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Peat for Environmental Applications A Review.

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Peat for Environmental Applications:
A Review
G. McKay
Dept of Chemical Engineering, Hong Kong University of Science
and Technology, Clear Water Bay, Kowloon, HONG KONG
ABSTRACT
Peat occurs abundantly in nature in various forms. Its chemical composition
varies widely with geographical region, age and within individual deposits due to
the chemical reactions resulting in its formation. However, it is now realized that
the basic chemical and physical properties of most peats are beneficial to sorption
and ion exchange processes. The potential range of environmental uses for peat
described in this paper vary from the treatment of oil spills and textile effluents to
VOC removal and metal ion removal from waste streams.
Keywords: Peat, Environment, Sorption, Oil Spills, Textile Effluents, VOC,
Odours, Colour, Metal Ions.
INTRODUCTION
Occurrence
Many nations have deposits of peat within their borders; some of the more
significant ones include Australia, Canada, China, Finland, Germany,
Ireland, New Zealand, USA and the countries of the USSR Peat has been
used traditionally for many years in agriculture and as a domestic fuel for
heating or as a commercial fuel mainly for electricity generation. However,
the nature of its chemical and physical characteristics are amacting
significant attention in recent years with regard to its potential applications in
environmentalareas.
127
0
Formation
Peat is 80-90 percent water in its natural state. The solid component is
derived from the partially decomposed residue of dead plants and the remains
of decayed microorganisms. Only a short depth below the surface of a peat
bog, there is virtually no oxygen because the wetness limits air access. This
oxygen deficiency gives rise to predominantly anaerobic decay processes in
the bog. The position of peat in the era of time is shown in Table 1, relative
to coal and lignite.
Table 1. Classification of Peat by Age.
Classification
Material
Aunvximate Age
~
Fossil
Bituminous coal
400million yean old
Aged
Lignite, brown coal peat
640million yean old
RMlt
Straw, wood, mmut shells, pith
10,ooO yean old
eac.
Chemical Composition and Other Properties
As with most microbial systems, the composition of the product - in this
instance peat - varies considerably, and this characteristic is an obvious
disadvantage for continuous large scale processing. Chemically, peats are
largely organic material. When burned, they leave little ash (2-10 per cent by
weight) and have calorific values of 8OOO k 500 kJ kg-'. The main varieties of
peat can be distinguished by their acidity and ash content. Characteristically,
high-moor sphagnum peats are only slightly decomposed, have a high
polysaccharide content, and possess relatively large oxygen contents and
correspondingly lower levels of carbon and hydrogen than low-moor peats.
Some comparative fuel properties are shown in Table 2.
128
Peat for environmental applications:A review
Table 2. Technological Properties of Solid Fuels.
Fuel
% Moisture
%Carbn
(W
Peat
>75
50-60
Calorific Value
&J kg")
7,000
Lignite
30-70
60-70
17,000
Binrminous coal
10
80
36,000
2
90
37,000
Anthracite
The living plants from which peat is derived contain principally proteins,
carbohydrates,lipids, and polyphenols such as lignin. Also,small amounts of
nucleic acids, pigments, alkaloids, vitamins and other organic substances are
present, along with inorganic materials.
During decomposition, triglycerides hydrolyse, yielding fatty acids and
glycerol. In anaerobic conditions of decay, the glycerol is readily consumed
as a carbon and oxygen some for micro-organisms. The residual fatty acids,
unaltered waxes and steroids persist as relatively stable components of the
peat. Collectively, these substances - and any other substance which can be
extracted with non-aqueous organic solvents - are called bitumens. Humic
acids are thought to originate directly from lignin or as a microbial product.
Bitumens are an important group of peat components and can be
dissolved from peat using organic liquids. Peat wax is the bitumen produced
when the extracting solvent is an aliphatic hydrocarbon,or when bitumens are
purified by removal of resins. The wax is a mixture of esters, acids, alcohols
and hydrocarbons. Asphalts are peat bitumens which are insoluble in light
petroleum ether or in hot methanol. The yields of bitumens from peats are
usually in the range 1-10per cent by weight.
So far, there is only one commercial peat wax plant in the world, in the
USSR and the wax is used commercially as a mould release agent in foundries
and as a release and anti-blocking agent on polyurethane surfaces. But major
developments in such systems will depend on the integration of such
129
G. McKay
extraction processes with other intended uses of peat, for example, fuel or
chemical feedstocks.
Organic chemicals can be derived from the solvent extraction of peat.
