Frequently Asked Questions

Phoslock was developed in Australia by the Commonwealth Scientific and Industrial Research Organisation (CSIRO), to remove phosphates from water. The active element in Phoslock is lanthanum (a rare-earth element) which has a strong affinity to bind with phosphate. This forms an insoluble and biologically inert compound, Rhabdophane. Phoslock is comprised of 95% bentonite and 5% lanthanum.  

See also: What is Phoslock

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. 

Phoslock is manufactured as a dry granule which makes it easy to transport and store.  

See also: What is Phoslock

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. This binding forms the mineral Rhabdophane (an insoluble and biologically inert compound) which strips phosphate from the water.

See also: What is Phoslock

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.

Phoslock has been applied to a wide range of waterbodies. Some applications have occurred to drinking waters, while others have occurred on waterbodies with high conservation importance.  Many applications have also occurred on lakes which are used for recreational water sports e.g. swimming.

Many eco-toxicity tests have been undertaken on a wide range of test species by a variety of independent and governmental research institutions over the last few decades. Collectively, these reports demonstrate that Phoslock is safe to use in all naturally occurring environmental conditions at the recommended dosages. 

A detailed overview of these studies has been produced by PET’s Technical Team and can be downloaded here (PDF).

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 accurately calculate the Phoslock dose for a water body.

Phoslock offers a wide range of benefits over other methods for reducing phosphorus concentrations in waterbodies. Some of the most important benefits 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 be 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.
• 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-toxicity safety | See FAQ above
• 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 | Several 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 effects | 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. The combination of these benefits makes Phoslock a unique and innovative solution to eutrophication control.

Phoslock is not an algicide so it does not directly control the occurrence and severity of algal blooms.  The effectiveness of a Phoslock application can be hampered if there are still high concentrations of external phosphorus entering a waterbody after a Phoslock treatment.

When used in lake restoration, Phoslock is usually added to a water body as a slurry. Phoslock is mixed with in-situ lake water and sprayed over the water’s surface.

As Phoslock sinks, it strips phosphate from the water column. Once settled on the sediment, Phoslock continues to bind phosphate released from the sediment, until the lanthanum binding sites are saturated, at which point no further binding will take place. 

See also: What is Phoslock

The best time to apply Phoslock is between late Autumn and early Spring in Europe. This is because during these periods the majority of the available phosphate is not bound in biomass such as algae or aquatic plants but mainly in the sediment. 

An application of Phoslock strips the water column of phosphate and then ‘caps’ the bed sediments, and this targets much of the phosphate in the system.

Every water body is unique and a water body-specific Phoslock dose would need to be calculated for each potential treatment but typically, 2 – 4 tonnes/hectare of Phoslock are applied but this can vary considerably for individual waterbodies as dosage is based on how much phosphate is in the water column, bound in the sediments and any external sources of phosphorus still entering the waterbody.

The cost of an application will vary considerably for a waterbody depending on the location, size, depth and amount of phosphate that would need to be bound to Phoslock.

As the cost varies considerably from waterbody to waterbody, it is necessary to have a certain amount of information available to create an estimated cost. 

If you are interested in applying Phoslock to your waterbody, please get in touch and we would be happy to provide an estimated quote.

Every lake is different and a range of information about the lake needs to be collected when considering a Phoslock treatment. For instance, it is important to determine the sources of excess nutrients to your water body; i.e. are the nutrients coming largely from internal sources (e.g. from phosphorus release from sediments) or from external sources (e.g. from diffuse or point sources in the catchment). This is critical to know as continued high external loads could impact the effectiveness of a Phoslock application.

Water quality data is vital to understand each water body's unique story. Long-term water quality monitoring data is unfortunately rare for many water bodies but regular monitoring helps us understand what management options may be best if nutrient pollution and symptoms of eutrophication have become a problem. Water quality monitoring data is also vital to help us understand if Phoslock is a suitable product to control nutrient pollution in a water body as there are many aspects that need to be considered before an application. 

In addition to water quality sampling, sediment sampling is often needed to estimate how much phosphorus is potentially releasable under normal lake conditions. This information, along with other nutrient calculations is used to make an informed dose calculation for a Phoslock application. 

Please get in touch if you are considering Phoslock to treat your water body or if you need further information.

We can offer advice and create a pre-treatment assessment, estimated dose calculations and costings for your water body.

Phoslock will continue to bind phosphorus until all lanthanum binding sites are saturated. Although the bond formed by lanthanum and phosphate is permanent and will not be broken under naturally occurring conditions in water bodies, the effects of a Phoslock application will depend on if any high external nutrient loads are entering the waterbody. 

If high loads are entering a water body following a Phoslock application, then repeated, usually smaller Phoslock doses, may need to be applied. 

Sometimes the presence of benthic feeding fish in high densities can impact a Phoslock application through continued sediment disturbance and potentially releasing phosphorus deeper than the Phoslock layer. High biomass of water birds at a water body can also have an impact of Phoslock efficiency through continued high nutrient concentrations after an application.

Eutrophication is a natural process which occurs over centuries due to the natural aging of water bodies where they are filled with sediments gradually over time. 

However, human activities have accelerated the level and scale of eutrophication through the supply of excess nutrients into aquatic systems. Excess nutrient input can be supplied to a waterbody from anthropogenic origins such as municipal wastewater discharges, industrial effluents and runoff from fertilizers and manure spread on agricultural areas.

Phosphate can be retained and released from lake sediments which can accelerate eutrophication and prolong its effects.

Phosphate is an inorganic nutrient.

Lanthanum is a rare earth element that occurs naturally in the environment in low concentrations. 

Lanthanum forms a very strong bond with oxidized anions, such as phosphates, carbonates and silicates, forming lanthanum salts. The bond between lanthanum and ortho-phosphate (LaPO4) is particularly strong and stable under many environmental conditions. 

Bentonite is a clay consisting of smectite minerals, the most common of which is usually montmorillonite. 

Bentonite is characterised by exchangeable sodium, calcium or magnesium cations which greatly influence the properties and commercial uses of the clay. Bentonite is often called the mineral of 1000 uses due to its wide range of applications. The major worldwide applications of bentonite are as an additive in foundry sands and drilling muds, as cat litter, as an additive to stock feed to aid digestion, as a binder in iron, as pelletisation processes and as a clarifying agent in wine making and edible oil refining. Bentonite is also commonly used in the paper industry and as an efficient material to line a seal dams and landfill sites.