The use of phosphorus-saturated
ochre as a fertiliser
S.T.D. Carr1, K.E. Dobbie1,2, K.V. Heal1 and K.A.
Smith1
1
School of GeoSciences, The University of Edinburgh, Crew Building, West
Mains Road, Edinburgh, EH9 3JN, UK
2
Current affiliation: SEPA, Heriot Watt Research Park, Avenue North,
Edinburgh, EH14 4AP, UK
ABSTRACT
Diffuse pollution is the major cause of water pollution worldwide. The longterm solution is a change in land-use practices and management. However, this
will take time to implement and become effective and a short-term, low-cost
treatment method is required. Ochre is formed in minewater settling lagoons as
iron-rich precipitates which can be air-dried and used as a filter substrate. Ochre
has a high adsorption capacity for P as it is comprised largely of Fe(OH)3 and
FeO.OH and contains other compounds known to adsorb P, such as aluminium
oxides and calcium carbonates. When the P-adsorption capacity of the ochre
filters has been reached, the substrate will offer a rich source of P (up to 30.5
mg P g-1) which it is proposed could be used as a slow-release fertiliser. The
studies reported here showed that the use of P-saturated ochre as a fertiliser
compared to conventional fertilisers, such as K2HPO4, had a tendency to
produce greater crop yields with no signs of stress, possibly due to the slow
release of P from the ochre matrix. Concentrations of potentially toxic elements
in the ochre-amended soil were within permissible standards and there was no
evidence of soil contamination.
© 2004 Copyright holder. Book title. Edited by Editor(s) name(s). ISBN: X XXXXX XXX X.
Published by IWA Publishing, London, UK.
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Book title
1 INTRODUCTION
After mines are abandoned, pumping of water ceases, often resulting in the
rebound of the water table and the release of water containing potentially toxic
elements (PTEs), especially iron (Fe). In order to treat these environmentally
damaging discharges, mine water treatment plants (MWTPs) have been
constructed across the UK. They are based around the oxidation of Fe(II) in
solution to Fe(III), to form a precipitate known as “ochre”. Ochre therefore
requires removal from MWTPs and is currently disposed of to landfill as no
alternative end-use is available. Ochre not only contains Fe, but, depending on
the geochemistry, other contaminants such as Al and As. It is estimated that at
least 3.7x 104 tonnes of ochre are produced in the UK each year from coal
MWTPs (Hancock, 2004), with associated processing and disposal costs
between £35 to £100 per tonne.
Due to a high content of elements known to adsorb P, such as Fe, Al, Mg and
Ca, ochre has been proposed as a material to remove phosphorus (P) from
eutrophic and nutrient-enriched waters, with this potential confirmed in
laboratory studies (Heal et al., 2003). Parfitt (1989) states that the predominant
mechanism of P sorption to goethite (noted by Heal et al.(2003) to be the major
component of ochre) involves rapid ligand exchange with surface OH groups at
very reactive sites and the formation of a binuclear bridging complex between a
phosphate group and two surface atoms. Following the exhaustion of very
reactive sites, weaker ligand exchanges occur at less reactive sites whilst over
time there is a slow penetration of phosphates into the solid matrix through
defect sites and pores.
After the capacity of ochre to adsorb P is exhausted it requires disposal.
Upon saturation, ochre will offer a rich source of P (up to 30.5 mg P g-1) which
could be used as a slow release fertiliser. Dobbie et al. (2005) showed that the
use of P-saturated ochre as a fertiliser resulted in higher yields at the same
application rate as conventional fertiliser, with no signs of stress to the
vegetation. Furthermore, there was no evidence of soil contamination since
concentrations of PTEs in the ochre-amended soil were within permissible
standards. Alternative methods of P recovery from P-saturated ochre are also
under consideration, such as subjecting it to reducing conditions causing the
release of P from the ochre matrix due to the reduction of Fe 3+ to Fe2+. A
laboratory experiment showed that placing ochre in such conditions produced a
leachate with a P concentration of 50 mg l -1 (Bozika, 2001). Hence the
recovered P-rich solution rich in P could be concentrated or converted into a
more conventional form for use in industry.
