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i
BIOMASS PRODUCTION
AND
WATER QUALITY IN
AN
ACIDIC SPOIL
AMENDED WITH Mg(OH)2-EMICHED BY-PRODUCT GYPSUM
Humberto Yibirin, R.C. Stehouwer,
J.
Bigham, and P. Sutton.
OARDC/School of Natural Resources, Williams Hall
Wooster, OH 44691
Keywords: GYPSUM, MAGNESIUM, SPOIL, LAND RECLAMATION
Gypsum (G) and Mg(OH), (FGD-Mg) are recovered from the
thickener overflow of an experimental wet FGD scrubber in the
Zimmer power plant of CINergy (Cincinnati Gas and Electric
Company). The purity of recovered G ranges from almost 100 to
92%, with impurities occurring mainly as Mg(OH),. Gypsum
contaminated with Mg(OH), is referred to as Mg(OH),-enriched
by-product gypsum (Mg-G)
.
Because
of
its Mg(OH), content, Mg-G can potentially be
used as a source of both Ca and Mg for green plants growing
on soils and spoils with low levels of available Ca and Mg.
In addition, the presence of Mg(OH), should allow Mg-G to be
used as an alkaline amendment for the reclamation of hyper-
acid spoils. The total abandoned surface coal mined land
needing reclamation in eastern
USA
has been estimated to be
more than 0.5 million ha (Sutton and Dick, 1987). Gypsum can
reduce
A1
toxicity by: (1)increased ratio of Ca to
A1
in the
soil solution (Kinraide et al., 19921, and (2) physical
removal of A1 from the soil profile through Ca exchange
(Wendell and Ritchey, 1993). The Mg(OH), in Mg-G may enhance
the effectiveness of these mechanisms for amelioration of
phytotoxic conditions below the zone of incorporation. The
presence of Mg and sulfate increases the potential for salt
loading (Stehouwer et al., 1995), which may enhance downward
movement of Ca and Al.
The purpose of this greenhouse study was to investigate
the effects of G and Mg-G, at three application rates, on
spoil and leachate pH and electrical conductivity (EC), and
movement of major and trace elements. Preliminary
observations on plant growth are also reported.
MATERIALS
AND
METHODS
Acidic minespoil was sampled from the upper 20
cm
of an
abandoned mineland
(AML)
site located at the Eastern Ohio
Resource Development Center (EORDC)
.
The samples were air-
dried and passed through a 12.7-mm sieve. Initial Bray #1 P
was 25
mg
kg"; NH,-acetate extractable Ca, K, Mg, and
A1
were
225, 54, 37, and 601
mg
kg-' respectively; CEC was 30 cmol,
kg-', and pH was 2.9.
Three by-product materials were used: G (98% gypsum),
and two Mg-G blends (4%Mg-G, and
8%
Mg-G) which contained 4
and
8%
(w/w) Mg(OH), equivalent. Reagent grade Ca(OH), was
also mixed with G to produce two mixtures (4%Ca-G, and 8%Ca-
G) with neutralizing potentials equal to 4%Mg-G and
8%
Mg-G
respectively.
Spoil material
(8
kg) was poured into PVC columns (60 cm
tall, 15 cm dim.) to
a
height of 36 cm forming an untreated
sub-surface layer.
A
15-cm surface layer of spoil (3.4
kg
)
was then thoroughly mixed with the various treatments and
placed over the untreated spoil. The columns were mounted on
flat PVC plates with a nipple in the center to allow for
leachate collection.
Treatments consisted of G, 4%Mg-G, 8%Mg-G, 4%Ca-G, or
8%Ca-G applied at rates equivalent to 145, 290, and 580 Mg
ha-'
.
These rates were Calculated to supply Ca from G in
amounts equal to 2.5,
5.0,
and
10.0
times the spoil CEC in
the treated layer. Control treatments included unamended
spoil and spoil amended to pH
7.0
with limestone (112 Mg ha-
I).
Treatments were arranged in randomized complete blocks
with three replications. Fertilizers were applied together
with the treatments in amounts of 0.5
g
NH,N03, 0.4
g
triple
superphosphate, and 2 g KC1.
