"Using iron nanoparticles in anaerobic digestion"


The transformation of biomass into methane (CH4) is one of the possible solutions to some of the energy and environmental issues the world is facing nowadays: decrease/consumption of fossil fuel reserves, a global increase in energy demand and the greenhouse effect, among others.

Meanwhile, the need to improve the processing of organic waste towards sustainable waste management continues to increase.


Biogas in Europe

oxide nanoparticlesThe EBA (European Biogas Association) published the following statement in its Biogas 2014 report: “The combined amount of electric and thermal energy that is now produced from biogas in Europe equals annual household consumption in Belgium and Slovenia taken together. Such production could replace 15 coal-fired power stations with an average capacity of 500 MWel (Electrical MegaWatt).”

The current reduction of oil prices is not only a geo-political issue, but also a clue that the model is starting to show signs of exhaustion. This is also closely related to new technologies, as well as the higher efficiency of new machines and devices, the severe effects of pollution and the climate change associated to fossil fuel consumption. Besides, biogas production appears as a circular economy model, as well as a solution for energy provision in areas that are isolated from the electricity network: a huge amount of organic residues (agro-food farms) are produced in such areas, and energy is needed for them to work (lighting, heating, etc.) 

These are some of the conclusions reached by the aforementioned publication. Nevertheless, some of the leading biogas producers, such as Germany and Italy, are already coming to a halt, and forecasts are not optimistic for the rest of Europe either (changes in the financial control and support systems are expected, or else they are already being introduced). In this context, the EBA will continue to work in order to ensure a steady growth of biogas-based energy in the whole continent, as it is an essential aspect for energy security in Europe, as well as for decarbonization. Clearly enough, pressing needs arise, due to the current situation and deficiencies, to develop advanced technologies for biogas optimized production.

According to the recently publish EBA Biogas Report, there are more than 14.500 biogas plants in Europe, and their number is increasing. Jan Štambaský, President of the EBA, summarized it as follows:

“The biogas industry is facing big policy changes, and it is our duty to offer the most reliable data to support our member associations in their political efforts in their countries of origin, to support our scientists in their research on new technologies and to provide our companies with up-to-date information. The radical change that started with the German Renewable Energy Law EEG 2012 remains a hindrance to the industry. In spite of that, biogas industry is growing in other European countries -United Kingdom, Italy and Denmark- and we expect the rest of Europe to follow this positive development.”

The biomethane industry follows the growing trend of biogas; 282 plants exist in Europe, and total production equals 1.375 millions m3. Several opportunities for utilization are appearing, such as the number of service stations, that doubled in 2013, or the increase in the proportion of biomethane used in transport (10% of the total of that produced in Europe). 


How is biogas produced? 

The biogas production process, methanogenesis, is performed by archaic microorganisms (Archaea) that play a fundamental role in the carbon cycle and in the decomposition of organic matter in natural anaerobic ecosystems such as sediments, reservoirs and waste waters. Nevertheless, energy recovery from biomass only reaches 30-40% of the theoretical estimates, which makes the process somewhat inefficient and unattractive from the economic point of view. For this reason, in order to try to achieve higher conversion rates, the behaviour of archaea colonies has been widely explored, both by selecting optimal bacterial consortia and by trying to boost its activity by means of biomass pre-treatment, selective hydrolysis, biomass heating or addition of iron salts. Until now, increases in production had been modest, and costly processes were required to achieve even small increases.

A few years ago, the potential toxic effects of inorganic nanoparticles that would reach waste waters was studied. Reaching waste waters might have been unintentional, as would be the case when they are used in consumer goods (such as TiO2 nanoparticles in beauty products) or intentional, after their use as waste water cleansing agents (nanoremediation); this would be the case when using iron NPs to absorbe and mineralize heavy metals. The presence of certain types of nanoparticles was shown to increase the generation of methane, and by optimizing nanoparticle design methane generation could even be fostered. The increase even reached a 200% proportion, a much better improvement than that provided by any other existing alternative.


Biogas plants in Europe


Iron nanoparticles in digestion 

Iron is an essential trace nutrient for all known organisms, as well as one of the most abundant elements in the earth’s crust. However, iron availability is limited by the low solubility and the slow solution kinetics in mineral phases that contain iron, specially in neutral or alkaline environments (in terms of pH) such as carbonate soils, sea water and anaerobic digestors, where pH must be carefully controlled. In fact, bacteria, fungi and plants have developed complex systems of iron acquisition in order to increase mineral iron bioavailability in such environments.

