An eco-environmental assessment of harmful algal bloom mitigation using modified clay
Introduction
Under the influence of human activities and climate change, harmful algal blooms (HABs) have become a global marine ecological disaster (Anderson et al., 2012; Yu et al., 2017; Yu and Chen, 2019). Outbreaks of HAB often cause deterioration of water quality, threaten marine life and human health, and lead to imbalances in the ecosystem structure, which are difficult to remediate within a short time. Currently, HABs cannot be accurately predicted. Therefore, in the event of HABs, timely and effective emergency disposal is urgently needed. In particular, effective prevention and control of HABs in coastal waters with special needs (such as breeding areas, large-scale sports competition venues, scenic tourist areas, etc.) are of great significance for protecting and restoring the functions of the water area.
In theory, many methods can be used to control HABs; however, each has its own limitations. Physical, chemical, and biological methods for HAB mitigation are difficult to implement in large areas due to their high cost, ecological risk and slow effects. Clay minerals (such as montmorillonite, bentonite, kaolinite, etc.) can provide an environmentally friendly method of HAB mitigation due to their flocculation properties (Choi et al., 1998; Park et al., 2013). The flocculation efficiency of natural clay, however, is limited, which results in a large dosage requirement and inconvenience in the field application. Researchers in China have used inorganic compounds that are commonly applied in drinking water treatment (such as polyaluminum chloride (PAC) and aluminum sulfate (AS), etc.) and organic compounds such as Gemini surfactant to change the surface properties of natural clay, and clay treated by the above modifiers is collectively referred to as modified clay (MC). MC exhibits great improvement in removal efficiency and a reduced dosage requirement. And now, a systematic theory and technical system of MC for HAB control have been established. This technology (MC technology) realizes both high efficiency and safety in HAB control. Currently, MC technology has been recommended by UNESCO and APEC (Anderson et al., 2001) and has been included as a national standard method (GB/T 30743-2014) in China. To date, MC technology has become the only large-scale field application method for HAB mitigation in China.
Although the selection of raw materials and preparation processes is based on the principle of environmental friendliness and no secondary pollution, the eco-environmental safety of MC in HAB mitigation in the field remains a focus of users and government departments. The most common concerns about the eco-environmental effects of MC include the following issues. Will the application of MC affect water quality parameters, especially nutrients? Will there be a risk of increasing or accumulating algal toxins when disposing of toxic algal blooms by MC? As an artificial additive, is MC toxic to nontarget aquatic organisms, including typical fish, shrimp, shellfish, and other organisms? Could the sedimentation of flocs cause significant changes in the benthic environment? In the emergency disposal of HABs, the application of MC is expected to prevent or at least reduce the negative impacts of HABs on the ecosystem. Researchershave carried out scientific research on subjects ranging from water to sediment and from HAB organisms to nontarget organisms for more than 20 years to answer the above questions. In this paper, the results of these studies are reviewed and summarized, mainly including works of researchers in China, as well as the latest research progress in related fields worldwide, to provide scientific guidance for mitigating HABs around the world using clay/modified clay.
Section snippets
Adsorption of major nutrients
Nutrients such as nitrogen, phosphorus and silicon provide the essential material basis for the growth of HAB organisms. The results have shown that clay can remove a certain degree of nutrients in the process of flocculating and settling HAB organisms (Yu et al., 1995; Lürling and van Oosterhout, 2013; Lu et al., 2015a; 2015b; 2017). It is well known and verified that clay has a strong adsorption capacity for phosphate due to the replacement of surface hydroxyl ions in clay with phosphate ions
Effect on algal toxins
Some HAB organisms can produce algal toxins, such as those producing paralytic shellfish poisoning (PSP), diarrhetic shellfish poisoning (DSP), amnestic shellfish poisoning (ASP), neurogenic shellfish poisoning (NSP), and ciguatera fish poisoning (CFP), causing the pollution of aquatic products and even threatening human health (Zhou et al., 2007). Most algal toxins exist inside microalgae cells, and some toxins can also be released into the water in different ways, such as through secretion or
Effects on nontarget aquatic organisms
The use of clay/MC for HAB mitigation involves artificially adding suspended particles into the water. In the process of effectively removing HAB organisms, the impacts of MC particles on nontarget aquatic organisms have always been the focus of research and application. Based on the fact that clay or modified clay is mainly used in HAB mitigation of coastal waters, which is also the location of most aquaculture areas, studies have paid much attention to the effects of clay and modified clay on
Dinoflagellate cysts
Many dinoflagellates can form cysts and deposit in the sediment under adverse environmental conditions such as nutrient limitation or temperature stress. When the environmental conditions are suitable, cysts in sediments can form vegetative cells through germination. Therefore, the formation of cysts is regarded as a potential result of HAB extinction, and the germination of cysts is considered one of the contributors to HAB formation. Compared with the natural extinction process of HAB, using
Field application
There have been many studies on the field applications of clay or modified clay since as early as the 1980s, such as those in the area of Kagoshima (Shirota, 1989). In 2002, yellow clay at a concentration of 10 g/L and sophorolipid-yellow clay (mixture of 5 mg/L sophorolipid and 1 g/L yellow clay) were used to treat the Cochlodinium bloom in the near sea of Miruk Island, South Korea (Lee et al., 2008). The results showed that the removal efficiencies of yellow clay and sophorolipid-yellow clay
Conclusions
Modified clay greatly improves the removal efficiency of HAB organisms by changing the surface electrical properties of natural clay, resulting in a low-cost and preferred choice for field application in HAB emergency control. As an artificial additive, however, the impacts of MC on environmental parameters, including water quality, algal toxins, typical economic organisms and benthic environments, have been studied in the laboratory and in the field for many years. This review summarizes the
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
The authors would like to acknowledge the contributions of Dr. Guangyuan Lu, Dr. Jing Li, and doctoral candidates Huihui Shen, Yu Ding, and Weijia Song for their assistance with supplying materials. This research was supported by the National Key Research and Development Program of China (2017YFC1404300), the National Natural Science Foundation of China (41976148), and the Taishan Scholars Climbing Program of Shandong Province of the year 2019.
References (71)
- et al.
Removal of chromium (VI) ions from aqueous solutions by using Turkish montmorillonite clay: effect of activation and modification
Desalination
(2009) - et al.
Clay flocculation effect on microbial community composition in water and sediment
Front. Environ. Sci.
(2018) - et al.
The influence of dilute clay suspensions on sea scallop (Placopecten magellanicus) feeding activity and tissue growth
Neth. J. Sea Res.
(1992) - et al.
Synthesis and characterization of a novel Mg–Al hydrotalcite-loaded kaolin clay and its adsorption properties for phosphate in aqueous solution
J. Alloys Compd.
(2015) - et al.
Toxin production and competitive abilities under phosphorus limitation of Alexandrium species
Harmful Algae
(2004) - et al.
Amino acid adsorption by clay minerals in distilled water
Geochim. Cosmochim. Acta
(1987) - et al.
High-silica zeolites for adsorption of organic micro-pollutants in water treatment: A review
Water Res
(2018) - et al.
Field experiments on mitigation of harmful algal blooms using a Sophorolipid—yellow clay mixture and effects on marine plankton
Harmful Algae
(2008) - et al.
Toxicity of clay flocculation of the toxic dinoflagellate, Karenia brevis, to estuarine invertebrates and fish
Harmful Algae
(2003) - et al.
Effect of modified clay on the transition of paralytic shellfish toxins within the bay scallopArgopecten irradians and sediments in laboratory trials
Aquaculture
(2019)