China.- With 27 tons of “magic soil”, modified clays developed by Chinese scientists, being stored in Chile, salmons there will enjoy their “summer vacation” of southern hemisphere without fear of toxic red tide, one kind of harmful algal blooms (HABs).
The red tide caused economic losses of 100 million U.S. dollars in Chile’s aquaculture industry from the end of 2015 to the early of 2016.
As the mix of modified clays and seawater are sprayed onto red tide by apparatus aboard, the modified clays will take the majority of algae down to seabed through flocculation and inhibit the growth of residual algae.
NEW HOPE FOR SALMONS IN CHILE
As the main production country of salmon and trout in the world, Chile’s aquaculture industry provides incomes for thousands of families. However, the toxic red tide has become a fatal threat to fishes and their livelihood.
After catastrophic economic losses in 2016, Chile sought solutions here and there, and modified clays used in China as a HABs mitigation strategy attracted their attention.
A Chilean delegation with members from government, aquaculture business and research institutes, visited the Institute of Oceanology of the Chinese Academy of Sciences (IOCAS), which owns the technology of modified clays, in October 2016.
The research group, led by Prof. YU Zhiming from IOCAS, illustrated the technology of modified clays for the delegation, which also observed manufacturing site of modified clays, special spray equipment and live dispersal demonstrations in Beihai and Fangchenggang cities of Guangxi Autonomous Region of south China.
Both sides agreed to bring the modified clays technology to Chile and apply it to emergency disposal of the red tide in coastal culture zones of Chile, according to the Memorandum of Understanding (MoU) they signed after the visit.
IOCAS and Virbac-Centrovet, a large aquaculture enterprise of Chile and representative of Simon Chile in the cooperation with IOCAS, reached an exclusive strategic collaboration agreement in June of 2017.
“As agreed, we developed special modified clays in full consideration of its application in Chile and tested its effect on salmons and shellfishes many times. It turns out that the new modified clays play no harm to the cultured species,” said YU.
After a third-party testing by the well-known Woods Hole Oceanographic Institution of America, 27 tons of modified clays were shipped from China on December 24 of 2017 and arrived in Valparaiso of Chile 38 days later.
YU Zhiming and three other researchers visited Chile for seven days in March, talking with members of maritime affairs, fishery and aquaculture in five cities, holding seminars on application details of modified clays, and demonstrating its dispersal aboard.
The 27 tons of modified clays will be dispersed onto the red tide when it appears in large scale in this November to protect salmons and other cultured species.
The cooperation between IOCAS and Virbac-Centrovet will continue in the coming years, according to their agreement.
SECRET OF CHINA’S MODIFIED CLAYS
Spraying clay, a basic component of soil, is one of only few mitigation methods for HABs ever applied in the field, but low flocculation efficiency has always been the most significant drawback associated with natural unmodified clays, leading to the requirement of an exorbitant amount of clays to achieve an effective efficiency.
The experiments by YU’s research team showed that the surface charges of HAB cells and natural clays particles were both electronegative and they had repulsive interaction, which caused low flocculation. It is one of the fundamental laws of electricity that like charges repel.
YU’s research team also found that the effective interaction radius of clay particles also contributed to an improvement in flocculation efficiencies as well.
Therefore, the research team managed to change surface properties of clay particles from negative to positive to increase attractive interaction between clays particles and HABs cells and enhance flocculation efficiency, through absorption or insertion of proper modifiers, such as poly-aluminum compounds (PAC) and aluminum sulfate.
The absorption and insertion are a series of physical and chemical processes.
“The modifiers can be both organic and inorganic matters, as long as they can change the surface charges of clay particles from negative to positive and have appropriate molecular chain length, which increases charge neutralization between clay particles and HABs cells,” YU said.
Production of modified clays has been industrialized in China, involving slurry water screens, magnetic separation and purification, surface modification, pressure filtration and dehydration, and establishment of ultrafine grinding, which guarantee the quality and stability of modified clay products.
WIPING OUT HABs
The modified clays take down the majority of HABs to sea bottom as the flocculation efficiency increases by dozens to several hundred folds, and the dosing requirement decreases sharply from 110-400 to 4-10 t/km2.
Based on field experiences, 70% to 80% HABs can be removed by modified clays. However, 20% to 30% initial algae could still be considered to be a blooming level.
But practices showed that the growth of the residual algae had rarely been observed after modified clays treatment.
