Improving the Environment Sustainably using Natural Zeolites

My research focuses on the properties of zeolites, which are natural environmental purification materials, with a view to using them in the recovery of hazardous substances. Zeolites are naturally-occurring minerals that are created through volcanic activity over a long span of time. They are characterized by their porous structure, which features tiny holes (micro-pores) of less than 2 nanometers* in diameter arranged in uniform rows. Because they adsorb other substances well, they have a wide variety of practical applications, including water purification, soil improvement, and deodorization.

Japan is a volcanic country and zeolites are readily available. In their natural state, however, zeolites contain impurities, so they are also fabricated artificially and classified based on the purpose of use. You can find these zeolites in some familiar places including the water purifiers in goldfish tanks, pet litter, gardening and agricultural fertilizers, and cleaning aids.

I’m researching one way that the adsorptive properties of zeolites can be used to improve the environment: removing radioactive cesium from the soil polluted by the Fukushima Daiichi Nuclear Power Plant accident. Ideally the cesium should be removed without altering the composition of the soil, a variety of methods are being explored, but a workable technique has not been devised yet. The cesium is easier to recover if the soil structure is broken down first, but the cost of doing so would be immense considering the area of soil that needs to be treated, and the destroyed soil would go to waste, unsuitable for reuse. The recovered cesium would also need to be stabilized.

Nature offers a hint as to how this problem could be solved. One type of zeolite, pollucite, contains cesium. This is proof that cesium in the soil can be adsorbed in zeolites and stored there in a stable form. So I am now researching ways to create a pollucite artificially to stabilize cesium.

I came across zeolites when I was a student at Hosei University. Why I chose them as the focus of my research on environmental materials was that they are a reusable and readily available natural substance. I believe that we can achieve sustainable environmental improvements by creating a resource cycle with environmentally-friendly, easy-to-use materials.

Zeolites are already in use as an ingredient in fertilizers designed to improve soil quality. This is because the minerals contained in natural zeolites can be stored in soil when mixed into the soil.

Building on this existing use, I’m also researching how zeolites themselves could be used as a habitat (growing medium) in place of soil. I’ve proven that it’s possible to grow leafy vegetables exclusively in a zeolite complex created to contain the compounds essential for plant growth, with the addition of water. In a joint research project with corporate partners, we’re fabricating sheets embedded with zeolite to be placed over the top of the soil. These sheets have the advantage of being light and portable. The properties of the zeolites enable stable delivery of water, oxygen and nutrients, so vegetables grow well in it.

The agriculture industry is vulnerable to natural disasters and environmental pollution, and production is unstable. I believe that in the future, by improving the functionality of growing media, the stable growth of high-quality agricultural produce will become a reality.

There are still many other possibilities for making use of the environmental purifying functions of zeolites. I expect that pursuing deeper research and improving functionality tailored to practical uses will yield “practical wisdom” that helps solve a variety of environmental problems.

I’m receiving Grants-in-Aid for Scientific Research (KAKENHI) for my research, so I try actively to share my findings with wider society. With this in mind, I participate in the Japan Society for the Promotion of Science’s HIRAMEKI ☆ TOKIMEKI SCIENCE – Welcome to a University Research Lab – KAKENHI program on an ongoing basis.

In the summer vacation period, I run the Experience a Water Environment Lab for a Day program, a science lab class for elementary school students. I have the participants construct a water purifier using zeolites, make observations using an electron microscope, and experience how interesting science can be.

I encourage the students in my own research lab to play the role of instructors in this program. In order to explain things in a way that elementary school students can understand, you need to clearly organize information in your head. Teaching other people helps to consolidate your own knowledge.

My students tend to be shy, but I hope that they will learn freely, make the most of the open environment that is Hosei University’s strength.

One of my new projects in recent years focuses on geothermal power generation using the geothermal energy in the magma layer deep beneath the earth’s surface. Generating geothermal power, which is a natural power source, involves pumping up geothermal water heated by the magma, using steam from the water to power a turbine and capturing the turbine’s energy. Currently geothermal power accounts for only 0.3% of overall electrical power generation in Japan, but there are high hopes for its future development as a clean, sustainable source of energy.

However, several problems have also been identified in relation to geothermal power. One is the cost blowout resulting from silica scale processing. Silica scale is a water deposit that accumulates through repeated exposure to air and evaporation of the silicic acid (silica) that is contained in the water. If silica scale formed on the pipes used in drawing up the geothermal water, heat efficiency is lost and there is a decrease in power output.

The deposited silica needs to be removed periodically and the pipes cleaned. The deposits removed in this process contain arsenic and other toxins, so need to be disposed of as industrial waste. This is an extremely costly process.

My research is looking into a possible solution for this silica scale problem by making use of the silica in the geothermal water to manufacture useful porous silica, before it forms deposits in the pipes. Porous silica can be used as an adsorbent and for other purposes, and contains zeolites, mesoporous silicates, and silica gel. If the silica in water for geothermal power generation can be removed and used to produce a useful material, we will be much closer to solving the silica scale problem. We are advancing research toward practical application of this technique through ongoing experiments at a geothermal power plant, and are applying for a patent on it.

Even if you can produce renewable energy as an environmental initiative, if it comes at massive costs it will be difficult to sustain. Cost is surely a key factor for promoting the use of natural energy. I believe that by using geothermal water to generate electricity, and cutting costs by producing a useful material at the same time as solving the problem of silica scale, we can look forward to a sustainable solution.

*Nanometer: a unit of measurement equaling one billionth of a meter.

Porous silica deposit experiment at the Yamagawa Geothermal Power Plant in Kyushu