Monday, March 7, 2011

Zeolite for lake remediation

Lake Okaro: core studies

The efficacy of modified zeolite on blocking the release of phosphorus from lake sediments was evaluated prior to conducting a whole lake trial (Gibbs and Özkundakci, 2010). Two cores were treated with two different grain sizes and dose rates of modified zeolite under both aerobic or anoxic water conditions.

The applied modified zeolite had a high affinity for phosphorus with a thin layer (~2mm) completely blocking the release of phosphorus from the sediment under aerobic and anoxic conditions. It also removed phosphorus from the overlying water in contact with the capping layer. Interestingly, it also absorbed mercury and arsenic from the geothermally influenced Lake Okaro sediments. Modified zeolite also completely blocked the release of ammoniacal nitrogen from the sediments as well as ammonia from the water in contact with it.

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Lake Okaro: 2007 application

Over four days in September 2007, Lake Okaro was treated with about 110 tonnes of modified zeolite to form a sediment cap. This modified zeolite was applied where the lake depth exceeded 5 m, equivalent to 20 ha of lakebed. Environment Bay of Plenty contracted NIWA to independently evaluate the efficacy of modified zeolite as a phosphate-inactivation agent and to identify any adverse effects (Gibbs et al., 2007). Their results demonstrated the modified zeolite applied as a capping agent is capable of blocking phosphorus released from the sediment and overlying water.

Despite the rapid reduction in lake water total phosphate following modified zeolite application, the trophic state level of the lake remained constant (Özkundakci et al. 2010b). The observed significant decline in P concentration in the bottom waters of Lake Okaro in response to the modified zeolite application suggests that sediment capping was the most effective amongst all of the restoration procedures carried out in this lake, which included a constructed wetland, farm nutrient management, and the riparian restoration.

A follow-up study in 2009 (Gibbs, 2009) showed the modified zeolite adsorbed about 50% of the available phosphorus in the sediment over 18 months from the September 20017 application. The P content in the sediment declined from 3.15g m-2 to around 1.5 g m -2 a reduction of around 50%. Measuring the P content of modified zeolite granules recovered from the lake indicated that most of the P reduction could be attributed to uptake by the modified zeolite.

The researchers speculated that the large grain size of the modified zeolite sank in to the sediment and effectively was only able to remove P from the sediment surrounding it. A further trial in 2009 used a smaller grain size and was expected to result in a more effective sediment cap.

The 2007 sediment capping of Lake Okaro using modified zeolite did not have any undesirable effect on zooplankton or phytoplankton community structure (Özkundakci, et al. 2009). In the long term, the zooplankton community is expected to change in composition with a reduction in nutrients and algal levels. Modified zeolite application had no effect on fish health in Lake Okaro (Landman and Ling, 2010).

Lake Okaro: 2009 application

In a second application in August 2009, the lake was treated with a further 44 tonnes of modified zeolite applied as a slurry (Gibbs, 2009). This application had a finer grain size (0.3-0.5mm) compared with the 2007 application (1-3mm). This fine grain size was able to absorb phosphorus from the water column, possibly a function of the lower settling rate. Results have not yet been reported on the efficacy of this application of modified zeolite as a sediment cap. There was strong evidence that the finer modified zeolite slurry acted as a flocculant as it precipitated organic matter, including algae out of the water.

Lake Rotorua trials

As part of the programme for the restoration of Rotorua lakes, Environment Bay of Plenty studied the use of sediment capping materials to reduce the amount of dissolved reactive phosphorus in the lake water column from sediment release. Of four P-inactivation products trialled (Gibbs et al. 2008), modified zeolite had a high P-binding rate and was the only capping material which reduced the released of ammonia nitrogen from the sediments.

The potential for modified zeolite as a sediment cap in Lake Rotorua has been trialled. Laboratory trials by NIWA and the University of Waikato (Gibbs and Hamilton, 2009) compared modified zeolite with two other potential sediment cap materials. All three performed well in the lab and suggested no phosphorus would be released from sediments following application of these capping materials.

Currently, consents are being applied for trials in a limited area in Lake Rotorua. A challenge will be the uneven application of the modified zeolite on the lake bottom due to lake depth, drifts and currents. Work is being progressed by Blue Pacific Minerals in this area with a site-specific water dispersible granule being developed.

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