CHEMICAL METHODS OF REMEDIATION


A feature of this literature review on the application of chemical methods of
Sediment remediation for eutrophied sediments has been the apparent paucity of a
Coordinated approach to research. Investigations of chemical methods of sediment
Remediation appear to have first began in earnest the early 1970s. The
motivation for This research was probably two-fold, the first being that with
the recognition that eutrophication of estuaries from around the world was
becoming common, potential Solutions to reverse the trend needed to be
investigated. The second motivation appears To have been driven by a desire to
find meaningful applications (in pteference to conventional landfill
disposal)for by-products of large scale industrial processes such as alumina
refining (bauxite residue/red mud) and power generation (fly ash). Continuing
research into chemical methods of sediment remediation for eutrophied sediments
appears, however, to have reached somewhat of a nadir in the late 1980s until
present , with many of the more obvious chemical (and in many cases physical)
remediation techniques having been evaluated and found to be inapplicable in
many aquatic systems.


APPLICATION OF ALUM



The use of alum [A12(SO4)3.14H20] to reduce concentrations of soluble phosphorus
in aquatic systems appears to have been first suggested by Lund (1955), with the
first documented application occurring in a Swedish lake (Jernelov,1971). Since
that time alum or similar aluminium based amendments (e.g.aluminium hydroxide,
Browman etal., 1977) have been applied to a number of aquatic systems, in
particular lakes, and has become an increasingly popular management tool, with
the most widespread use in North America (e.g. Kennedy and Cooke, 1982, Connor
and Smith, 1986, Welch etal (e.g. Haumann and White, 1978, Wolter, 1944). A
proposed trial in Lake Turingen, Sweden will use an alum gel in an attempt to
cap mercury comtaminated bottom sediments (Bergman and Pestonk,1977).


A prime concern over the use of aluminium or related compounds to treat sediment
to control soluble phosphorus concentrations in aquatic systems is the potential
toxicity of AL(M) and related species to both fish and plants (e.g.Baker and
Schofield, 1982, Neville, 1985, Cleveland et al, 1991, De Lonay et al., 1991,
Faust et al, 1995). Particularly in acidic waters. Hence, there has been
considerable effort in monitoring Al concentrations in natural and drinking
waters (e.g. Van Benschoten and Edward, 1990, Coller and Lin. 1997). It has also
been demonstrated that dissolved aluminium concentrations may significantly
increase when fluoride concentrations are above 0.8 mg/L ( Zhang et ab, 1994).
This factor may be particularly important when salinities are high in the Swan
River Estuary as average fluoride concentrations in seawater are typically 1.3
mg/L (Zhang et al., 1994). This factor may be particularly important when
salinities are high in the Swan River Estuary as average fluoride concentrations
in seawater are typically 1.3 mg/L (Stumm and Morgan, 1955). It has also been
suggested, however, that in waters of high TOC concentrations or where adequate
dissolved silica is available, potential aluminium toxicity may be significantly
reduced due to compexation (Driscoll et al., 1980, Birchall et al., 1989).
Alternatively, it has been suggested that the complexation of aluminium by humic
substances may enhance the transport of this metal, particularly in acidic
environments (Howe et al., 1997). At the pH of most natural waters (ca. 6-8), an
insoluble form of polymeric aluminium hydroxide [A1(OH)3] dominates which has
the potential to scayenge dissolved phosphorus, while in the pH range 4-6
various soluble species are present , and at pH ‹4 hydrated (soluble) A1(III)
dominates. At a pH above 8, however, the amphoteric nature of aluminium
hydroxide results in the formation of the soluble aluminate ion [A1(OH)4] which
may potentially lead to phosphorus (re)release (Cooke and Welch,1993).



The formation of polymeric forms of Al in aquatic systems may also have a time
flependency. Research has shown that it may take in excess of a year to
stabilise polymeric forms of aluminium in natural waters. The consequences of
this reaction interms of long term phosphorus release are poorly understood
(Burrows, 1977), however, one study suggests that the effectiveness of
phosphorus retention by polymeric aluminium flocs may decrease over time
(Kennedy,1978). In addition, it has been also suggested that the overall
effectiveness of alum may be reduced over time due to the dispersal of the
aluminium floc layer in deep sediment and coverage by new phosphorus-enriched
sediment (Welch es al, 1986). Limited studies also have demonstrated that the
removal efficiency of phosphorus from waste water is enhanced when alum is used
in combination with other materials such as clay (bentonite) and polyelectolyte
flocculants (Jorgensen et al., 1973). Studies of the fate of anthropogenic
aluminium in natural waters suggest that although there may not be significant
differences in water column concentrations (due to complexation and removal of
aluminium by dissolved ions), there may be substantial accumulation of aluminium
in bottom sediments (e.g. Abdullah, et al,1995).