Many small scale extraction processes for producing organic chemicals from
peat have been proposed. One lab0mm-y system illustrates the wide range of
organics in peat, and presents particular methods of isolating various
components. Figure 1 shows this scheme as developed by Lucas (1970) and
gives some indication of peat’s potential as a chemical feedstock, if such
processing is economically feasible.
Figure 1. Feat Fractionarwn Scheme.
ReactionlHts04
carbohydrates,
-
amino acids
Reactor
r
water
NaCO,
Peat for environmental applications: A review
Yeast cultivation is a further option. Yeast culture has long been
regarded as the best way to use the sugars released by peat hydrolysis. The
traditional connection between yeast and sugar has been based on
fermentation. A second, and now more important, type is the production of
high protein yeasts suitable for the diets of animals and humans. Processing
plants in the USSR are capable of producing 10,OOO to 50,000 tonnes of high
protein yeast a year at production rates of 200 kg yeast per lo00 kg dry peat.
Finally, other applications that affect human healthcare. Various Bvitamins (B1,
&, Blz for example) have been detected in peat by Raitsinu and
Evdokimova (1971). The presence of such substances in peat has been
atmbuted to synthesis by actinomycetesand other fungi, and by bacteria. The
vitamin contents in peats, however, vary widely, e.g. vitamin B1 contents
range from 0.34-60mg kg-' of organic material and vitamin Bz contents range
from 0.2 to 0.9 mg kg-'.
Several physical characteristics of peat have been determined; the flow
properfies of water through beds of peat have been established by Poots and
McKay (1980) and the effects of pressure drop, particle size, water velocity
and filter bed depth for peat systems has been studied.
CONVENTIONAL USES OF PEAT
Peat may be used directly for horticultural applications or it may be blended with
other components including lime and fertilizers.
Further uses include heat and power generation, and metallurgical
applications. A modified flowsheet for a peat coke production plant in operation
at the Technical Research Centre in Finland is shown in Figure 2 by Kadner and
Fisher (1961).
131
G.McKay
Figure 2. Flow Diagram for Peat Coke, Briquette and
Power Plant.
Peat
Fines
saeening
Sod peat (30-40per cent water) is crushed, screened and sent to storage before
controlled rotary drying is canied out to inhibit break-up of the peat. Further
screening occurs and all fmes are sent to a briquetting unit. The dried coarse peat
is coked in a counter-currentlyfved rotary kiln, which is fxed with fuel oil or peat
dust in the temperature range 500-900"C.Exit tars and gases are used to drive a
small generating plant prior to exit gas cleaning. The hot coke is m l e d in a third
unit using a direct water spray followed by screening and storage.
132
Pear for environmenral applications: A review
ENVIRONMENTAL APPLICATIONS
In the past 20 years the chemical and physical characteristics of peat, and in
particular sphagnum peat moss, have generated significant interest in the
environmental field. Several applications have been studied and tested and a
number have been installed for full scale plant operations. These applications,
some old and some new, are reviewed in this section.
Oilspillages
The problems of significant oil spills in the ocean and their subsequent
migration onto beach and coastal areas are well documented. Some estimates
indicate that spillage and dumping of oil into the ocean is in excess of one
million tonnes per year. Research on absorbents and dispersants is being
extensively carried out but the implications of using additional chemicals as
dispersing agents is still not considered a realistically acceptable solution in
the mediumfiongterm.
It has been shown by Ekman and Sandelin (1971) and D’Hennezel and
Coupal(l972) that peat moss has a much stronger absorption affmity for oil
than straw, which is often used for oil spill treatment and can absorb five
times its own weight of oil. Microscopic studies of peat moss show a highly
porous and fibrous material, and this property, associated with the
phenomenon of selective absorption, has been used to absorb oil. Laboratory
experiments indicated that peat moss absorbs up to eight times its weight of
Oil.
A limited number of field tests have been undertaken, but it has been
found that beach cleaning is relatively easy with peat moss. Raking the peat
moss and the oil provides mechanical energy which eases absorption. The
mixture does not stick to the tools and the technique is valid where raking is
possible.
For the treatment of oil slicks at sea,future proposals include the design
of absorption booms of peat and finding a use for the peat/oil mixture.
Burning is a possibility, but this can contribute to the air pollution.
133
G.McKay
Peat Biological FiltedBiosorbers
Peat biofilters can be used for odour removal from gaseous effluent and for the
removal of a range of organic compoundsfrom vapours.