This paper examines the potential for using ochre to recover P from Pcontaminated water. The potential of ochre for P removal is demonstrated and
The use of phosphorus-saturated ochre as a fertiliser
3
ongoing research is presented which aims to improve understanding of the
processes affecting P-removal by ochre. Finally the use of P-saturated ochre as a
slow release fertiliser is shown.
2 OCHRE AS A PHOSPHORUS ADSORBENT
Prior to experimentation and use, ochre is usually air-dried to transform it from
sludge with up to 80-95% water content to a dry powder or granular form.
Depending on the geochemistry of the mine water, the MWTP design and
operation ochres from different sites (see Table 1) have different chemical
compositions and particle size distributions and hence a varying ability to
adsorb P. Ochres can therefore be selected for different P-removal applications
based upon their characteristics. For example, fine-grained ochre from the
Minto MWTP is suitable for dosing applications due to its fine particle size,
whilst granular ochre from the Polkemmet MWTP, with a higher permeability,
is more suitable for use in P filters.
Table 1: Chemical and physical properties of air-dried ochres from two MWTPs,
Scotland (modified from Heal et al., 2004).
pH (in distilled water)
% Fe1
% Al1
% Mg1
% Ca1
Dry bulk density (g cm-3)
Particle size range (mm)
Saturated hydraulic conductivity (m day-1)2
Polkemmet
7.2
65±0.5
0.7±0.02
0.6±0.01
7.0±0.1
1.8
0.25-10
26-32
Minto
6.9
67.5±3
0.1±0.01
0.8±0.04
11.8±0.4
0.8
<0.25-1
0.7-1.7
1
Mean of triplicate samples ± standard error. Determined by atomic adsorption
spectrophotometry of acid digests (concentrated nitric and hydrochloric acid additions) of
ashed samples. 2 Determined in columns over 32 days using the falling head method and
Darcy’s Law
2.1 Laboratory experiments
Laboratory batch experiments have demonstrated the capability of ochre to
adsorb P from artificial solutions. The maximum P adsorption capacity of
ochres from Polkemmet and Minto MWTPs was determined as 26 and 30.5 mg
P g-1, respectively (Heal et al., 2003). Minto ochre has a smaller particle size
than Polkemmet ochre and thus a larger reactive surface area and this is the
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likely reason for its higher P adsorption capacity. In these experiments the rate
of the reaction was found to be rapid, with almost all P adsorbed within the first
hour by Polkemmet ochre and within a few minutes by Minto ochre. The
adsorption capacities of ochre are far greater than for other materials that have
been investigated for P adsorption (Table 2).Ochres have also been shown to be
efficient adsorbents of P in non-agitated mixtures. When P solution was added
to Polkemmet ochre in a beaker P concentrations were reduced from 5 to <0.01
mg P l-1 within 8 minutes (Heal et al., 2003, 2004).
Table 2: Maximum P adsorption capacities of different wetland substrates (Heal et al.,
2003)
Substrate
Gravel
Bottom ash
Steel slag
Blast furnace slag
Fly ash
Shale
Laterite
Zeolite
Polkemmet ochre
Minto ochre
Adsorption capacity (mg P (g substrate)-1)
0.03/0.05
0.06
0.38
0.4-0.45
0.62
0.75
0.75
1
26
30.5
2.2 Ochre as a filter substrate
The long term suitability of Polkemmet ochre to adsorb P was examined over a
nine-month period by pumping a P solution (20 mg P l -1) onto a gently angled
trough packed with 10 kg ochre (Heal et al., 2003). The solution was pumped at
a flow rate of 1.2 l hr-1 ensuring a contact time of 4.5 hours. The reduction in P
concentration during the experiment is shown in Figure 1. Passage through the
trough reduced the P concentration of the solution by up to 99.8%, with the
lowest efficiency being 95.2%. A similar experiment but on a far larger scale
was conducted by Dobbie et al. (accepted) using over one tonne of ochre from a
variety of MWTPs to provide tertiary treatment of sewage effluent in southern
Scotland. During the 27-month trial P concentrations were reduced by up to
80% in optimal flow conditions, with no detectable release of PTEs from the
ochre. P removal rates declined over time, probably due to clogging of the filter
unit.
