moisture content, “doped” at saturated extraction with 14.8,29.6 or 44.4 mg B/l. Curves X, Y and Z are the Freundlich isotherm solution to the CAM equations for each “doping” solution.
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66 (I 995) 99-111
For Crowedge minespoil “doped” with boron solutions containing either 14.8, 29.6 or 44.4 mg B/l (equivalent to additions of 5, 10 and 15 mg B/kg at saturated extraction, respectively) the computed change in boron concentration as a function of moisture content was calculated slightly differently. Instead of using the measured boron concentration of the saturated extract (BP) as an input parameter, this was calculated from the Freundlich adsorption isotherm and then subsequently used to compute boron concentration change as a function of moisture content. These data are shown in Fig. 2, and illustrate a good agreement between predicted and experimental results. When the minespoil was “doped” with 14.8 mg B/l the Freundlich solution to the CAM equations produces a R* value of 0.957 (p < 0.001). For “doped” concentrations of 29.6 and 44.4 mg B/l the Freundlich isotherm solution to the CAM equations gave R2 values of 0.979 and 0.908, respectively (p
6. Discussion Both the Langmuir and Freundlich solutions to the CAM equations were calculated from adsorption constants determined at saturated extraction and consequently at the pH of the resulting extract. Changes in moisture content relative to saturated extraction are likely to change both the hydrogen ion concentration and the concentration of other soil species which may compete with boron for adsorption sites. Because boron adsorption is known to be pH dependent, with boron adsorption decreasing with acidity (Hingston, 1964; Okazaki and Chao, 1968; Goldberg and Forster, 1991)) such concentration changes could explain some of the discrepancy between the theoretical variation in boron concentration predicted by the CAM equations and the experimental data. Additionally, differences in the particle size distribution between individual samples of each minespoil must also be considered as a source of possible deviation from the predicted relationship between boron concentration and moisture content. The Freundlich isotherm was originally an empirical equation which mathematically describes quantitative experimental results. However, it can be shown to conform to a model of adsorption in which the affinity term decreases exponentially as the amount of adsorption increases (Hayward and Trapnell, 1964). By contrast the Langmuir isotherm was developed to describe the adsorption of gases on solid surfaces, and has been adapted to describe adsorption of many soil species (Harter and Baker, 1977). However, for any particular species the affinity and capacity factors of the Langmuir isotherm are pH dependent. For boron adsorption the affinity factor decreases with pH, whereas the capacity factor increases (Hingston, 1964). Thus, neither isotherm can predict adsorption as a function of pH. By comparison the phenomenological equation developed by Keren et al. ( 1981) is able to predict boron adsorption as a function of both the boron concentration in solution and pH. This equation assumes that B (OH) 3, B (OH); and .OH- compete for adsorption sites and that the pH of maximum adsorption is a function of the ratios of affinity coefficients of these three species (Keren and Mezuman, 198 1) . However, both the Langmuir and Freundlich isotherms are still commonly employed to describe boron adsorption, partly because they are relatively easy to determine and partly because they provide simple parameters which can be related to other soil properties (Barrow, 1978).
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The minespoils in this present study are typical of minespoils in which pyritic oxidation following tipping, produces a highly acidic substrate. Crowedge minespoil is most acidic at pH 3.2, and Woolley Edge and Hoyland Common minespoils less acidic at pH 4.8 and 4.7, respectively. Accordingly boron adsorption will predominantly be that of boric acid. Practical implications of the low boron adsorption capacities of the three minespoils studied and an indication of the usefulness of the CAM equations relate to the consequences of ameliorating these minespoils with PFA. The three acidic minespoils in this study have very low maximum boron adsorption capacities (the Langmuir b constants) of 0.770,1.748 and 7.057 mg B/kg, which upon application of PFA could have severe boron phytotoxicity consequences for the successful establishment of vegetation. Very little boron would be adsorbed, most would pass into solution and be potentially available for plant uptake in the form of molecular boric acid. For Crowedge minespoil, studies between 1990 and 1994 have shown bulk moisture content to vary between 166 l/m3 during summer and 498 l/m3 during winter. Taking a 10% by weight application of PFA containing 150 mg B/kg as an example, during summer months the minespoil solution could contain 94 mg B/l (see Fig. 2). During winter this would reduce to 33 mg B/l. However, both concentrations would be far in excess of the 6 mg B/l suggested as the limit above which boron phytotoxicity can be considered a problem (Gupta et al., 1985). For PFA containing 50 mg B/kg, a 10% application rate could result in boron concentrations of 42 mg B/l during summer and 15 mg B/l during winter, which again are likely to be highly phytotoxic. Because the parameters defining both the Langmuir and Freundlich isotherms were determined for the three minespoils at a pH corresponding to that of the equilibrium solution at saturated extraction, several problems persist, which require further research. British PFAs are alkaline, so upon addition to an acidic minespoil some neutralisation occurs and the resultant substrate would have an increased pH. For the CAM equations to be applicable to the resulting PFA-minespoil substrate the isotherm parameters would need to be determined for the resultant pH. However, the results of field trials conducted at the University of Huddersfield have shown several complications. Namely that, (a) boron dissolution from PFA added to a minespoil is significantly greater than that determined by cold-water extraction, (b) following PFA addition to a minespoil re-acidification continues due to pyritic oxidation, (c) decreases in bulk density lead to increased leaching of boron from the ameliorated horizon, and (d) during summer months evaporation causes the upward movement of boron. Work is presently in progress to relate these factors and the CAM equations into a predictive model of long-term boron concentration changes following PFA amelioration of acidic minespoils.
