About Phoslock

This page explains what Phoslock is, how it works and what its benefits are.

What is Phoslock?

Phoslock is a product designed for phosphate removal from water.  Its active component is lanthanum which reacts with phosphate forming an insoluble and biologically inert compound.   Its ability to bind phosphate makes it an effective tool in lake restoration.

Phoslock was developed in Australia by the CSIRO as a way of utilising the ability of lanthanum to bind phosphate but at the same time making it safe for use in natural aquatic systems.

When used in lake restoration, Phoslock is usually added to the lake as a slurry.  As this sinks through the water column it binds phosphate as it goes.  Once settled on the sediment it can continue to bind phosphate released from the sediment, thus controlling one of the major blocks to lake restoration - sediment phosphorus release.

The ability of Phoslock to bind phosphate is important because phosphate is the form of phosphorus which is biologically available and used by plants - especially algae in lakes, to promote growth.  Excessive amounts of phosphate entering water courses is one of the main causes of deterioration of water quality in freshwaters today because of the way it drives algal productivity and undesireable changes in the balance of aquatic life.  The process is known as eutrophication.  The ability to remove biologically available phosphorus in a lake is therefore a major step towards improving water quality.

Phoslock can be used in lakes and ponds as well as other standing waters (e.g. reservoirs, fish hatcheries) where high phosphate levels are causing problems.  It is not usually applied to flowing waters.

Phoslock - technical information

Phoslock is comprised of 95% bentonite and 5% lanthanum.  It is manufactured through a controlled ion-exchange process whereby cations within the bentonite are exchanged with lanthanum cations.  The result is that lanthanum held within the bentonite structure retains its ability to bind phosphate, but does not readily dissociate, i.e. will not form free ions in water.

phoslockstructure.gifGranular Phoslock 







Phoslock is manufactured as a dry granule which makes it easy to transport and store.  It is usually applied as a slurry, being mixed first with some in situ water before being spread across the surface of the water body being treated.  

How does Phoslock Work?

Phoslock works by utilizing the ability of lanthanum to react with phosphate.  Removal of phosphate by lanthanum is highly efficient and has a molar ratio of 1:1 which means that one ion of lanthanum will bind with one ion of phosphate as follows;

La3+ + PO43- => LaPO4

The product of the reaction is lanthanum phosphate (LaPO4).  The important point is that lanthanum phosphate is highly insoluble which means it won't dissolve, which in turn means it is not biologically available.  This is the process by which phosphate (which is dissolved and therefore biologically available) is rendered non-biologically available.

Lanthanum phosphate is analogous to a highly insoluble, naturally occurring mineral known as Rhabdophane.  Lanthanum is also present in the sediments of many lakes, with typical background concentrations of around 40 mg/kg dry weight.

Click here for more information on bentonite and lanthanum.

How does Phoslock work within a water body?

Phoslock granules are generally mixed with in situ water and applied to a water body as a slurry.  This has the effect of creating many tiny particles of Phoslock which sink slowly through the water column.  As the slurry settles, phosphate in the water column binds to lanthanum contained within the Phoslock, stripping most of the phosphate from the water column.  Once settled on the sediment it forms a very thin layer, usually less than 1 mm thick. There it will continue to bind phosphate coming out of the sediment, until the lanthanum binding sites are saturated, at which point no further binding will take place.

 How phoslock works

Once on the sediment, Phoslock, and the lanthanum phosphate within it, remain as in inert mineral component of the sediment.  The nature of the bond between lanthanum and phosphate is such that it will not be broken under a pH range wider than found in almost any natural lake (pH 4-11).  Even more importantly, the bond will not be broken under anoxic (low redox potential) conditions, which are prevelant in most lake sediments and many overlying waters.  Most other phosphorus binders, especially those containing iron, will release phosphate under anoxic conditions.

How much phosphate does Phoslock remove?

One tonne of Phoslock is capable of removing 34 kg of phosphate (PO4) or 11 kg of phosphorus (P).  With this information and a knowledge of the amount of biologically available phosphorus in the water and surface sediments of a lake, it is possible to fairly accurately calculate the Phoslock dose for a waterbody.

Phoslock and algal growth

algal bloomPhosphate is an important food source of algae (particularly blue green algae).  By removing phosphate from the water column and preventing its release from sediments, Phoslock essentially deprives algae of an important food source and the proliferation of algae, particularly blue green algae, is reduced.

Both nitrogen and phosphorus are important nutrients for algal growth, however the growth of blue green algae is generally not limited by the concentration of nitrogen as many species of blue green algae can fix and store nitrogen from the atmosphere.  The proliferation of blue green algae in a water body is related to the competitive advantage they have over other phytoplankton groups (such as diatoms and green algae) when excess phosphorus is available in the water. 

The reduction of phosphate levels in a water body and its sediments through an application of Phoslock will therefore result in a significant reduction of blue green algae and the restoration of healthy ecological conditions which favour the growth of diatoms, green algae and macrophytes.

Benefits of Phoslock

Phoslock offers a wide range of benefits over other methods of reducing phosphorus concentrations in lakes and reservoirs.  The combination of these benefits makes Phoslock a unique and innovative solution to eutrophication control. 

Some of the most important benefits of Phoslock include:

Reduction of phosphate

The ability of Phoslock to reduce phosphate to levels close to or below standard detection limits (< 10 µg/L) has been well demonstrated in a large number of both laboratory and field applications.  Importantly, dosages can also adjusted to achieve phosphorus concentrations that fall within a particular target range (e.g. 20-30 µg/L).  This approach is used when it is necessary to reduce phosphorus concentrations to levels that are sufficiently low to prevent blooms of blue green algae but high enough to ensure that the natural level of productivity in the system is retained.

Rapid uptake of phosphate

The kinetic uptake of phosphate by Phoslock varies to some degree according to water chemistry, however in most situations, more than 90% of available phosphate is bound within three hours of an application of Phoslock.

Eco-toxicological safety

mayflyMany eco-toxicity tests have been undertaken on a wide range of test species by a variety of independent academic and governmental research institutions over the past ten years.  Collectively, these reports demonstrate that Phoslock has a very low toxicity and is safe to use in all naturally occurring environmental conditions at the recommended dosages.  A detailed overview of these studies has been produced by Phoslock Water Solutions' Technical Team and can be downloaded from the Eco-Toxicity Assessment page.

In addition, unlike other methods of binding phosphate (such as iron and aluminium salts), an application of Phoslock will have no effect on either the pH or the conductivity of the water body. 

Stability under varying conditions

Unlike other methods that could be used to immobilize phosphorus in water and sediments, Phoslock is insensitive to the range of redox, temperature and pH conditions that are naturally found in lakes and reservoirs.  Furthermore, buffering is not required prior to an application.

Resistance to resuspension

lake in finland with pierSeveral studies have been conducted to determine the resistance of a Phoslock sediment capping to resuspension at different flow velocities. All studies have shown that Phoslock has substantially higher resistance to resuspension than aluminium or iron salts, which can also be used to immobilize phosphorus in sediments. This is not surprising given that the density of Phoslock is much higher than flocculants and highlights the suitability of Phoslock for use, even in shallow lakes.

Long term effect

Following the application of Phoslock to a water body, any phosphate that has reacted with the lanthanum in the clay will remain permanently bound.  Furthermore, any lanthanum sites in the clay matrix that have not reacted with phosphate remain active and will continue to bind phosphate (from both external and internal sources) until saturated.  This means that an application of Phoslock can be designed to achieve a sustained reduction in phosphate levels.