Microorganisms are capable of oxidising many compounds and therefore
have the potential for use in VOC abatement systems. The principle involves
passing the VOC containing gases through a mimbially active bed of
particles where the VOC’s are adsorbed, and then oxidised to simpler
compounds such as carbon dioxide and water, by microorganisms aaached to
the surface of the particles. The mechanism is complex but the first step is
considered to be the dissolution of the VOC’s and oxygen in water and their
subsequent reaction via several stages. A large gas-liquid interface is required
and an extensive and active microbial flora.
Figure 3 shows a typical peat biofdter for mating oxidizible VOC’s. The
vapour stream is fed through a humidifier before passing through a bed of
peatheather, with various mixtures of peat/heather/s~aw/compos~l~tic
granules.
Figure 3. Biojilter Schematic.
HUMIDIFIED GAS STREAM
VOC
CONTAINING
134
I
PURIFIED AIR
Peat for environmental applicarions: A review
As the process is usually designed for aerobic operation, the vent stream
must contain sufficient oxygen and the beds must be kept moist to sustain the
microorganisms. Problems may occur when acid metabolites are produced,
then biofiltration can only be sustained if the pH of the bed is maintained.
Easily biodegradable VOC’s such as alcohols, esters and ketones
generally degrade on natural packing materials. The maximum concentration
a biofilter can sustain for these compounds is around 1500 mg/m’.
VOC’s
that are less easily biodegradable, such as aromatics and chlorinated
hydrocarbons, generally require innoculation of the bed with specifically
cultivated organisms.
For these non-chlorinated more difficult VOC’s
biofdters can handle concentrations around 500 mg/m’ and for chlorinated
VOC’s around 20 mg/m3. For organics which are not mixable or soluble in
water, a water/oil emulsion may be used as the absorption fluid.
The benefits of the biofilter are that several organics can be treated,
installation cost is fairly low and operating costs are inexpensive. However,
the beds must be monitored carefully and good dismbution is required to
avoid channelling and excessive pressure drops, humidity control is important
and nutrients are often required. The problems and techniques of maintaining
the moisture balance around the bed have been discussed by Marsh (1992).
Some Design Features to be Considered:
BedStructwe
The bed should be porous but capable of bringing about good contact between
the fluids and the microorganisms. This is achieved by having altemative
layers of peat and heather fibre or expanded polystyrene beads.
Bed Conditioning
The beds contain several types of microorganisms in general and to add
specific microorganisms to the bed may require the addition of nutrients.
Beds take 2 - 4 weeks to condition naturally.
135
G.McKay
Operating Conditions
Beds require a moisture content of 40 - 60% w/w. This can be achieved by
spray nozzles located above the bed or by passing the vent stream through a
wetting column up-stream of the bed. The moisture content can be monitored
using load cells if required.
Microorganisms are active in general between 15OC and 35°C and
therefore, it may be necessary to preheat or cool the vent stream prior to
entering the bed.
Bed heights are normally about 1 m to maintain low pressure drops, and
the maximum superficial velocity is about 0.03 m/s.
Biojiker Design Theory
Starting from the proposition of Ottengraf and Van den a v e r (1983) that the
biological degradation process is of zero order, i.e. the rate of degradation is
independent of the concentration of the components being degraded, it is
possible to express the mass balance over a small part of the biofdter as
follows:
where:
W
= gas ratio [rn3/m2s];
C,
h
= component concentration in gas phase [g/m3];
N
= m a s flow of component into filter material [g/m2s];
A,
= specific surface area of filter material [m2/m3].
= depth of filter material [m];
The gas ratio (w> is defined as the volumetric gas flow rate per unit
transverse cross-sectional area of the filter material. This equates to the gas
velocity through the filter material.
The mass flow(N) represents the rate at which a component is degraded
by the microorganisms. It depends on the biological degradation velocity
constant for the specific component,and the effective biofilm thickness.
136
Peat for environmental applications: A review
N = k,,f,
where k,, = biological degradation constant for a compound [g/m3s];
and
f, = effective biofilm thickness [m].
Transport phenomena determine the effective biofilm thickness. The
components to be degraded enter the water phase and diffuse into the water
film surrounding each particle of filter material. As diffusion takes place,
components are being degraded so that at a certain distance into the water
film degradation is complete. No components are left and the process ceases,
the effective biofdm thickness is thus that part of the water layer in which
biological activity takes place. It is always less than or equal to the actual
thickness of the water film (fJ.
Production of Active Carbons
Potential processes are peat pyrolysis and peat coke production. Pyrolysis
converts peat from a substance containing much hydrogen and oxygen into
one with a very high carbon content. Peat coke may be used as a decolourising and de-odourising agent and as a filter medium.