7. Conclusions The CAM equations are presented as a predictive tool which allow the boron concentration of a minespoil (or soil) determined at saturated extraction to be related to concentration under field moisture conditions. For two of the minespoils studied the Freundlich isotherm solution to the CAM equations described the experimental variation in boron concentration
110
P. V. Perkins / Geoderm
66 (1995) 99-l 11
with moisture content better than the Langmuir isotherm solution. For one minespoil the Langmuir isotherm provided a better description. Utilising soil parameters which are relatively easy to determine, the CAM equations could form the basis of models which relate leaching rate and moisture variation to boron concentration and ultimately to the prediction of possible plant response. A practical application of the CAM equations is for prediction of changes in the boron concentration of minespoils ameliorated with PFA, as a function of moisture content. Work is presently being undertaken to test the validity of the CAM equations for different soil types and the possible applicability of the equations for the description of moisturedependent concentration variations for other soil species (i.e., phosphorus).
Acknowledgements The work presented in this paper was supported by research contract number LWA/B/ 0014 with National Ash (National Power U.K.). Thanks must go to Hepworth Building Products Ltd. and British Coal for site access and assistance. Thanks also go to Dr. T. Vann and and Dr. T. Pearson for their supervision of this reseach.
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Keren, R., Bingham, F.T. and Rhoades, J.D., 1985. Plant uptake of boron as affected by boron distribution between liquid and solid phases in soil. Soil Sci. Sot. Am. J., 49: 297-302. Keren, R., Gast, R.G. and Bar-Yosef, B., 1981. pH-dependent boron adsorption by Na-montmorillonite. Soil Sci. Sot. Am. J., 45: 45-18. Keren, R. and Mezuman, U., 1981. Boron adsorption by clay minerals using a phenomenological equation. Clays Clay Miner., 29: 198-204. Nicholaichuk, W., Leyshon, A.J., Jame, Y.W. and Campbell, C.A.. 1988. Boron and salinity survey of irrigation projects and the boron adsorption characteristics of some Saskatchewan soils. Can. J. Soil Sci., 68: 77-90. Okazaki, E. and Chao, T.T., 1968. Boron adsorption and desorption by some Hawaiian soils. Soil Sci., 105: 25% 259. Plass, W.T. and Capp, J.P., 1974. Physical and chemical characteristics of surface mine spoil treated with fly ash. Soil Water Conserv., 29: 119-121. Prasad, M., 1978. Extent of boron adsorption and its relationship to soil properties in some West Indian soils. Commun. Soil Sci. Plant Anal., 9: 203-214. Rhoades, J.D., 1982. Soluble salts. In: A.L. Page (Editor), Agronomy: Methods of Soil Analysis. Part 2. American Society of Agronomy, WI. Rimmer. D.L., 1982. Soil physical conditions on reclaimed colliery spoil heaps. J. Soil Sci., 33: 567-579. Sankary. M.N., 1985. Species distribution and growth on salt-affected land in Syria. Proc. Research for Development Seminar on Forage and Fuel Production from Salt-Affected Wastelands, Cunderlin, W. Australia. Shaw, P.J.A. and Moffat, A.J., 1993. Establishment of trees on mixtures of pulverised fuel ash and gypsum. Part II. Nutrition and Trace Elements. Land Degradation and Rehabilitation, 4: 123-129. Singh, M., 1971. Equilibrium studies of boron in soils and clays. Geoderma, 5: 209-217. Tanji, K.K., 1970. A computer analysis on the leaching of boron from stratified soil columns. Soil Sci., 110: 4451. Townsend, W.N. and Hodgson, D.R., 1973. Edaphological problems associated with deposits of pulverised fuel ash. In: R.J. Hutnik and G. Davis (Editors), Ecology and Reclamation of Devastated Land. Gordon and Breach, New York, pp. 45-55. U.S. Salinity Laboratory Staff, 1954. Diagnosis and Improvement of Saline and Alkali Soils. USDA Agricultural Handbook No. 60. Vann, A.R., Hannington, L., Smith, S., Howard, J.A. and Hughes, I., 1988. The effect of ripping, ploughing and pulverised fuel ash on the bulk density of opencast minespoil. Soil Tech., 1: 263-269. Warren, C.J., 1992. Some limitations of sluiced fly ash as a liming material for acidic soils. Waste Manage. Res.. 10: 317-327. Wolf, B., 1974. Improvements in the azomethine-H method for the determination of boron. Commun. Soil Sci. Plant Anal., 5: 3944.
