Team:HBUT-China/Description

Background

At present, heavy metal pollution is quite serious, and causes great harm to the environment, organisms, and humans. It can easily pollute the food chain. Therefore, scientists all over the world have paid great attention to solving this urgent problem.

Heavy metal pollution is different from that of other organic compound pollutants. Many organic compounds can be reduced or eliminated by the physical, chemical or biological purification of nature itself. Heavy metals are enriched and difficult to degrade in the environment.

Due to their exploitation, smelting and processing in China, many heavy metals such as lead, mercury, cadmium, cobalt and nickel enter the atmosphere, water and soil, causing serious environmental pollution. For example, heavy metals discharged with effluent, even if their concentration is low, can accumulate in algae and sediment. They are then consumed, or adsorbed through the bodies of fish and shellfish, resulting in food chain concentration, thus causing public hazards. HBUT-China iGEM team noted the seriousness of heavy metal pollution in the environment(Ojuederie and Babalola, 2017) and focused on the treatment of one of the heavy metals—nickel.

Further research

Nickel is one of the main elements causing serious heavy metal ion pollution. It is a good industrial raw material and widely used. It is mainly used to manufacture stainless steel and other corrosion resistant alloys(Zuolo and Walton, 1997).

It occurs naturally in the environment, with an average level in the Earth's crust (which is rich in trace elements) of 75 ppm. It mainly exists in the form of nickel sulfide ore and nickel oxide ore. In many metals – iron, cobalt, copper and some rare earth ores – nickel often coexists. Petroleum contains nickel, and most coal contains trace nickel; this is then released through combustion and are the main sources of nickel in the atmosphere. Nickel in soil mainly comes from rock weathering, atmospheric dust fall, and irrigation water (including nickel-containing effluent). Nickel ion pollution has gradually become an environmental issue of public concern(Raval et al., 2016).

Can we help? Yes, we can!

We firmly believe that for environmental pollution, we must follow the environmental protection policy of "giving priority to prevention and combining prevention and control".

The first task is to control and eliminate pollution sources. The main source of heavy metal pollution is industrial pollution, mostly discharged into the environment through waste residue, wastewater and waste gas, which concentrates in people, animals and plants, thus causing great harm to the environment and human health. Treatment of industrial effluent can reduce this pollution through expensive and time-consuming technical methods and management measures, thus ultimately meet the national pollutant discharge standards.

One of the main sources of nickel pollution in the environment is nickel-containing effluent. In a meeting with the staff of the local Environmental Protection Department, we learned that nickel ions are often contained in effluent from industries engaged in nickel plating, machine manufacturing, and metal processing. They use a high alkali process to treat industrial effluent by causing it produce a nickel hydroxide [Ni(OH)2] precipitate, and then remove it. Through further investigation, we found that the nickel ion concentration after traditional physicochemical methods is still high(Sujatha et al., 2012).

We contacted a factory that treats nickel effluent, and arranged a visit.We found that the total amount of pollutants discharged by the factory is quite large,and the concentration level is 30 times the newest Chinese national standard. This demonstrates why the final effluent must be strictly controlled in order to reduce ion pollution in the environment.

Therefore, we combine our project with the existing treatment methods to reduce the final concentration of nickel ions to the lowest level more efficiently and cheaply. In 2017 and 2018, the HBUT-China team devoted themselves to the construction of nickel ion detection devices in the environment, with detection accuracy ranging from 10^-4 to 10^-7mmol/l. But if we want to solve the pollution problem, detection is far from enough.

Last year, we envisaged a method for the absorption and treatment of nickel ions. So, with last year's future plan as the blueprint, this year our goal was born.

How did we do it?

The sludge in the activated sludge process consists of a variety of microorganisms, which originally include yeast, so there is a realistic basis for treating effluent with yeast. It is just that yeast is used to treat domestic effluent(Jafari et al., 2014).

Due to its excellent heavy metal tolerance, our project has enabled it to actively adsorb heavy metals, thereby expanding the range of yeast treatment. In the yeast genome, 28% of the nucleotide sequence does not encode an expression cassette. This property facilitates the insertion of the gene of interest into the genome and the expression of the corresponding protein.

We used Saccharomyces cerevisiae as a biological chassis this year to store nickel ions in vacuoles rather than in their cell environments in order to improve their tolerance to nickel ions. Three genes were introduced to enable engineered yeast to actively remove nickel ions in the environment. In order to achieve this goal, we first use the surface display system of MFα1+hexa-his+ AGα1 to capture and bind nickel ions(Kuroda et al., 2001; Kuroda et al., 2002; Kuroda and Ueda, 2003). We also use NixA to transfer nickel ions into cells(Deng et al.,2013), and then use TgMTP1t2 to transfer nickel ions from the cells into the vacuoles(Persans et al., 2001).

Under the coordination of these three sets of genes, our engineered yeast can actively bind or absorb nickel ions, and its tolerance to nickel ions is greatly increased.

After achieving these goals, we also sought a way to recover and recycle the absorbed nickel ion, so the original waste can regain its value.

In order to control the distribution of engineered yeast in effluent, we used immobilization technology for the cells. After consulting many documents and teachers, we finally chose PVA materials to encapsulate yeast into gel balls(Liang and Huang,2009).

In the field investigation of the factory, we observed the treatment workshop. Based loosely off the design of the effluent treatment plant, we replaced most of their system with our simpler device. We made a model mini factory physical object through 3D printing technology to demonstrate our concept.

The nickel-containing effluent flows into the Collection tank. When we are ready, the treatment starts. Our automated system pumps effluent into the treatment pool to a predetermined water level, automatically stops. Our treatment yeast gel balls (contained in a straining vessel) are then lowered into the tank, then the system starts blowing air from below (as a physical stirring mechanism, not for aeration), while the yeast absorbs the nickel ions. When the treatment is completed (after 45 to 60 minutes), the vessel containing the yeast gel balls is lifted out, and the system pumps the treated effluent into an outlet sump. Here, the success of the treatment could be tested/verified. We also added semi-permeable membranes to this tank's outlet to prevent microbial escape to the environment.

The used gel balls can later be purified by enzymatic cleavage, ion exchange and nickel electrolysis to reclaim pure nickel.

References:

[1] Ojuederie, O. B., & Babalola, O. O. (2017). Microbial and plant-assisted bioremediation of heavy metal polluted environments: a review. International journal of environmental research and public health, 14(12), 1504.

[2] Zuolo, M. L., & Walton, R. E. (1997). Instrument deterioration with usage: Nickel-titanium versus stainless steel. Quintessence International, 28(6).

[3] Raval, N. P., Shah, P. U., & Shah, N. K. (2016). Adsorptive removal of nickel (II) ions from aqueous environment: a review. Journal of Environmental Management, 179, 1-20.

[4] Sujatha, P., Kalarani, V., & Kumar, B. N. (2012). Effective biosorption of nickel (II) from aqueous solutions using Trichoderma viride. Journal of Chemistry, 2013.

[5] Jafari, N., Soudi, M. R., & Kasra-Kermanshahi, R. (2014). Biodecolorization of textile azo dyes by isolated yeast from activated sludge: Issatchenkia orientalis JKS6. Annals of microbiology, 64(2), 475-482.