Carbonized peats or activated carbons are prepared in different grades
from peat.
Different qualities are required for different duties
-
water
purification, the removal of organics from starch and sugars, colour removal,
and gas and vapour adsorption for example.
Peat cokes prepared at
temperatures of 500°C and less are best suited for activated carbons. [The
activation process depends 011 the oxidation of carbon from the pore walls
using steam, carbon dioxide, air or chemical activation.] The conditions for
producing carbonized peat chars vary slightly depending on the end use and
required characteristicsof the char. Re-treatment of the peat is usual since it
increases the activity and active surface area of the char. Some of the more
commonly used pre-treatment agents include the following metal salts: zinc
chloride, ZnCb femc suplhate, F@(sO.&.x&o,
ferrous sulphate,
FeS04.7H,0 and potassium chloride, KCl. These salt solutions would be
137
G.M c K q
made at 520% concentration by weight, and peat addition to make a slurry
varies between 20-50% by weight. The activation process itself is usually
carried in the temperature range 600-800"C for 20-30 minutes depending on
the furnace design and contacting method. Generally, large surface area
activated chars can be produced in these operating ranges and the chars
contain significant amounts of micropores and mesopores. Activation at
900°C or above usually results in low surface area chars due to the char
forming the micropore walls burning off, leaving a macroporous sorbent
compound. Steam or carbon dioxide may be used to enhance the formation of
the char and post charring acid washing may be used to enhance surface
activity. Future developments in this area will include the use of other pretreatment materials and optimization of process conditions to produce harder
chars and greater surface area products. Detailed studies are required to
minimise energy usage and to minimise emissions from such plants.
The Irish Ceca company produces 1000-1500 tomes per year of activated
carbon from peat, and the Dutch Norit plant produces around 30,000 t o ~ e ~
Per Year.
Phosphate Removal
The use of soils for wastewater treatment is an old practice which is currently
receiving increasing attention as a solution to nutrient removal problems. A
paper from Finland by Surakka and Kamppi (1971) describes the use of a
ditched peatland area for wastewater treatment at the village of Kesalahti
since 1957.
The infiltration of wastewater through peat soil was
accomplishedby pumping raw sewage to a large storage ditch in the peat bog,
and the wastewater then percolates through the peat u)intercept ditches 20 m
away. Results have been good in the removal of phosphorus (82 per cent),
nitrogen (90 per cent), BOD (95 per cent) and pathogenic bacteria (99+ per
cent) after 14 years of operation.
The occurrence of phosphorus in virgin peat soils is largely in organic
form in the residues of plants and microorganisms (Kaila, 1956). The effects
of various cations, anions and organic compounds on the phosphate retention
138
Pear for environmental applications: A review
of soils has been studied, particularly under laboratory conditions by Larsen
(1967), and the effect of superphosphate on the retention of phosphorus by
peat soil was investigatedby Kaila (1959).
A peat and a peat-sand filter has been used to remove phosphorus and
organic matter from wastewaters on a pilot plant scale. A full-scale field
plant has been proposed by Farnham and Brown (1972) and is shown in
Figure 4. The system is simple and relatively easy to operate, it comprises a
mound type (golf-screen type) filter system utilising grass vegetation and an
irrigation system.
Figure 4. Goy-Green Type Filter Bed.
r-12” PEAT OR
PEAT-SAND
SPRIhXLER
MlVmlDI2
I
6‘ PEATROCK OR
COARSE GRAVEL
YFINESAND
OUTLET PIPE
More recently Viraraghavan and Kikkeri (1988) have used peat filters for
the purification of wastewaters from the food industry, and Viraraghavan and
Ayyaswami (1989) have studied peat filtration of septic tank effluents.
Treatment of Slaughterhouse Wastewaters
Wastewaters from slaughterhouses contain proteins, iron, fats and
phosphorus. Different slaughterhouse wastes will also differ in quality from
each other because of many factors, for instance, the quality and quantity of
139
C.McKay
animals to be slaughtered, various procedures, working time, and the quantity
of water used.
A common property of slaughterhouse wastewaters is,
however, that if their proportion in the total wastewaters amount is great
enough, they may cause disturbances in communal purification plants if they
are fed to the sewer without any precursory treatmenl
Some experiments by Silvo (1972) were made on slaughterhouse wastes
with Sphagnum peat and the following conclusions were made.
A 0.15 m deep layer of peat removes disturbing impurities up to 90% and
after filtration the wastewater resembles household wastewater.
When strong slaughterhousewastewater is in contact with loose Sphagnum
peat a remarkable purification takes place, although the purification is
not as good as in a light peat filter.