The consideration of soil boron adsorption and soil solution boron concentration as affected by moisture content P.V. Perkins Department of Geographical and Environmental Sciences, The University of Huddersjield, West Yorkshire, HDl3DH, UK Received 21 February 1994; accepted after revision 21 September 1994
Abstract Concentration-adsorption-moisture equations (CAM equations) were developed that enabled adsorbed boron and the boron concentration at any soil moisture content to be calculated from the measured boron concentration and moisture content of a saturated water extract, the bulk density and the measured coefficients defining the Langmuir and Freundlich boron adsorption isotherms. The
CAM equations were used to calculate the variation in boron concentration as a function of moisture content for three minespoils. Both Freundlich and Langmuir solutions to the CAM equations were tested against experimentally determined boron concentrations of the minespoils for moisture contents between 200 and 1200 l/m3. Both isotherm solutions to the CAM equations explained the experimentally determined variation in boron concentration as a function of moisture content to a high degree of accuracy. For two minespoils the Freundlich isotherm solution to the CAM equations provided the best fit to experimental data, for one minespoil the Langmuir solution to the CAM equations was better. Also the CAM ‘equations were used to predict potential boron phytotoxicity problems associated with seasonal moisture content variations within one of the minespoils and the implications of ameliorating the minespoil with pulverised fuel ash (PFA).
1. Introduction Boron is known to be highly phytotoxic and can cause severe problems to plants grown on some saline soils (Sankary, 1985) and soils irrigated with high-boron water (Gupta et al., 1985), and plants growing on Pulverised Fuel Ash (PFA or “fly ash”) or PFA ameliorated soils (Adrian0 et al., 1980; El-Mogazi et al., 1988). Boron can be extracted from soil using a wide range of extractants but extraction with cold water (Bingham, 1982) or hot 0016-7061/95/$09.50 0 1995 Elsevier Science B.V. All rights reserved .SsoIOO16-7061(94)00065-4
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water (Wolf, 1974; Gupta, 1979) are most widely used. Cold water extraction was used in this present study for two reasons: (i) boron adsorption studies are conventionally undertaken at room temperatures (Prasad, 1978; Evans and Sparks, 1983; Keren et al., 1985), and (ii) the boron contents of saturated extracts have been correlated with yield reductions for a wide range of plant species (U.S. Salinity Laboratory Staff, 1954; Bingham, 1973). A long-standing problem concerns how the boron concentration of a soil solution determined at saturated extraction (or some other suitable extraction ratio) is related to the boron concentration of the soil at different moisture contents within the soil under actual field conditions. It is this problem which this paper attempts to address. Equations using the Langmuir adsorption isotherm have been derived by Jame et al. ( 1982) which allowed the concentration of boron in soil solution under actual field moisture conditions to be estimated from laboratory determinations at saturated extraction. These equations for the Langmuir isotherm have been further developed in this paper to estimate the actual concentration of boron in soil solution at any moisture content. In addition, equations have been developed for the Freundlich isotherm which have been shown to provide a better description of boron adsorption for certain soil types ( Singh, 197 1; Elrashidi and O’Connor, 1982; Nicholaichuk et al., 1988). Both sets of equations utilise easily measurable properties; the boron concentration and water content of the saturated extract, dry bulk density and measured values of the parameters defining the Langmuir and Freundlich isotherms.