Laboratory investigations revealed that the collision between modified clays and HABs could generate large amount of reactive oxygen species (ROS), which was widely used in sterilization and inhabited HABs’ growth and proliferation.
As we know, the modified clays take down HABs to seabed, so can the organisms escape from the flocs or become “seeds” of HABs next year? Long term filed observation showed no evidence of a second bloom from the sediments after a modified clays treatment.
It suggests it is difficult for HABs organisms to escape from flocs, and even they may have escaped, their growth and survival environment would have been severely compromised by modified clays.
The clay loading modes in the field include powder and suspension. Clay powder is directly dispersed onto HABs from planes, while the second mode needs to prepare suspension first and then spray them on the bloom by ships.
Although the efficiency of powder is much greater than the suspension, but powder in the air affects the surrounding environment and requires a specific apparatus. Therefore, suspension spraying is the most popular method.
WOULD MODIFIED CLAYS KILL OTHER FLORA, FAUNA?
Since HABs often occur in aquaculture regions, the effects of modified clays on surrounding flora and fauna are also a concern, especially its influences on shellfish and other filter-feeding organisms.
Studies showed that the modified clay didn’t pose any acute or chronic toxicity concerns, as a very high concentration (far more than 1000 g/m3) of ball clay is needed before the level becomes toxic to milkfish, sea bass and rabbit fish.
“The effective dosage is only at 4-10 g/m2, so the modified clays to mitigate HABs have no negative influences on natural organisms,” YU said.
Furthermore, both laboratory and field results have shown that appropriate dosage of modified clays could achieve marked improvement in water quality while mitigating HABs.
Experiments showed that clays had a high absorption capacity for phosphate (30%) and surface modification of clays can increase their absorption capacity for DIP (dissolved inorganic phosphorus) by 25% to 50%. In addition, modified clays can also remove particulate nutrients by flocculation to make water quality better.
However, could the algae settled by modified clays cause a secondary eutrophication as a result of nutrients released from the high biomass on the sea bottom?
“The answer is no,” YU said, “The algal organic matters combine with the clay particles to form a sandwich-matrix in which organic phosphorus and other nutrients are effectively sealed and blocked.”
APPLICATION OF MODIFIED CLAYS IN CHINA
After first applied in the Xuanwu Lake of Nanjing in 2005, the modified clays technology has been successfully applied to mitigate HABs in many important events and more than 20 regions along China’s coastlines.
In August 2008, a massive HAB with an impacted area of 86 km2 occurred near the Olympic Sailing Center of Qingdao, threatening the sailing events of the 2008 Beijing Olympic Games.
About 360 tons of modified clays were continuously dispersed into the bloom water within 40 hours, and the green algae decreased by 1-2 orders of magnitude and water quality was restored the day after treatment. The potential threats to the sailing events were successfully eliminated.
In August 2016, a red tide occurred along the Qinghuangdao cost of China’s Hebei Province, which impacted several hundred square kilometers and posed a serious threat to adjacent aquaculture.
Approximately 100 tons of modified clays were sprayed to form an isolation belt to block the bloom waters into the aquaculture areas, and the density of dominant species declined by 96.8%. Water quality also improved as the Chemical Oxygen Demand (COD, an index of water pollution) decreased from 2.32 mg/L to 1.08 mg/L in two days.
It’s well known that HABs are capable of destroying ecosystems, impacting aquaculture, and even threating human health. However, it is unknown that HABs could threaten the safety of nuclear station.
The water cooling system of Fangchenggang nuclear power plant in Guangxi was blocked by one kind of algae named P.golobosa, and the modified clay was selected as the sole method for HABs treatment in cooling water intake areas of the power plant by the national nuclear safety administration of China.
During the P.golobosa bloom from December 2015 to February 2016, 210 tons of modified clays were sprayed, and the algal colonies were removed and the power plant’s safety was secured.
The modified clays technology was also applied to block HABs into the nearby waters of the BRICS Xiamen Summit’s venue in September 2017 and the Shanghai Cooperation Organization (SCO) Qingdao Summit’s conference center in June 2018.
Since 2014, modified clays have been included as a standard method in the “Technical Guidelines for Treatment with Red Tide Disaster” in China. Currently, it is the only large-scale filed application method for HABs mitigation in the nation.
Contact
YANG Fengfan
Institute of Oceanology
E-mail: fengfanyang@qdio.ac.cn
Source: Chinese Academy of Sciences
