Reclaiming the health of British riversOptimising conditions in low-cost systems for treating diffuse water pollution

Carr1, S., Heal1, K.V., Lumsdon2, D., and Vinten2, A.1The University of Edinburgh, Edinburgh. 2The Macaulay Institute, Aberdeen.

1 Research rationaleThis project focuses upon the design and implementation of a suite of filters comprising ‘iron’ ochre, suitable for removing phosphates from anthropogenically enriched surface waters.

Ochre (amorphous Fe precipitates) is formed when treating mine-water which is contaminated with potentially toxic elements such as Fe, Al and Ca. At least 2.7x104 tonnes of ochre is generated p.a. in the UK with no current end-use.

Ochre is largely comprised of Fe(OH)3 and FeO.OH with smaller proportions of aluminium oxides and calcium carbonates. Due to this ochre has a high adsorption capacity for phosphorus (up to 26mg P g-1 ochre) and is therefore proposed as a suitable filter substrate for phosphorus adsorption.

Selection of OchreAcross the UK there are over 177 abandoned metal and coal mines producing discharges affecting over 600km of river reach. To alleviate this, mine-water treatment plants have been created across the UK. These act to settle potentially toxic elements out of the water column prior to discharge to natural water course, leaving behind an iron rich sludge; ochre.

The elemental and structural composition of ochre is highly site specific, depending upon factors such as mine chemistry and the conditions of settlement.

Ochres were chosen for this project based upon;

•Phosphorus adsorption capacity and elemental composition

•Body of previous research

•Availability of the ochre

For

more

deta

ils c

on

tact

Ste

ph

en

Carr

at

s.t

.d.c

arr

@sm

s.e

d.a

c.u

k

Photograph: Polkemmet, west Lothian mine-water treatment plant settlement pond

Figure: Location of mine-water treatment plant ochres used for this project

Ochre characterisationPrevious research on ochre has shown its high capacity and ability to remove phosphorus from solution under laboratory conditions. Expansion to field trials has been less successful, with far lower rates of phosphorus adsorption. This may be for a range of hydrological and chemical reasons. A detailed chemical and physical examination is ongoing in order to characterise and develop a more through understanding of ochre. This can then be used to optimise filters of ochre to remove phosphorus from aquatic environments.

Project MethodologyThe project has been divided into a series of experiments; laboratory, computer and field based which when completed will achieve the objectives of the research.

pH equilibrium experimentspH equilibrium experiments have been conducted for each ochre over an 8 day period. Ochre was suspended (15mg l-1) in 30ml 0.1M NaNO3 solution with the addition of 1ml of either HCl or NaOH to adjust the pH of the solution over the desired range (pH 4-10). The solution was then allowed to equilibrate. The sweep of pHs was achieved by conducting 9 trials with varying acid/base concentrations for each ochre.

ConclusionA range of ochres from across the UK are being characterised chemically and physically. This will form the basis of a chemical model using the software ORCHESTRA. From this filter design will be optimised and implemented.

Shaking experiment (with complex

solutions)

Column experiments

Complex column

experiments

ORCHESTRA filter modelling

Arsenic trial: Validate the model

Filter design

Field implementation

Diffusion into a sphere

BET surface area

Acid digest

Filter evaluation

Pellet stability

Density/ porosity

experiments

Oxalate-extractable

Fe

X-ray diffraction

pH equilibrium

Isotherm exp.

Shaking experiment

Flow chart of intended experimentation for the project

Figure: pH equilibrium for Acomb unpelletised ochre. Legend denotes strength of acid or base (-) added. Data points are the mean of triplicate experiments with the error bars indicating 1σ.

pH equilibrium experiment: Acomb unpelletised

2

4

6

8

10

12

0 24 48 72 96 120 144 168 192

Time (Hours)

pH

0.12

0.10

0.08

0.06

0.04

0.02

0.00

-0.01

-0.03

2

3 5

4

pH equilibrium experiment

-0.1

0

0.1

0.2

0.3

0.4

0.5

2 3 4 5 6 7 8 9 10 11 12

pH

Aci

d/ b

ase

add

itio

n (

Mo

lar

stre

ng

th)

Acomb unpelletised

Acomb pellets

Third order polynomial (Acomb pellets)

Third order polynomial (Acomb unpelletised)

Figure: Final pH value after equilibration for Acomb pellets and Acomb unpelletised. Y-axis shows the strength of either acid (HCl) or base (NaOH) addition. Data points are the mean of triplicate experiments with the error bars indicating 1σ.

6