The necessity of reducing internal P loading for lake eutrophication control has been well accepted by scientists and lake managers, and the in situ immobilization is a commonly considered method for the pollution control. In general, the theoretic basis of the in situ method is to reduce the release of P from lake sediment by dosing P-inactivation agents to induce the limitation of bioavailable P in lake water, and then to inhibit phytoplankton breeding. Accordingly, successful applications of the in situ method have been reported worldwide, e.g., for the restoration of Lake Rauwbraken and Lake De Kuil in the Netherlands. However, the in situ immobilization remains contentious because of the variable results reported in literatures.
In Nanjing Institute of Geography and Limnology, Chinese Academy of Sciences, Dr. Changhui Wang and Helong Jiang's one of the main research areas is to determine the potential uncertainties of the in situ immobilization for lake internal P loading control. They first demonstrated the limited impacts of some physicochemical factors (e.g., organic matter and aging effects) on the effectiveness of the in situ method, and then literature review was performed, finding that information about P bioavailability in amended sediment is lacked. Accordingly, a comprehensive analysis of P bioavailability in lake sediment amended with two promising P-inactivation agents, namely La-modified bentonite clay and drinking water treatment residue (DWTR), was investigated. The results showed that reduction of cyanobacteria available P by DWTR and La-modified bentonite clay (< 58%) were clearly less than the mobile P immobilization (> 75%) at recommended dosages. The reasons may be related to the capability of cyanobacteria to utilize various fractions of P (except the residual P) in amended sediment under proper illumination; whereas, DWTR and La-modified bentonite clay, as well as other P-inactivation agents such as Al, Fe, and Ca compounds that were commonly investigated, were hard to transform mobile P to residual P in lake sediment. Furthermore, the slow release of P from algae, the induced anaerobic conditions, the released organic matter from algae, and the increased sulfide in the overlying water and sediments during algae sedimentation also can reduce the efficiencies of P-inactivation agents for P immobilization. Therefore, the potential bioavailable P may pose uncertainties for eutrophication control in lakes under proper illumination, typically with regular sediment re-suspension. Overall, an evaluation of bioavailable P pool in lake ecosystem treated with P-inactivation agents should be essential for successful application of the in situ immobilization.
These works were supported by grants from National Natural Science Foundation of China, Natural Science Foundation of Jiangsu Province, and CAS Interdisciplinary Innovation Team, and also have been published in Water Research, Science of the Total Environment, Critical Reviews in Environmental Science and Technology, Journal of Environmental Management, and Environmental Science Pollution and Research. The results will provide theoretical support for the in situ method to control eutrophication.
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