2. Theoretical background The total potentially BT = B& + B’A
available boron ( BT) in a soil can be expressed as: (1)
where B’& is the total boron in the soil solution [ mg B /m3], and Bx is the total adsorbed boron [ mg B/m3]. An equation similar to ( 1) in conjunction with the Langmuir adsorption isotherm was used by Bums and Collier ( 1980) to predict the leaching rate of boron (borate) from soils and PFA disposal sites. The total boron in solution will depend upon the boron concentration of the soil solution and the moisture content: B&,,=B,xO
(2)
where B, is the boron concentration of the soil solution [ mg B/l] , and 0 is the moisture content [l/m31 The total adsorbed boron will depend upon the dry bulk density of the soil p, and the adsorbed boron on the soil moisture content 8: BT,=pXB:
(3)
Assuming that the total potentially available boron does not change between field moisture condition (F) and saturated extraction (Se), Eq. ( 1) can be rewritten as:
P. V. Perkins / Geodemta 66 (1995) 99-11 I
B~x~+~xB~=B~xe~,+pxB~
101
(4)
This assumption implies that any change in the boron concentration results from changes in the moisture content of the soil, either as an increase resulting from rainfall, or as a decrease resulting from evaporation. It does not however allow for changes in the boron concentration resulting from solution movement (i.e., leaching). Rearranging Eq. (4) for B& the concentration of boron in the soil solution under field moisture condition, produces: BE= [BF-p/&,x Letp/&=
Wand
BE= [BP-
(B:-By)]
&,/&=X,
x f&,/f&
(5)
so Eq. (5) becomes:
W(Bx-By)]X
(6)
Eq. (6)) having been solved by Jame et al. ( 1982) for one field moisture content, which was assumed to be 250 l/m3, compared to a moisture content of 500 l/m3 at saturated extraction. For r3r< t?,, the increased concentration of the boron in the soil solution would cause a corresponding increase in the boron adsorbed, so let the difference in the adsorbed boron under the two moisture conditions be Z BFA =BSe+Z A
(7)
Substitution of Eq. (7) into Eq. (6) produces: B;= [BF-WZ]X
(8)
For &> es, the decreased concentration of boron in the soil solution would cause a decrease in adsorbed boron, so the difference in adsorbed boron (Z) can also be written as: BF =BSe-Z A A
(9)
Substitution of Eq. (9) into Eq. (6) produces: Bg= [B,s”+ WZ]X
(10)
2.1. Langmuir adsorption isotherm Applying the following form of the Langmuir isotherm: B; = kbB,Bf( 1 + kB,B)
(11)
where Bi is adsorbed boron at moisture content 8 [mg B/kg], Bg is the equilibrium concentration of boron at moisture content 8 [ mg B/l], b is the maximum boron adsorption capacity [ mg B/kg], k is a constant [ I/mg] . For S, < 19,~Eq. (7) can be written as: B:=B:+Z=kbg‘l(l
+kB;)
For $ > es, Eq. (9) can be written as:
(12)
102
P. V. Perkins / Geodema 66 (I 995) 99-11 I
B;=BS,“-Z=kbBg/(
1 +kB;)
(13)
An equation similar to ( 13) was utilised by Tanji ( 1970) together with a chromatographic equation to predict the leaching of boron applied to soils in columns. This approach allowed the boron in solution between saturated extraction and a 1: 1 soil:water extraction ratio to be predicted. However, it did not allow the boron in solution below saturated extraction to be predicted. This is of more practical importance because reduced moisture contents are likely to increase the concentration of boron in solution and exacerbate any boron phytotoxicity problems. Substitution of Eq. (8) into Eq. ( 12) gives: Bs;:+Z=kb[BFRearrangement
WZ]X/(l
and expansion
+k[Bp-
WZ])
(14)
of Eq. ( 14) gives:
- kxwzz + Z( 1 + kXBp - kXWBs,” + kbXW) + BF + kXB?BS,’ - kbXBp = 0
(15)
Let: A = - kXW,
B = 1 + kXBp - kXWBp + kbXW;
C = B”A” + kXB?BF
- kbXB;
Eq. ( 15) is a quadratic in Z, the solution to which is given by: Z=(
-B+(B2-4AC)“2)/2A
For &> (!I,, substitution
of Eq. ( 10) into Eq. ( 13) gives:
BF-Z=kb[Bp+WZ]X/( Expansion
(16)
1 +k[Bp+
WZ])
(17)
of Eq. ( 17) gives:
-kxwz2+Z(
- 1 -kXBp+kXWBF-kbXW)
+BF+kXBpBS,“-kbXB$==O (18)
Let A and C take the same values as for Eq. ( 15) but: B= -I-kXBF+kXWBF-kbXW Eq. (18) can now be solved by: Z=(
-B-(B2-4AC)“*)/2A
(19)
3. Freundlich adsorption isotherm solution Using the following form of the Freundlich B;=k(B:)”
adsorption isotherm, (20)
where Bz is adsorbed boron at moisture content 13 [ mg B/kg], Bl is the = equilibrium concentration of boron at moisture content 0 [ mg B/l], k and n are experimentally determined constants.