During the rreatment of strong waste, clogging of the peat filter may occur.
The success of making use of Sphagnum peat for precursory treatment of
slaughterhousewaters before leading to a general sewage treatment plant
is therefore dependent on finding the correct technical solution.
Textile Effluents
Textile effluents contain a wide range of chemicals from processes such as
sizing, scouring, dyeing, bleaching and finishing. The effluent from textile
houses has long been a prime target in efforts to clean up waterways, and was
first investigated by Ottemeyer (1930).
A process using peat has been
developed for the treatment of such effluent and is called the HussongCouplan Water Treatment System developed by Leslie (1974).
The
purification process is based on the scrubbing action of a moving mat of peat
as the effluent is passed through. The process involves chemical adsorption
and complexing. An insoluble complex salt is formed by the ionic and
mrdinate bonding peculiar to the molecular structure of the peat.
The Hussong-Couplan System comprises three basic functions; first, the
peat must be prepared for the process; secondly, the effluent must be contacted
with peat; and thirdly, the spent peat must be carried away. The process is
shown in Figure 5.
140
Pear for environmental applications: A review
Figure 5. Using Peat to Treat Textile Effluents: The Hussong-Couplan
System.
Peat
water
I,
I
("&"I
-7
L
dlI
Holding tank
Dram
Common peat is dumped into a large hopper to which some water is
added to settle the peat dust and to facilitate handling. Controlled amounts of
this dampened peat and additional water are fed into a mixing tank at a ratio
of approximately 100 parts water to one of peat. The slurry is deposited
evenly on a moving perforated belt which permits the excess water to drain
off, leaving a homogenouspeat mat.
The conveyor moves this mat along under discharge pipes from which the
effluent is sprayed on to the mat, through which it passes on to collection
trays. The effluent passes through the mat in a two-stage process with raw
effluent first contacting partially spent peat which has already completed
141
G.McKay
purification of partially treated effluent. Small holding tanks store effluent
between the first and second stages to compensate for minor irregularities in
flow rates.
The liquid, after the second stage, has completed the purification process
and may be recycled through the plant or may be discharged. The spent peat
is safe to handle or store and may be disposed of in a number of
environmentally acceptable ways. It may be deposited in a sanitary landfill
with no danger that the metals, now in highly insoluble state, would leach out.
Or the peat may be dried and simply burned. In large installations, recovery
of the heavy metals from burned peat can be profitable.
In tests performed on actual textile effluents with an operational water
treatment system, colour was reduced by as much as 99.6 per cent and
turbidity by 100 per cent, other data are listed in Table 3. Further tests on
textile effluents indicate the success of the system in treating heavy metal
pollutants and the results are shown in Table 4.
Table 3. Test Resultsfor a Typical Textile Efluent.
QlaractaiStiC
1200
After
85
150
8
1200
146
f3CfOE
COD (ppn)
33.6
0.76
4
216
Table 4. Test Results for Metallic Pollutants in Textile Ejguents.
Metal
142
EPA Effluent
Limiratians Schedule A
BdOR
After
Cadmium
0.1
25
0.1
chromium (4)
0.05
300
0.04
Chromium (+3)
0.25
300
0.25
copper
0.2
250
0.2
kon
0.5
31.5
0.25
Lead
0.05
8.4
0.025
Nickel
1
.oo
67.5
0.05
Peat for environmental applications: A review
Colour Removal from Aqueous Effluents
Several papers have been published demonstrating that peat is a very effective
adsorbent for the removal of dyestuffs in aqueous solutions. A thorough study
involves determining equilibrium isotherms, batch contact time studies and
fmed bed column breakthrough curves. One of the earliest papers specifically
dealing with peat for colour removal was by Dufort and Rue1 (1972).
400
300
200
100
n
Basic blue 3
0 Basic yellow 21
0 Basic red 22
0
Figure 6. EquilibriumIsothermsfor VariousDyes on Peat.
Figure 6 shows the adsorption isotherms for Basic Blue 3, Basic Red 22,
and Basic Yellow 21 onto Sphagnum peat by Allen (1987) and Allen et al.
(1988). These plots represent the amount of dyestuff adsorbed per unit mass
of peat in equilibrium with the dye solution. The adsorption of Basic Blue 3
on peat in a batch adsorber is shown in Figure 7 by Allen and McKay (1987).
The breakthrough curves for dye adsorption in fmed beds of peat are
illustrated in Figure 8 by Allen et al. (1986). These results all indicate the
ability of peat to treat colored effluents with reasonable success.