The use of phosphorus-saturated ochre as a fertiliser
5
Figure 1: Phosphorus removal and % adsorption capacity used in Polkemmet ochre after
nine months of trough experiment (Heal et al., 2003).
2.3 Ongoing research on the use of ochre as a filter substrate
Current research is developing a process-based understanding of the factors
affecting P removal by ochre in order to inform the design of a suite of ochrebased filters to treat water polluted with P over a range of flow conditions and P
concentrations. The filter design will take into account the need for continued
reactivity between water and filter matrix and the avoidance of clogging.
Sorption-desorption between reactive surfaces and solution, dissolutionprecipitation equilibria and kinetics will also be investigated. Ochres with
different chemical and physical properties from seven sites in the British Isles
are being investigated. The presence of competing ions and dissolved organic
carbon (DOC) on the magnitude and rate of P adsorption by ochre will be
assessed in batch experiments. An aspect of concern for the long term
implementation of ochre filters, the potential release under anoxic conditions of
PTEs, such as As, will be investigated through batch and column experiments.
A chemical model using the software ORCHESTRA (Meeussen, 2003) will
be calibrated using the results of the batch and column experiments for the
seven ochres and will form the basis for designing ochre-based filter units for
field implementation. The development of a robust model will allow a range of
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scenarios to be tested which would otherwise not be possible, e.g. ten year filter
implementation.
3 USE OF PHOSPHORUS-SATURATED OCHRE AS A
FERTILISER
After P-saturation ochre removal from filter units and disposal will be required.
Due to its high concentration of up to 30.5 mg P g-1 this ochre could be
employed as a fertiliser. A similar approach was considered by Hylander &
Simàn (2001) who investigated whether P adsorbed by blast furnace slag was
plant-available. Other waste materials, such as steel slag without additionally
adsorbed P, have been used as P fertilisers in agriculture (MacNaeidhe, 2001)
and forestry (Jandl et al., 2003). However, even if waste products rich in P are
effective fertilisers, application cannot proceed if it leads to soil contamination.
Therefore two main issues require addressing for the successful use of Psaturated ochre as a fertiliser. Firstly, whether crop productivity resulting from
application of P-saturated ochre matches that from conventional fertilisers, and
secondly whether the application of P-saturated ochre leads to soil
contamination from PTEs.
3.1 Materials and methods
3.1.1 Pot experiments
Research to address these issues was conducted by Dobbie et al. (2005) using
ochre from Polkemmet MWTP which was air-dried, coarsely crushed and then
saturated with P using solutions of KH2PO4. Pot experiments were conducted
using agricultural soil collected in central Scotland. The soil was air-dried,
sieved (4 mm) and mixed with sand to give it the texture of sandy loam.
Available P, K and Mg were then measured in the mixture to determine the
appropriate fertilizer application rates which were 85 kg P 2O5 ha-1 and 90 kg
K2O ha-1. Four litres of the soil-sand mixture were placed in each of 60 5-litre
pots. The pots were sub-divided into two crop types, barley and grass, with five
replicates of six different P treatments for each crop type. The six different P
treatments used were: a control with no added P, a conventional P treatment
using KH2PO4 at the recommended application rate, and four treatments using
the P-saturated ochre at 0.5, 1, 2 and 5 times the recommended rate, as
determined from the acetic acid extractable P content of the P-saturated ochre.
Additionally 0.29 g K and 0.2 g N were added to all the pots in the form of
K2SO4 and NH4NO3. A further 0.06 g of N was added in solution after three
The use of phosphorus-saturated ochre as a fertiliser
7
weeks in the form NH4NO3. The pots were sown to standard agricultural
practice equivalents; for the barley 8 seeds (equivalent to 200 seeds m-2) and the
grass 0.16 g of seed (40 kg ha-1).The pots were distributed randomly in an
unheated greenhouse and redistributed every 2 weeks. Soil water content was
determined gravimetrically and tap water added as required to maintain soil
water content at approximately 80% field capacity. The experiment was
conducted from July to October 2002, until the barley heads had ripened. The
mass and total P content of the barley and grass at the end of the experiment
were determined. In addition, at the start and end of the experiment. The soils
were analysed for total and available P and total PTEs (Al, As [at end only], Cd
[at start only], Cr, Cu, Fe, Mn, Ni, Pb and Zn).