P.V. Perkins/Geodemu66(1995)9%111
103
For $ < es, substitution of Eq. (20) into Eq. (7) gives: Bx=p,“+Z=k(B;)”
(21)
and substitution of Eq. (21) into Eq. (8) gives: Bp+Z=k(
[BP-WZ]X)”
(22)
Taking logs of both sides produces: log(BT+Z)
=nxlog(XBp-WXZ)
+log k
(23)
For $ > 13,~substitution of Eq. (20) into Eq. (9) gives: B;=psA”-Z=k(B;)’
(24)
and substitution of Eq. (24) into Eq. ( 10) gives: BF-Z=k(
[Bp+WZ]X)”
(25)
Taking logs of both sides produces: log( BF -Z) = n x log( XBF + WXZ) + log k
(26)
Eqs. (23) and (26) can be solved to calculate Z by simultaneous computer iteration. The right-hand sides of Eqs. (23) and (26) becoming:
G, (r) = n X log(XBp - wXZ( t) ) + log k G2(f) =nXlog(XBp+WXZ(t))
+log k
and the left-hand sides becoming: H,(t) =log(B:+Z(t)) Z&(t) =log(B:-Z(t)) Computer based iterations allows the absolute value of the difference between the two sides to be less than a small specified value. A suitable condition for the iteration is IGr (t) -H,(t) I < 0.0005 for r increasing in increments of 0.00025 and similarly for G2( t) and H*(f).
4. Experimental method Three acidic minespoils were selected as part of an extensive study of potential physicochemical amelioration with pulverised fuel ash (PFA) . The potential benefits of PFA application to badly compacted minespoil include the neutralisation of acidity (Warren, 1992), improved moisture holding properties (Plass and Capp, 1974) and a reduction of bulk density (Vann et al., 1988). However, the phytotoxicity of boron released from the PFA has for a long time been recognised as a major problem to the establishment of vegetation (Townsend and Hodgson, 1973; Gutenmann et al., 1979; Shaw and Moffat, 1993).
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66 (I 995) 99-1 I 1
Table 1 Selected properties of minespoils Source of minespoil
Woolley Edge Hoyland Common Crowedge
pH
Boron concentration extract
(1:l)
Bulk density Moisture content of saturated extract (kg/m’) (l/m3)
4.8 4.7 3.2
1695 1580 1660
1.6 0.9 2.7
536 490 560
of saturated
(mg B/l)
British minespoils tend to suffer the twin extremes of summer drought and water-logging during winter (Rimmer, 1982). This wide variation in moisture content needs to be taken into account in order to relate the boron content of a minespoil (or PFA ameliorated minespoil) determined under laboratory conditions to changes in the actual field moisture content. Three minespoils from the Yorkshire area were selected for study. All three are compacted and consist of shales and clay interspersed with coal fragments and sandstone. These “soils’ ’ are azonal and pedogenetically very immature (Table 1) . Crowedge minespoil is overburden resulting from opencast mining of fireclay (Ordnance Survey 110; Grid Reference 185042). Woolley Edge minespoil (O.S. 110; 305115) and Hoyland Common minespoil (O.S. 110; 355013) result from the deep mining of coal. To allow some consistency between field determinations of bulk density and moisture content on unsieved minespoils, all experiments were conducted on 10 mm sieved samples oven-dried at 110°C for 24 hours. 4.1. Experiment
1
Preliminary investigations had shown that for all three minespoils, shaking for 24 hours with boron solutions between 0 and 50 mg B/l was sufficient to ensure “near” equilibrium conditions at room temperatures. Indeed, no statistically significant difference in the boron concentration of the equilibrium solution was apparent between 18 hours and 14 days. However, between 18 hours and 42 days a reduction in the boron concentration did occur, but this was only significant for solutions in excess of 30 mg B/l for Hoyland Common and Crowedge minespoils and