143
G.McKay
1.00
0.90
0.80
0.70
0.60
8
\
L1
0.50
U
0.40
0.30
0
30 60 90 120,150 180 210 240 270 300
Time (min)
~
~~~~
Figure 7. Dimensionless Liquid-Phase Concentrationversus Timefor Basic
Blue 3 on Peat.
1
1.00
0.90
0.80
0.70
0.60
0.x)
Flow rate
0.85 d / s
Particle size 355-500 m'crans
co
200 mg/
Temperature
15°C
0.40
2
0
d
0.30
0.20
0.10
1
0.00
0
5
10
1
15
1
20
v o l m treated (&)
1
25
4
30
.
Figure 8. Fired-Bed Dimensionless Liquid-Phase Concentration versus
Volume Treatedfor Acid Blue 25 on Peat.
144
Peat for environmental applications: A review
Dye Removal from Wastewaters and Adsorbent Costs
Many factors influence the rate and extent of dye uptake on adsorbents,
including pH, temperature, particle size, initial dye concentration, and
adsorbent mass. A series of experiments have been undertaken using
similar conditions, and using Basic Blue 3 (Astrazone Blue) to compare
adsorbent costs using several materials. A uniform range of adsorbent
particle size, between 150 pm and 250 prn, was selected for all materials.
Various masses of adsorbent were agitated with a constant volume and
constant concentration of dye solution. The isotherms for the various
dyes on different adsorbents were determined and plotted as q against C,
where q is milligrams dye adsorbed per gram adsorbent and C is the
concentration of dye at equilibrium in solution. Experimental conditions
were selected so that the isotherms reached a plateau in order that the
saturation value of the adsorptive capacity, q, could be determined. The
values were used to assess the quantity of adsorbent required to remove
1 kg of dye. These quantities have been used as a basis for costing the
adsorption process.
In this simplified approach by Allen (1995), no account has been
taken of contact time data, and it was assumed that the adsorbent was
saturated with dye. In addition, regeneration costs have been neglected,
since very few figures are available.
The relative costs of the adsorbents for the two basic dye systems are
shown in Table 5, together with the adsorption costs in removing 1 kg of
dye. Carbon was taken as a standard, having a comparative cost of unity
per kilogram, and the relative costs of the other adsorbents are shown in
the third column of Table 5. The mass (kg) of adsorbent required to
remove 1kg of dye is obtained from the dye isotherms and, consequently,
using columns 3 and 5, a comparative cost of different adsorbents to
remove 1 kg of dye may be obtained; this is shown in column 4. The
benefits of using peat, based on this simple analysis, are apparent.
145
G.McKay
Table 5. Adsorption Costs of CI.Basic Blue 3.
Adsorbent Material
Units
(s)
Commtive Cost
Per kg of
To Remove
Adsorbent
1kg of BB3
Mass of Adsorbent
Requiredm
h o v e lg of Dye
Activated cartxm
448
1 .oo
1.OoO
2.232
Peal
375
0.04
0.048
2.667
Silica
87.5
1S O
7.682
11.430
Pith
62
0.04
0.286
16.000
Lignileactivaled~
560
0.85
0.674
1.770
Char
125
0.25
0.896
8.000
Lignite
392
0.15
0.1714
2.55 1
Fuller’s eanh
500
0.66
0.60
2.000
Removal of Metal Ions
The ~
~capacity
a of peat
l for heavy metal retention has been recognised in
studies which have investigated metal contaminants in peat bogs. Pakarinen
et al. (1980) studied the vertical distribution of trace heavy metals in
Sphagnum peat of southern Finnish ombrotropic bogs. They found selective
sorption of Pb > Cu > Zn > Mn in the surface of the bogs. They found that
most of the flow occurred across the surface and in the top 30 cm of the bog.
This flow pattern resulted in 30 to 100% removal of trace metals from
solution: >30% removal of Ni, and >99% removal of Cu. Glooschenko and
Capobmco (1982) found low and consistent concentrationsof Zn,Pb, Cr, Cu
and Hg in several peatland ecosystems located in Ontario, Canada. In
addition to studying the natural constituents of peat bogs, considerable
attention has been focused on the potential of peat as a commercial adsorbent
for the removal of toxic metals in contaminated water. As far back as 1939,
Nikol’skii and Paramonova realised the potential of peat as an ion exchange
medium for metals such as copper, zinc, lead and mercury.