3.1.2 Field trials
Field trials were conducted using the same crop types as in the pot experiments,
for barley at a farm in central Scotland, and at a nearby acid grassland which
had a low P status soil. At both sites four replicates of three P treatments were
established: a control with no added P, a conventional P-application (triple
superphosphate, TSP) and a P-saturated ochre amendment, which, as in the pot
experiments, contained the same amount of acetic acid extractable P as the
conventional P-treatment. For the barley trial, P was applied by broadcasting the
conventional fertiliser and the P-saturated ochre by hand over their respective
plots of 3 m x 2 m at an application rate of 85 kg P2O5 ha-1. Following this the
seedbed was cultivated to a depth of 15 cm before barley seeds were sown at a
rate of 200 kg ha-1 prior to the plots being rolled. Additionally, at the start of the
experiment 60 kg N ha-1 in the form of NH4NO3 and 60 kg K2O ha-1 were
applied to the plots, with 60 kg N ha-1 applied again 3 weeks into the
experiment. P-saturated ochre and TSP was applied to the smaller grassland
plots of 1 m2 on an existing sward at 30 kg P 2O5 ha-1 along with N at 62 kg N
ha-1 in the form of NH4NO3. The plots were established in March 2003 and
allowed to grow until August 2003. For the grassland, vegetation was sampled
from an area of 25 cm x 25 cm in each plot in March prior to treatment and also
in June and August. After each sampling the plots were mown and the
remaining grass discarded. Barley was sampled from a 50 cm x 50 cm area in
each barley plot in August. Vegetation samples were dried and weighed with
total P determined. Total Cd and Pb concentrations were also measured in the
barley grain. At the end of the trials, soil cores were taken in each plot from the
top 15 cm (barley) and 7.5 cm (grass) and analysed for available and total P and
for the same PTEs as described for soils in the pot experiments.
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3.2 Results
3.2.1 Soil P concentration
The addition of P-saturated ochre in the pot and field experiments led to a
significant (P<0.05) increase in the amount of plant-available P as well as total
P in the soil. Over the course of the experiments, the concentration of available
soil P decreased in all treatments, with the largest reduction in the conventional
fertiliser treatments as shown in Figure 2 for the barley pot experiments.
Figure 2: Available soil P concentration (± s.e.) in the barley soil at the start and end of
the pot experiment. CO=unfertilised control; CP=conventional fertiliser; O(0.5)O(5)=ochre applied at 0.5-5 times available P in the CP treatment. (adapted from Dobbie
et al., 2005)
In the pot experiments, the uptake of P by vegetation in the control and all Psaturated ochre treatments substantially exceeded the depletion of available P in
the soils. In the conventional fertiliser treatment, the P uptake by grass was
slightly greater than the depletion of plant-available P in the soil, with the
depletion greater than the amount of fertiliser added. In contrast, the barley from
the conventional fertiliser treatment contained less P than was depleted from the
soil, indicating a net fixation of P by the soil minerals.
The use of phosphorus-saturated ochre as a fertiliser
9
The explanation for soil P depletion not being as high as plant P uptake in the
P-saturated ochre treatments and control is the conversion of initially
unavailable P in the ochre and soil into plant-available forms, possibly due to
organic acids released from the root zone dissolving unavailable Fe, Ca and Al
phosphates (Dakora & Phillips, 2002). In this way the P-saturated ochre acts as
a slow release fertiliser throughout the growing season, resulting in larger
amounts of residual P in the amended soils for the next growing season. This
reduces the need for a repeat application of P-fertiliser in the following growing
season. From the field trial results it is suggested that the addition of 40 t Psaturated ochre ha-1 would supply enough P for at least two growing seasons.