above 40 mg B/l for Woolley Edge minespoil. For adsorption studies, 100 g samples of the three minespoils and sufficient boron solution (containing 0, 5, 10, 20, 30, 40 and 50 mg B/l as boric acid) to produce saturated extracts (Rhoades, 1982) were continuously shaken for 24 hours at room temperature, after which time the boron concentration of the filtered equilibrium solution was determined using the azomethine-H spectrophotometric technique (Bingham, 1982). Using standard graphical techniques (Evans and Sparks, 1983) adsorption constants were determined for the Langmuir isotherm (linear plot of C/x/m vs. C) and the Freundlich isotherm (linear plot of log x/m vs. log C) , where x/m and C represent the amount of boron adsorbed by unit mass of minespoil and the equilibrium concentration in the solution, respectively. A statistical analysis of the 95% confidence limits of the true gradient of the regression lines for the linear plots of both isotherms was undertaken. This demonstrated
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66 (I 995) 9%I 1 I
105
that a correction of the O-50 mg B/l solutions, achieved by taking into account the boron found naturally in the minespoils, did not significantly alter the calculated gradient of either plot, compared to uncorrected plots (p > 0.05). Consequently, no correction was made for naturally occurring boron. Adsorbed boron was therefore calculated from the difference in the concentration of the added solution (O-50 mg B/l) and the boron concentration of the equilibrium solution. For moisture content-boron concentration studies, sufficient distilled water was added to 2 kg of each minespoil to produce a saturated extract, mixed several times over 24 hours and stored in a plastic container. A 100 g sample of the saturated extract was removed, filtered and boron determined using azomethine-H. To produce samples with reduced moisture contents, 100 g samples of the saturated extracts were weighed into plastic specimen tubes and allowed to air-dry for between 1 and 10 days. After which time a sub-sample was oven dried to determine moisture content and the solution from the remaining sample extracted for boron analysis. Solution less than 0.2 ml/g minespoil, was extracted by ceramic pressure plate apparatus set at - 1.5 MPa (equivalent to permanent wilting point), and solution greater than 0.2 ml/g was extracted by vacuum filtration. To produce samples with increased moisture contents, distilled water was added to 100 g samples of the saturated extracts, stirred several times and stored in sealed plastic containers for three days. After which time samples were weighed to determine moisture content and the solution extracted by vacuum filtration used to determine boron concentration. 4.2. Experiment 2 British PFA samples have been shown to contain between 3 and 150 mg B/kg of coldwater soluble boron (Townsend and Hodgson, 1973). Taking a possible application rate of 10% by weight as an example, this would introduce between 0.3 and 15 mg B/kg into a minespoil. For the Crowedge minespoil, which has been studied extensively between 1990 and 1994, the following experiment was designed to study the consequences of boron additions over this possible range. For moisture content-boron concentration studies 2 kg of Crowedge minespoil were mixed with boron solutions containing either 14.8,29.6 or 44.4 mg B/l to produce saturated extracts (equivalent to additions of 5, 10 and 15 mg B/kg, respectively). The boron concentrations of 100 g samples of these “doped” saturated extracts were then determined using the procedures outlined for experiment 1 over a range of moisture contents.