575
Columns were then leached with 2.5 L of deionized water
and the first leachates (150-200
ml)
were collected.
Following this first leaching, 5 g of soil was collected from
the surface 5-cm depth of each column for pH and EC
measurements, and each column was planted with 30 seeds of
Orchardgrass (Dactylis glomerata
L.).
After an initial 80-d
growing period, orchardgrass was harvested monthly for a
total of 4 harvests. Leachates were also collected at the end
of the study. Columns were watered daily with deionized water
such that the amount of water applied during the study was
equivalent to the average annual rainfall
for SE Ohio (=lo00
m).
Leachates were analyzed for pH, EC, and for As, B, Ba,
Be, Ca, Cd, Co, Cr, Cu, Fe,
K,
Li, Mg, Mn, Mo, Na, Ni,
P,
Pb,
S,
Sb, Se, Si, and
Zn
by inductively coupled plasma emission
spectrometry. Data analysis was conducted using analysis of
variance procedures, and single degree of freedom orthogonal
contrasts.
RESULTS
AND
DISCUSSION
The Mg-G and Ca-G amendments were equally effective at
increasing the pH
of
the acidic minespoil in the treated
layer (Table
1).
None of the amendments used, however,
increased the pH of the first
or
final leachates compared to
the untreated spoil. Increases in spoil pH led to increased
growth of orchardgrass, however, the largest application
rates of
4%
and
8%
Mg-G suppressed yield in the first two
harvests. This initial yield suppression was associated with
large increases in EC.
Mg-G increased spoil and leachate EC more than any other
treatment (Table
1).
The use of 4%Mg-G increased spoil EC
1.23 times compared to 4%Ca-G, 8%Ca-G, or gypsum, and 3.4
times compared to either the untreated or limed spoil. These
differences were larger when the spoil was amended with 8%Mg-
G.
Leaching of salts out of the column during the course of
the study reduced the EC in the final leachates, but Mg-G
effects
on
EC were still present. Stehouwer et al. (1995)
showed that the solubility of Mg in a spoil amended with
materials containing CaSO, and Mg(OH), may be controlled by
epsomite (MgS0,.6H20) which is
fl
300
times more soluble than
gypsum.
Enrichment of gypsum with Mg, therefore, increased
the potential for salt loading. Gypsum-containing treatments
increased the leachate concentrations of Ca, Mg, Al, Fe,
Mn,
and
S
(Table
2),
and of Cd, Cr, Pb, Cu, and B (Table
3)
relative to the control treatments. The largest increase
occurred with Mg-G. Most cations were much less mobile with
limestone and Ca-G treatments. The use of G alone increased
the movement of Al,
S,
Fe,
Mn,
Cr, and B compared to Ca-G.
The Ca(OH), in Ca-G reduced the movement of metal cations in
the spoil compared to G alone, thus making Ca-G behave more
like limestone than
G.
By contrast, the Mg(OH), in Mg-G
increased the movement of metal cations in the spoil compared
to
G
alone, limestone,
or
Ca-G. Effects of Mg-G
on
cation
movement were most likely not due to pH differences since Mg-
G and Ca-G had similar effects on pH. The increased transport
of these metals with Mg-G appeared to be due to large
concentrations of Mg2' in solution. We believe the increases
in metals
in the leachates were due to mobilization of metals
present in the spoil. These were brought into solution
through exchange reactions with Mg2' and then transported
downward.
Amendment with Mg-G increased pH, thus allowing
revegetation of otherwise phytotoxic spoils.
In
addition to
being a source of Ca and Mg, Mg-G enhanced downward
movement of Al, and Fe which may promote root penetration in
untreated subsurface layers, and improve the chances of
reclamation success. Amendment applications, however, should
be limited to rates that will not cause phytotoxic salt
concentrations, excessively high pH,
or
increase the
concentrations of heavy metals in water to harmful levels.
516
Table 1. Initial and final electrical conductivity (EC) and
pH in the spoil and leachate, averaged across rates, as
affected by wet FGD Mg (OH) ,-enriched
gypsum, gypsum,
Ca (OH) ,-enriched
gypsum,
and calcitic limestone'.