The importance of iron minerals as nutrient sources for the acquisition of biologically active iron has been confirmed many times in microbial culture studies. In most soils, Fe oxides are the most common source of Fe for bacterial and plant nutrition. Given that Fe must be supplied in a soluble phase, solubility and dissolution rate of Fe oxides are essential for Fe supply. Hydrolysis constants and solubility products (Kps) are available for well-known Fe oxides present in soils, such as goethite, hematite and ferrihydrite. Nevertheless, Kps may be increased by several orders of magnitude for each kind of mineral when crystal size is reduced. Soluble Fe supply is regulated by dissolution rate as well as solubility. Fe oxides may be dissolved either by protonation, complexation or most often reduction (favoured in anaerobic environments) and Fe2+ is several orders of magnitude more soluble than Fe3+. Besides, organic anions, such as oxalate (which are adsorbed on the nanoparticle surface) may also weaken Fe3+-O bonds, thus increasing reductive dissolution and, as a consequence, be an iron supply for the biological reactions around the particle. Solubility at pH 7 is 0.1 M for Fe2+ and 10-18 M for Fe3+. 

In this way, an additive containing iron NPs -designed and functionalized so that they supply active iron ions at the required doses for bacteria in the anaerobic digestor- promotes biogas production up to 200% (60 days are required when cellulose is used as the starting material). Iron dissolution in the culture medium is an advantage, as it provides iron ions; besides, NPs ensure iron ions will be distributed homogeneously. Thus, NPs are used to maintain a given concentration in the digestor in a sustained manner. Such a sustained concentration effect prevents the concentration of iron ions being too high at inoculation, and it also prevents its concentration falling too fast. It should be noted that too high iron concentrations cause cytostatic effects at first, which are followed by cytotoxic effects later on; this means that high amounts of inactive iron that transform into active iron ions at the required doses for the metabolism of anaerobic bacteria are to be introduced, never to reach undesired maximum levels.

At the end of the process, the residue mass (digestate) is reduced due to the improved biogas production, and the additive breaks down into non-toxic iron salts, which results in a lower amount and a better quality of fertilizer after composting. In any case, iron NPs of the sort used here are well-known for its non-toxicity in pluricellular organisms within the application ranges.

Finally, working with an additive brings the advantage of the product being suitable for use in current digestors, either introduced simultaneously with biomass or previously (by spraying iron nanoparticles on biomass). As a matter of fact, technology is directed towards bacteria, not biomass itself, which means it may be applied to cellulose, dung, urban residues and other sources of potential material.


Prof. Dr. Víctor F. Puntes (ICREA Research



Funded by Bill Gates

The technology involved in the use of such metallic nanoparticles in biogas production by means of anaerobic digestion has been developed and patented by the team formed/led by the ICREA Professor Víctor Puntes at the Instituto Catalán de Nanociencia (ICN2, Catalan Institute for Nanoscience) and Dr. Antoni Sánchez at the Universidad Autónoma de Barcelona (UAB, Autonomous University of Barcelona). The project was initially funded by the former Ministerio de Medio Ambiente Rural y Marino (Ministry for Rural and Marine Environmental Issues); later on, by the Bill and Melinda Gates Foundation, and finally by the Secretaría General Iberoamericana (SEGIB; General Iberoamerican Secretary).

Recently, Biogas Plus was chosen by the Entrepreneurial Fund of Repsol Foundation (third call) because of the launching of a spin-off, Applied Nanoparticles, which is intended to transfer laboratory results to an industrial scale. Among the co-founders there are scientists in the aforementioned institutions, international experts in RRI (Responsible Research and Innovation), experts in e-communication and experts in business development and knowledge transfer. In this sense, Applied Nanoparticles is a nanoparticles engineering study that currently explores the use of iron oxide nanoparticles in applications such as the improvement of bacterial activity in anaerobic digestors. This is also open to other areas such as catalysis, environmental remediation, energy storage, drugs, imaging technologies as contrast, substance or hyperthermia agents and bacteriostatic and bactericide agents.