Studies On Removal Efficiency Of Nitrogen And Phosphorus From Eutrophic Seawater With "solidified Slag Concrete-Macroalgae" System

In recent years, with the development of economy, there is a series ofeutrophication in our coastal waters. Eutrophication causes blooms of algae and otheraquatic organisms. Not only the aquaculture industry but also the development ofcoastal tourism industry sufferes huge losses, and eutrophication even causesecological imbalance. Steel slag is a kind of smelting slag, with a hard texture,porosity and specific surface area and other characteristics. National Developmentand Reform Commission has proposed to promote the comprehensive utilization ofslag. Slag was used in the manufacture of cement and bricks. The adsorption of heavymetals, ammonia and phosphate of slag was also explored. The removal efficiency ofnitrogen and phosphorus of eutrophic seawater with “steel slag–Gracilarialemaneiformis” system has been investigated, and the adsorption properties ofmodified steel slag for removal of mercury and arsenic from seawater have beenexplored. However, whether the solidified slag concrete could be used for seawatereutrophication control is unclear. Therefore, in our study, the slag and macroalgaewere adpopted experimental materials, and a series of single-factor experiments fromsteel slag and cement ratio, several different particle sizes, and different shapes werecarried on to optimize the solidified slag concrete. And then the removal efficiency ofnitrogen and phosphorus from eutrophic seawater with the solidified slag concrete,macroalgae and solidified slag concrete-macroalgae system was detected. Finally,security analysis was conducted. This study provides the basic data for the practicalapplication of solidified slag concrete in seawater eutrophication control. Our resultsindicated that:(1) The removal rate of phosphates and nitrates in eutrophic seawater withsolidified slag concrete was researched. And we found that solidified slag concrete could effectively remove the phosphates and nitrates. When using the7and80meshes, with the appropriate ratio of steel slag and cement and triangular prism shapeto make the solidified slag concrete, it showed the highest removal rate for phosphatesand nitrates, which were53.84%and62.55%, respectively.(2) The removal rate of phosphates and nitrates in eutrophic seawater withSargassum thunbergii and Ulva pertusa were studied. And we found that Sargassumthunbergii and Ulva pertusa can effectively remove the phosphates and nitrates. Ulvapertusa can remove71.27%of the phosphates and26.82%of the nitrates. And theSargassum thunbergii can remove66.17%of the phosphates and26.78%of thenitrates. The removal of nutrient by Ulva pertusa is slightly better than that ofSargassum thunbergii.(3) The removal rate of phosphates and nitrates in eutrophic seawater withsolidified slag concrete-macroalgae system was studied. And we found thatsolidified slag concrete-macroalgae system can effectively remove the phosphatesand nitrates. And the removal rate was significantly higher than that of singlesolidified slag concrete or single macroalgae. However, the solidified slag concreteshowed negative impacts on Sargassum thunbergii and Ulva pertusa. It made algaeweight reduced and made Ulva pertusa rot. Therefore, we should deal with thesolidified slag concrete and select an appropriate amount before its application.Otherwise, it will cause certain influences on the macroalgae.(4) In safety evaluation experiments, solidified slag concrete could release smallamounts of copper, lead and cadmium into the seawater, and solidified slag concretecould make pH and COD values rise, since that solidified slag concrete containedsome metal oxides and basic oxides. Still, the experimental seawater met Sea WaterQuality Standard GB3097-1997level two. Therefore, we should deal with thesolidified slag concrete and select an appropriate amount before its application.Otherwise, it will cause certain influences on the seawater.In summary, solidified slag concrete-macroalgae system can effectively removethe phosphates and nitrates. And we can solve the utilization of waste slag and avoidthe slag pile pollution. Solidified slag concrete can also be used as artificial reef. It can play a fish collection function and provide growth and breeding grounds for fish.It can be seen that the application of solidified slag concrete-macroalgae system isvery broad.