Spoil
Leach Leach
Spoil Leach
Lehcii
Init
First
Filial
Init
First Final
EC,
s
,-I
-----
PH
------
-----
-----_
4 %Mg-G 5.60 2.27 2.58 0.27
0.64 0.32
8% Mg-G 7.31 2.26 2.67
0.32 0.64
0.29
4%Ca-G 5.82
2.27
2.55
0.23 0.44
0.24
8%Ca-G
7.23
2.26 2.55 0.21 0.47
0.21
Gypsum
3.09 2.26 2.60 0.22
0.53 0.28
Untreated 2.87
2.33 2.78
0.07
0.30 0.09
Limestone 7.09
2.33 2.71 0.09
0.30
0.10
avq
.
5.71 2.27 2.61 0.23 0.52 0.25
lsd
0.05' 0.31
NS
0.05 0.03 0.09 0.04
7
lsd
=
least significant difference.
*
leachate and initial are abbreviated as leach and init.
Table 2. Major element composition of first leachate,
averaged across rates, as affected by wet
FGD
Mg(OH),-
enriched
gypsum, gypsum,
Ca
(OH)
,-enriched
gypsum,
and
calcitic limestone.
4%Mg-G
124 301 866 142 3 1821
8%
Mg-G
105
249
950
150 3 1903
4%Ca-G
64 53
418
72 2 646
8%Ca-G
69
56
432
76
2 617
Gypsum
78
108
700
114
3 1201
Untreated
40 35
227
48
1
440
Limestone
36
37 246
53
1
479
avg
.
82 139 622 104 2 1146
lsd'
0.05 22 57
170
22
1
258
t
1sd
=
least significant difference.
Table 3. Trace element composition of first leachate,
averaged across rates, as affected by wet FGD Mg(OH),-
enriched
gypsum, gypsum,
Ca
(OH)
,-enriched
gypsum,
and
calcitic limestone.
As
Cd Cr Pb cu
B
--_-______-__-__
mg
L-1
--__---------__
4%Mg-G
<0.04 0.05 0.28 0.22 0.64 4.41
8% Mg-G <0.04 0.05 0.30 0.21
0.65
3.91
4%Ca-G <0.04 0.04 0.14
0.11
0.50 0.40
8%Ca-G CO.04 0.04 0.15
0.09
0.52 0.35
Gypsum
<0.04 0.05 0.24 0.18 0.63 1.30
Untreated <0.04 0.03 0.09 0.08 0.35 0.27
Limestone <0.04 0.03 0.08 0.19 0.32 0.23
avg
.
CO.04
0.05
0.21 0.16 0.56 1.86
lsd' 0.05
NS
0.01 0.05 0.08 0.1 0.7
3
ACKNOWLEDGMENT
This study is part of an ongoing research program
conducted at The Ohio State University. Support for this
research was provided by The Ohio Coal Development Office,
CINergy (Cincinnati Gas
h
Electric Co.), and Dravo Lime
Company.
LITERATURE CITED
Kinraide, T.B., P.R. Ryan, and
L.V.
Kochian. 1992.
Interactive effects of Alt3, H+, and other cations on root
elongation considered in terms of cell-surface
electrical potential
.
Plant Physiol. 99:1461-1468.
Stehouwer, R.C, P. Sutton, R.K. Fowler, and W.A. Dick.
1995. Minespoil amendment with dry
flue
gas
desulfurization by-pr0ducts:Element solubility and
mobility.
J.
Environ.
Qual.
24:164-174.
Sutton, P., and W.A. Dick. 1987. Reclamation of acidic
mined lands in humid areas. Adv. Agron. 41:377-405.
Wendell,
R.R.,
and K.D. Ritchey. 1993. Use of high-gypsum
flue gas desulfurization by-products in agriculture, p.
40-45.
In
Shiao-Hung (ed.), Proceedings
of
the Tenth
Annual International Pittsburgh Coal Conference.
September 20-24, 1993. Pittsburgh, PA.
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