|dc.description.abstract||Phosphorus (P) pollution of waterbodies is a global issue with detrimental
environmental, social and economic impacts. Low-cost and sustainable P removal
technologies are therefore required to tackle P pollution, whilst also offering a
technique for reclaiming P. Ochre, a waste product from minewater treatment
plants (MWTPs), has been proposed as a suitable material for the removal of P from
enriched waters due to a high content of Fe, Al, Ca and Mg, which have high
affinities for P removal.
Whilst a range of studies have been conducted investigating ochre as a P adsorbent,
most of these are large-scale field experiments and lack understanding of the
underlying processes of P removal by ochre. There have also been very few detailed
comparisons of different ochre types. The primary focus of this thesis is thus to
provide a process-based understanding of P removal by various ochres, in order to
investigate the optimal conditions for the use of ochres in the treatment of aquatic P
pollution. Seven ochres from six MWTPs in the UK and Ireland were investigated,
one of which was in a pelleted form. The ochres were largely comprised of Al, Ca,
Fe and Mg (42-68 % by dry weight), had a high B.E.T. surface area, 56-243 m2 g-1,
and contained mineral surfaces with a high affinity for P adsorption, such as
goethite and calcite.
A novel batch experiment methodology was utilised to calculate the adsorption
characteristics of ochre at discrete pH conditions. The variation of these
characteristics with pH indicates the importance and requirement for such a method
to study adsorption by materials at the expected pH conditions of application. At
the pH conditions of wastewater streams (~pH 7), the P adsorption capacities of the
ochres, determined from fitting adsorption isotherms, was 11.8–43.1 mg P g-1.
Results of P adsorption batch experiments were modelled in ORCHESTRA, wherein
P removal by the ochres was described well by adsorption onto hydrous ferric
oxides. Three of the ochres contain relatively high calcite contents and due to a poor fit of the model to the observed datasets at high pH conditions, with equilibrium P
concentrations lower in the batch experiments than the modelled result, adsorption
onto calcite is suggested as a P removal mechanism for these ochres at pH > 7.
Environmental application of ochre filters will require P removal under flow-through
transport conditions. Column experiments were therefore conducted using
two ochres, coarse-grained Polkemmet ochre and Acomb pellets (column volume
1055 cm3, pore space 490-661 cm3, typical pore volumes of experiments: 220-400). P
removal efficiency increased with contact time, and the presence of competing ions
had only marginal effects on P removal. Resting the column substrate for 48 hours
between P applications greatly increased the P removal efficiency of a packed
column of Polkemmet ochre, resulting in 81 % of influent P removed over 1000 pore
volumes of operation (7.68 mg P g-1). Acomb pellets had a lower P removal
efficiency than Polkemmet ochre. It is suggested that the high calcium content of the
pellets, as a result of the pelletisation process, has created a substrate where the
dominant P removal mechanism at neutral pH conditions is adsorption to calcite,
which has slower reaction kinetics than adsorption onto goethite. Therefore, this
pelleted ochre requires a higher contact time for adsorption reactions to occur.
It is suggested that ochre filters are most suitable for application in situations where
flow rate is constant or can be controlled e.g. septic tank effluent. Ochres which dry
to a coarse particle size are preferred for use as a substrate as pelletisation requires
capital, expertise and can produce substrates with slower P sorption kinetics.
Resting the filter substrate between P application regenerates surface sites for
adsorption, and filters should be run in parallel to maximise P removal efficiency.
Acomb pellets, which are a mix of iron hydroxides and alkaline materials, may have
potential application as a permeable reactive barrier substrate to treat P enriched
ground waters. Further research utilising fine-grained ochres as an additive to P rich
fertilisers or for use in continuously stirred tank reactors is recommended.||en_US