The insolubility of the P bound to the ochre also means that it is less available in
surface run-off than conventional water-soluble fertiliser.
3.2.2 Plant response
In the pot experiments crop yield from the ochre treatments was greater than
from the conventional fertiliser and control treatments, although this was not
always statistically significant. Yields from the P-saturated ochre treatment of
half the recommended P application were higher (though not always
significantly) than from the conventional fertiliser treatment, which initially
contained twice the amount of available soil P. This confirms the conclusion
that P was gradually released from ochre during the experiment, providing the
vegetation with continuous nutrition. Results from the field trials also indicated
that yields were greater in the P-saturated ochre treatment than in the
conventional fertiliser treatment, though again these results were not statistically
significant. It is therefore concluded that the use of P-saturated ochre in place of
conventional P fertiliser has no adverse effect on crop yield and in fact there is a
tendency towards an increase in yield with a less frequent application rate
required. Furthermore P-saturated ochre could constitute a more sustainable
source of P fertiliser than conventional fertilisers derived from finite mineral
resources.
3.2.3 Potentially toxic elements
To assess whether ochre addition introduced contaminants to the soil in these
experiments, PTE concentrations were measured in soil samples from the
different treatments and compared with maximum permissible concentrations of
PTEs in soils for the application of sewage sludge to agricultural land (MAFF,
1998). Soil PTE concentrations did not exceed the permissible concentrations
with the exception of Ni in the pot experiments. However, the permissible
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concentrations for Ni were exceeded in soils from both the ochre and control
treatments and were not significantly different, showing that exceedance was
not due to the application of ochre. Plant uptake of Pb and Cd was examined by
measuring the concentrations of these elements in the barley seed heads. The
concentrations in barley grown in the ochre-treated soils were considerably
lower than the maximum permissible levels in foodstuffs set by the European
Commission (Commission Regulation (EC) No. 466/2001) and were not
significantly different from those measured in the barley grown in the
conventional fertiliser treatment. At the recommended application rate of 40 t
ha-1, ochre addition to this soil in the field would result in soil metal
concentrations considerably below limits set out for the application of sewage
sludge to agricultural land, with the exception of Ni. With respect to Fe, no
maximum application rates are given within the UK (MAFF, 1998). Although
the addition of Fe to soil in the field trials seems large (10.9 t Fe in 40 t ochre),
it is only equivalent to raising the soil Fe concentration from 2.4 to 2.6% for the
barley trial and 3.1 to 3.4% for the grassland trial, well within the usual range
for this area of Scotland (Paterson et al., 2003).
4. CONCLUSIONS
The ability of ochre to rapidly adsorb P has been demonstrated through
laboratory experiments and, with a high P-adsorption capacity (up to 30.5 mg P
g for Minto ochre) it is an excellent candidate for use as a filter substrate.
Sustained removal of P from sewage effluent by ochre has also been shown in
large scale field trial. Dobbie et al. (2005) showed that ochre can be used as a
slow-release fertiliser after P saturation has occurred, with no negative impact
upon crop yield and soil quality. The P-saturated ochre lead to the continued
release of P into the soil, with the potential for a single application to meet the
needs of several growing seasons. Current research is focusing upon testing and
developing ochre-based filters to adsorb P under a range of conditions with
varying flow rates, P concentrations and competing ions. The environmental
acceptability of ochre as a filter substrate will also be investigated by assessing
the release of PTEs in laboratory experiments and through chemical modelling.
ACKNOWLEDGEMENTS
The UK Coal Authority funded preliminary experiments and supplied ochre.
Some of the work reported here was conducted by Karen Dobbie who was
funded by EPSRC Grants GR/R73539/01 and GR/R73522/01 and Enviresearch
Ltd and supported by Scottish Water. Technical support from Andy Gray,
Robert Howard, John Morman and Graham Walker (The University of
The use of phosphorus-saturated ochre as a fertiliser
11
Edinburgh) is acknowledged as is advice from Alison York regarding current
legislation. Stephen Carr is funded by NERC and the Macaulay Institute,
Aberdeen.
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