Schwarrz (1968)found that peat was capable of removing Ca and Mg
ions from wastewater in laboratory column studies. He concluded that cation
146
Peat for environmental applications: A review
removal was due to a weak acid ion exchange mechanism. In batch and
column studies, Lalancette and Coupal (1972) found peat to be an efficient
means of removing Hg from water. Coupal and Lalancette (1976) found that
Hg, Cd, Zn, Cu, Fe, Ni, Cr(VI), Cr(III), Ag, Pb and Sb can be treated
efficiently by contacting wastewater with peat moss. They found sorption to
be quite high due to the polar character of peat. They concluded that the main
advantages of utilising peat for wastewater treatment are its broad scope in
terms of pollutants eliminated and its ability to accept rather wide variations
of effluentcomposition.
In a later study, Zhipei et al. (1984) investigated the removal of Pb, Cd,
Zn, Ni, and Cr from wastewater and concluded that, in treating water
containing heavy metal ions with peat, the best results were obtained when the
ion concentrations in wastewater were low. Parkash and Brown (1976) in
their investigation of the sorption of zirconium and titanium from aqueous
solutions by peat, found that peat accommodated almost four times more
zirconium than titanium. The data obtained by Smith et al. (1976, 1977)
showed that peat treated with sulphuric acid or a sulphuric/phosphoric acid
blend possessed enhanced cation exchange capacities.
Loading
Coupal and Lalancette (1976) concluded that peat can adsorb most metals
in a very efficient way, up to 4% of the weight of dry peat - that is, for
solutions of metals less than IOppm, 0.68 kg of peat can purify 0.95 m3 of
waste water. It was also noted that the metal removal efficiency in
unbuffered solutions is significant in a very large concentration range
from 0.01 to 100 mmol range. In a study of various classifications of
peat, Gossett et al. (1986) revealed that the maximum binding capacities
in 10 mmol 1” metal cation solutions were very similar regardless of
metal or type of peat used: all values fall in the range 180-200 mmol 1-’
dry weight.
Clymo (1963) proposed that there is a good correlation between the
content of unesterified polyuronic acids in the cell wall of Sphagnum peat
147
G.McKay
and the cation exchange capacity. De Mumbrum and Jackson (1956)
proposed that the sorption of copper and zinc ions occurs by the
formation of complexes with the carbony1 and nitrile groups in peat.
Kashirtseva (1960) pposed that the presence of humic acids in peat
were primarily responsible for its ability to sorb metals. Ong and
Swanson (1966) studied the sorption of copper onto peat and, using IR,
concluded that humic acids could complex with copper in solution.
However, they also confirmed that this was not the sole mechanism for
copper removal. Furthermore many workers have implicated carboxylic
acid (COOH) groups in the reaction of divalent metals with humic acids
(Gamble et al., 1970 Schnitzer, 1978; Schnitzer & Kahn, 1972;
Stevenson & Ardakani, 1972; Van Dijk, 1971; Vinkler et al., 1976; Boyd
et al., 1981). They support the general view that the reaction of metal
ions, such as Cu and Fe, with humic acids is one of chelate ring
formation involving adjacent aromatic carboxylate COOH and phenolic
OH groups or, less predominantly, two adjacent COOH groups which
participate in ion exchange reactions by binding metal ions with the
release of
H‘ ions. Others believe that there is no direct evidence for
chelation: NMR studies (Gamble et al., 1976; Deczky, 1978) and an ESR
study (Alberts et al., 1976) have shown that Mnz+ion does not form an
inner sphere and is bound electrostatically. This was supported by Bloom
and McBride (1979) who, after extensive investigations with acid washed
peat concluded that peat and humic acids are likely to bind most divalent
metal ions, with the exception of Cu”, largely as hydrated ions. The
binding of copper appears to involve the exchange of one or two aquo
ligands by wboxylate oxygens. Thus neither chelation by adjacent
functional groups nor heterogeneity with respect to acidity constants can
be postulated to explain the binding of metal ions by peat and humic
acids.
Sharma and Forskr (1993) have studied the sorption of hexavalent
chromium onto peat, and Ho et al. (1995) have studied nickel sorption
148
Pear for environmental applicanons: A review
onto peat. Several other papers during the past ten years report the
sorption capacity of various metal ions on peat. However, there is very
limited information on sorption kinetics. Gosset et al. (1986). Chen et al.
(1990) and Ho et al. (1994) have developed and successfidly used a
kinetic expression assuming a chemical rate-limiting step. No attempt
has been made to study if diffusioncontrol is limiting, particularly at long
contact times. The major omission in this area of application is the lack
of detailed, fundamental design data on the sorption of multicomponent
metal ions (or organics) onto peat.