5. Results A computer programrne was developed to calculate adsorbed boron and the boron concentration in the minespoil solutions for different moisture contents. Input data were measured and consisted of the boron concentration of the saturated extract (@) , moisture content at saturated extraction ( &.), dry bulk density (p), and the Langmuir and Freundlich adsorption constants (listed in Tables 1 and 2). Output data consisted of adsorbed boron at saturated extraction (By), adsorbed boron at field moisture condition (Bz), the boron
P. V. Perkins / Geodenna 66 (1995) 99-l 11
106
Table 2 Adsorption constants of minespoils Source of minespoil
Adsorption constants Freundlich
Langmuir
Woolley Edge Hoyland Common Crowedge
b (mg B/kg)
k Nmg)
”
k
1.748 7.057 0.770
0.898 0.053 1.070
0.694 0.615 0.510
0.392 0.541 0.193
concentration of the minespoil solution at field moisture condition (BEj and the total potentially available boron ( BT). The concentration-adsorption-moisture equations (CAM equations) were solved for both adsorption isotherms in the following order: Freundlich isotherm equations (26) and (24) for & > es,, (23) and (21) for & < I!&. Langmuir isotherm equations ( 19), ( 17) and ( 10) for &> es,, ( 16), ( 14) and (8) for 10
1
9
1
6
1-r
0
I
200
400
600
800
1000
1200
Fig. 1. Calculated and experimentally determined variation in the boron concentration of three minespoils as affected by moisture content. L = Langmuir solution to the CAM equations. F= Freundlich solution to the CAM
P. V. Perkins / Geodenna 66 (1995) 99-I 1 I
107
Experimental data and predicted curves calculated from the CAM equations are shown in Fig. 1 for the three “undoped” minespoils. For moisture contents less than at saturated extraction, the Freundlich isotherm solution to the CAM equations predicts boron concentrations slightly lower than the Langmuir solution. For moisture contents greater than at saturated extraction, the Freundlich solution predicts boron concentrations which are slightly higher than the Langmuir solution. The boron adsorption characteristics of the three minespoils exert a considerable influence upon the boron concentration of the solution as the moisture content changes. If no such influence occurred a halving of the moisture content would result in a corresponding doubling of the boron concentration. This is not the case. The minespoils tend to buffer the solution against decreases in the boron concentration by a decrease of adsorbed boron, so tending to attenuate any changes. Both isotherm solutions to the CAM equations fit the experimental variation in boron concentration with moisture content. For Woolley Edge minespoil R2 = 0.915 (p < 0.001) for the Freundlich solution and R2 = 0.833 (p < 0.01) for the Langmuir solution. For Crowedge minespoil R2 = 0.889 (p
125-
loo7 H g75-
so-
2s
05
0
100
200
300 400 0 (l/m31
500
600
Fig. 2. Experimental boron concentration of Crowedge minespoil solution as a function of moisture content, “doped” at saturated extraction with 14.8,29.6 or 44.4 mg B/l. Curves X, Y and Z are the Freundlich isotherm solution to the CAM equations for each “doping” solution.
108
P. V. Perkins / Geodem
66 (I 995) 99-111
For Crowedge minespoil “doped” with boron solutions containing either 14.8, 29.6 or 44.4 mg B/l (equivalent to additions of 5, 10 and 15 mg B/kg at saturated extraction, respectively) the computed change in boron concentration as a function of moisture content was calculated slightly differently. Instead of using the measured boron concentration of the saturated extract (BP) as an input parameter, this was calculated from the Freundlich adsorption isotherm and then subsequently used to compute boron concentration change as a function of moisture content. These data are shown in Fig. 2, and illustrate a good agreement between predicted and experimental results. When the minespoil was “doped” with 14.8 mg B/l the Freundlich solution to the CAM equations produces a R* value of 0.957 (p < 0.001). For “doped” concentrations of 29.6 and 44.4 mg B/l the Freundlich isotherm solution to the CAM equations gave R2 values of 0.979 and 0.908, respectively (p
6. Discussion Both the Langmuir and Freundlich solutions to the CAM equations were calculated from adsorption constants determined at saturated extraction and consequently at the pH of the resulting extract. Changes in moisture content relative to saturated extraction are likely to change both the hydrogen ion concentration and the concentration of other soil species which may compete with boron for adsorption sites. Because boron adsorption is known to be pH dependent, with boron adsorption decreasing with acidity (Hingston, 1964; Okazaki and Chao, 1968; Goldberg and Forster, 1991)) such concentration changes could explain some of the discrepancy between the theoretical variation in boron concentration predicted by the CAM equations and the experimental data. Additionally, differences in the particle size distribution between individual samples of each minespoil must also be considered as a source of possible deviation from the predicted relationship between boron concentration and moisture content. The Freundlich isotherm was originally an empirical equation which mathematically describes quantitative experimental results. However, it can be shown to conform to a model of adsorption in which the affinity term decreases exponentially as the amount of adsorption increases (Hayward and Trapnell, 1964). By contrast the Langmuir isotherm was developed to describe the adsorption of gases on solid surfaces, and has been adapted to describe adsorption of many soil species (Harter and Baker, 1977). However, for any particular species the affinity and capacity factors of the Langmuir isotherm are pH dependent. For boron adsorption the affinity factor decreases with pH, whereas the capacity factor increases (Hingston, 1964). Thus, neither isotherm can predict adsorption as a function of pH. By comparison the phenomenological equation developed by Keren et al. ( 1981) is able to predict boron adsorption as a function of both the boron concentration in solution and pH. This equation assumes that B (OH) 3, B (OH); and .OH- compete for adsorption sites and that the pH of maximum adsorption is a function of the ratios of affinity coefficients of these three species (Keren and Mezuman, 198 1) . However, both the Langmuir and Freundlich isotherms are still commonly employed to describe boron adsorption, partly because they are relatively easy to determine and partly because they provide simple parameters which can be related to other soil properties (Barrow, 1978).