Application of alum is generally undertaken (in lacustrine systems)using a
submerged manifold through which the alum (sourced from a barge mounted of
lakeside tank) is pumped (e.g. Kennedy and Cooke, 1982), however, modified
aquatic weed harvesters have also been used (Connor and Smith, 1986).
Application at the surface is generally avoided as this may potentially expose
surface biota to soluble aluminium salts and possibly low pH. In addition,
submerged (bottom water) application of alum delivers the treatment closest to
the problem area (Cooke and Welch, 1993). The development of ferric alum blocks
which dissolve and are periolically replaced has also been tested of (May,1974).
There are, however, no known documented applications of alum or related sediment
amendments in either riverine or esmarine systems.



Dosage rates of alum are in general estimated based on the physico-chemical
characteristics of the water body, with alkalinity being the most important
parameter ( Kennedy and Cooke, 1982). The optimum dose rate is usually
determined by diration of a standard alum stock solution with a water sample
with a stable endpoint of pH 6 (Cooke and Welch, 1993). As discussed above,
below this pH there is a possibility of the formation of potentially toxic
dissolved aluminium species. This problem may, however, be minimesed by the use
of sodium aluminate in combination with alum to provide a buffer in waters of
low alkalinity/acid buffering capacity (Connor and Smith, 1986).




POTENTIAL OF ALUM FOR SEDIMENT REMEDIATION IN THE SWAN RIVER ESTUARY




The application of alum or related sediment amendments to a number of lake
systems in both Europe and North America has frequently been demonstrated to be
highly effective in reducing water column levels of phosphorus and as a
consequence phytoplankton biomass. There are potentially two major advantages in
the use of alum in the Swan River Esmary.


                A major advantage of using alum or related sediment amendments
in the Swan River Estuary is that is both relatively cheap and easy to apply.


                 There is an extensive literature database on the use of alum
and case histories documenting its apparent effectiveness in lacustrine systems.



There are a number of aspects , however, that may potentially preclude the use
of alum or related materials as a amendment for sediments in the Swan Estuary.



                  As outlined above there is considerable concern in relation to
the toxicity of aluminium compounds to aquatic biota, and as such it has been
recommended that alum only be used in systems where pH is maintained between
6-8. Furthermore, it is likely that there would be a negative public perception
if alum or related sediment amendments were used in the Swan River Estuary as
there is considerable scientific evidence suggesting that long term (>15 years)
ingestion of aluminium may be a cause of Alzheimers disease (Walton and
Bryson-Taylor, 1995) and or renal failure (Broe and Caburn, 1990). These two
factors alone make the use of alum of similar aluminium-based amendments
controversial, not only in terms of scientific constrains, but also because of a
perceived public concern.



                    Limited evidence suggests that flocs formed from the
coagulation of alum would not be hydrodynamically stable and thus it is likely
that they would be significantly redistributed/disaggregated during moderate to
high flow (flood) events.



                      It has been demonstrated that there is a strong dependency
of water column pH for the effectiveness of alum treatments and that at low of
high pH release of not only soluble phosphorus but also of potentially toxic
aluminium species may occur.



APPLICATION OF LIMESTONE/DALAMITE/LIME/MAGNESITE



The coprecipitation of phosphorus with calcite (limestone [CaCO3] or other mixed
/impure carbonites) in aquatic systems is a well known phenomena (e.g. Effler
and Driscoll, 1985). Indeed , there is evidence that algal biomass is often
regulated in hardwater lakes due to absorption of P during carbonate
precipitation (Otsuki and Wetzel, 1972). Hence , it has been postulated that
with this mechanism it may be possible to manage aquatic systems by
significantly reducing phytoplankton biomass via phosphorus removal. As a
consequence, the use of limestone (CaCO3) or lime (CaO) has been extensively
tested for use in the removal of soluble phosphorus both natural (e.g. Harvey
River, Weston Australia, Jack and Platell, 1983, Rara River, Futaedani et al,
1992, Dagowsee Lake, Dittrich et al, 1997) and waste waters (e.g. piggery
effluent, Weaver and Ritchie, 1987) soils (Anderson et al 1995), in laboratory
trials (e. g. Ho and Monk, 1988) and even retention of phosphate within nutrient
rich soils (Anderson et al, 1995), Indeed, recent research in Canadian lakes
(Prepas et al, 1990) and prairie water dugouts (Murphy et al, 1990) has
demonstrated that with the addition of limestone and or lime, it was possible to
significantly decrease solution phosphorus concentrations (ca.30%), chlorophyll
a concentrations (ea 12%)