Figure 9 from Brown et al. (1992) shows the equilibrium isotherms
for the sorption of copper and cadmium ions onto peat at 20°C. The
figure shows the amount of metal ion sorbed (Q,poVg) at the liquid
phase equilibrium metal ion concentration (C, mmol/dm3).
The
maximum saturation capacities for copper and cadmium ions on peat are
270 and 180 WoVg respectively. These results indicate the considerable
potential for peat in metal ion removal.
Mo
100
0
*Peat
a-Peat
0
0
1
2
3
4
Ce ( m l / d m 3 )
Figure 9. Sorption of Cd and Cu onto Peat.
149
G. McKay
Figure 10 by Brown et a l (1992) shows concentration versus time
decay curves for the sorption of copper ions onto peat using an agitated
batch adsorber. The series of curves show the effect of different peat
masses on the rate of sorption of copper ions.
Time (min.)
Figure 10. Mass Effect on Uptake of Copper by Peat.
Figure 11 by Brown et al(1992) shows the results of the Sorption of
copper ions by peat in a fixed bed sorption column. The breakthrough
curves were measured for a series of bed heights and demonstrate that
peat has the potential as a sorbent for metal ions in fixed beds.
150
Peat for environmental applications: A review
200
0
400
600
Contact Time (minutes)
Figure 11. Breakthrough Curves for Cu on Peat.
0
Regeneration of Spent Peat
It is now well established that peat has excellent sorption/exchange properties
for many compounds including metal ions, dyestuffs and organics. The key
phase in assessing the commercial potential of such systems is to study the
regeneration or disposal of the spent peat. There is practically no literature
available on the subject and extensive research into this area is required. The
most likely options are briefly reviewed.
Combustion
The spent peat, if loaded with organics or dyes, could be burned as an
energy source. The specific calorific value of the peat is slightly
increased by the presence of dyes as demonstrated by Poots and McKay
151
G.McKay
(1979). However, the cost of drying wet peat is a critical factor, although
in hot countries the peat could be spread and partially dried in the sun.
This process is currently practised in parts of Egypt using waste bagasse
pith from the sugar cane industry.
Desorpilon
Desorption, leaching or stripping compounds Erom spent peat is the most
likely option. Organic solvents could be used for dyes, and acids to leach
out metals. The sensitivity of metal sorption to solution pH has been
demonstrated by Ho et al. (1974) and from this data it would appear that
many metals could be desor&ed from the peat by using an acidified
solution, thus regenerating the peat.
La*ll
Several of the sorption isotherms for basic dyes onto peat indicate highly
irreversible sorption and therefore this material would be suitable for
landfill. Many other sorbed compounds on peat need to be subjected to
leaching tests to assess their suitability for landfill. In fact, McLellan and
Rock (1988) have used peat for the treatment of landfill leachate.
Digestion
A solution of biological fluid could be passed through a fmed bed to
regenerate peat laden with digestible organics. An extension to this
would be to use biosorption in which the peat-fixed sorption bed is
innoculated with microorganisms, so there is sorption plus biological
digestion in the same contact bed.
Cryogenic Fluids
These fluids offer considerable potential in the area of sorbent
regeneration and sorbate recovery. The limitation will be that the value
of the recovered material justifks the cost of using cryogenic fluids.
152
Peat for environmental applications:A review
CONCLUSIONS
The potential for peat utilisation in several environmentalapplicationshas been
discussed with respect to :
Oil Spills
VOC/Odour Treatment Using Biofiltration
Production of Active Carbons
Phosphate Removal
Treatment of Slaughterhouse Wastewaters
Textile Effluents
Colour Removal
Removal of Metal Ions
The sorption capacity for colour, certain organics and particularly metal ions,
indicates peat has considerable potential in these applications. A number of
factors still require detailed attention before peat will be accepted widely in the
industrial market place.
1. The spent peat must be disposed of or regenerated.
This problem is typical of any sorbent material, however, each sorbent
material has its own chemical specificity therefore regeneration and reuse must be studied in detail.
2. Extended use of peat has been tested and proved feasible with peat for gas
phase applications in biofilters. However, for liquid phase applications
with intermittent regeneration peat has received limited attention.
Several key factors require further study, including: sorbent bed pressure
drop, friability (as peat is a soft material), compressibility/expandability
of the bed due to the prolonged effect of water on peat
3. The problems associated with leaching of humic, tannin and lignin stains
from the bed which occurs for approximately 10% of fmed bed operations
needs additional study.
153
G.McKay
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155
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