P.V. Perkins/Geodemw66(1995)
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109
The minespoils in this present study are typical of minespoils in which pyritic oxidation following tipping, produces a highly acidic substrate. Crowedge minespoil is most acidic at pH 3.2, and Woolley Edge and Hoyland Common minespoils less acidic at pH 4.8 and 4.7, respectively. Accordingly boron adsorption will predominantly be that of boric acid. Practical implications of the low boron adsorption capacities of the three minespoils studied and an indication of the usefulness of the CAM equations relate to the consequences of ameliorating these minespoils with PFA. The three acidic minespoils in this study have very low maximum boron adsorption capacities (the Langmuir b constants) of 0.770,1.748 and 7.057 mg B/kg, which upon application of PFA could have severe boron phytotoxicity consequences for the successful establishment of vegetation. Very little boron would be adsorbed, most would pass into solution and be potentially available for plant uptake in the form of molecular boric acid. For Crowedge minespoil, studies between 1990 and 1994 have shown bulk moisture content to vary between 166 l/m3 during summer and 498 l/m3 during winter. Taking a 10% by weight application of PFA containing 150 mg B/kg as an example, during summer months the minespoil solution could contain 94 mg B/l (see Fig. 2). During winter this would reduce to 33 mg B/l. However, both concentrations would be far in excess of the 6 mg B/l suggested as the limit above which boron phytotoxicity can be considered a problem (Gupta et al., 1985). For PFA containing 50 mg B/kg, a 10% application rate could result in boron concentrations of 42 mg B/l during summer and 15 mg B/l during winter, which again are likely to be highly phytotoxic. Because the parameters defining both the Langmuir and Freundlich isotherms were determined for the three minespoils at a pH corresponding to that of the equilibrium solution at saturated extraction, several problems persist, which require further research. British PFAs are alkaline, so upon addition to an acidic minespoil some neutralisation occurs and the resultant substrate would have an increased pH. For the CAM equations to be applicable to the resulting PFA-minespoil substrate the isotherm parameters would need to be determined for the resultant pH. However, the results of field trials conducted at the University of Huddersfield have shown several complications. Namely that, (a) boron dissolution from PFA added to a minespoil is significantly greater than that determined by cold-water extraction, (b) following PFA addition to a minespoil re-acidification continues due to pyritic oxidation, (c) decreases in bulk density lead to increased leaching of boron from the ameliorated horizon, and (d) during summer months evaporation causes the upward movement of boron. Work is presently in progress to relate these factors and the CAM equations into a predictive model of long-term boron concentration changes following PFA amelioration of acidic minespoils.
7. Conclusions The CAM equations are presented as a predictive tool which allow the boron concentration of a minespoil (or soil) determined at saturated extraction to be related to concentration under field moisture conditions. For two of the minespoils studied the Freundlich isotherm solution to the CAM equations described the experimental variation in boron concentration
110
P. V. Perkins / Geoderm
66 (1995) 99-l 11
with moisture content better than the Langmuir isotherm solution. For one minespoil the Langmuir isotherm provided a better description. Utilising soil parameters which are relatively easy to determine, the CAM equations could form the basis of models which relate leaching rate and moisture variation to boron concentration and ultimately to the prediction of possible plant response. A practical application of the CAM equations is for prediction of changes in the boron concentration of minespoils ameliorated with PFA, as a function of moisture content. Work is presently being undertaken to test the validity of the CAM equations for different soil types and the possible applicability of the equations for the description of moisturedependent concentration variations for other soil species (i.e., phosphorus).
Acknowledgements The work presented in this paper was supported by research contract number LWA/B/ 0014 with National Ash (National Power U.K.). Thanks must go to Hepworth Building Products Ltd. and British Coal for site access and assistance. Thanks also go to Dr. T. Vann and and Dr. T. Pearson for their supervision of this reseach.
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