In the first part, this review has explained the environmentally benign products and processes that constitute the essence of green technology. In this part, we will conclude the review by focusing on three cardinal areas bearing on the challenge of green technology: Alternate Fuels, Green Buildings and Green Nanotechnology.

Green alternatives to fossil fuels

Energy is one of the grand challenges of the present century. Research focus worldwide has perceptibly shifted to renewable energy, and solar energy is unquestionably one of the most promising large-scale routes to this. Solar energy can be transformed into electricity through photovoltaic cells or solar power plants.

1. Research on photovoltaic (PV) cells addressed the need to make current technology cells cheaper and more efficient, as well as developing new technologies based on new solar cell architectural designs. It also delves into the development of new materials to serve as light absorbers and charge carriers. In this regard, one-dimensional semiconductor nanowires have received much attention.
2. There is also a joint effort to tap solar energy to produce solar fuels from carbon dioxide reduction (fuel products include carbon monoxide, formic acid, methanol and methane) and water splitting (fuel product is hydrogen). The chemical bonds of these fuels effectively capture solar energy.
3. Various technologies are known for post-combustion CO2 capture. Alstom-DOW leads the industry in carbon capture efficiency with their advanced amine process (~90 per cent with 99.5 per cent purity of CO2).
4. Numerous procedures for CO2 utilisation depend on energy-intensive processes, but recent efforts have been dedicated to catalytic CO2 usage under ambient reaction conditions. Two noteworthy results are:
   • the economic conversion of CO2 into valuable polycarbonates and polyesters by copolymerisation with epoxides; and
   • the use of simple molten carbonate salts to promote C-H carboxylation and CO2 hydrogenation.
5. An attractive process envisioned for the creation of a clean and affordable source of hydrogen is the photocatalytic splitting of water under solar light. Much effort here is directed to synthesising novel visible-light-active semiconductor photocatalysts by bandgap energy engineering and exploring the possibility of using organic pollutants or industrial wastes as the needed sacrificial reagents or electron donors.
6. Harnessing energy from renewable plant biomass indeed offers a potential carbon-neutral replacement for fossil fuels. This has underpinned the drive to develop new methods to engineer energy crops with the desired chemical composition and physical characteristics, depolymerise lignocellulose to fermentable units, and program microbial metabolism for efficient conversion of sugars to ethanol.

Presently, biocatalysis using genetically engineered fungal and bacterial microorganisms remains the most cost-effective pathway for obtaining biofuels from biomass. For example, genetically engineered fungi e.g. Trichoderma reesei is used to produce large volumes of cellulase, xylanase and hemicellulase enzymes. These enzymes then convert cellulosic biomass into fermentable sugars. As cellulose, in contrast to starchy feedstocks, contains large amounts of 5-carbon sugars, fermentation to cellulosic ethanol is best achieved using genetically engineered Escherichia coli bacterium.

1. Although not well established as of yet, microbial fuel cells (MFCs) have in recent years evoked interest as a green technology. MFCs use bacterial metabolism to produce an electric current from a wide range of organic substrates, including renewable biomass resources and domestic wastewater. Among the microbes used, the genetically-engineered Geobacter species have received prominence.

Green buildings

Green buildings have been shown to save on average between 30 – 40 per cent energy and carbon emissions every year, and between 20 – 30 per cent potable water annually, when compared to the industry norm.

1. A wide array of renewable and new materials now goes into the construction of green buildings, among them earthen materials, wood, bamboo, non-VOC paints, low-emissivity glass, and several new structural materials containing recycled components.
2. A more durable green concrete has been now added to the list. Obtained by adding water containing suspended graphene nanoparticles to the cement-sand-gravel mix, the new composite has 400 per cent decreased water permeability than traditional concrete and is also 146 per cent stronger. This process reduces the need for frequent replacement of structures with consequent savings in cement usage (the cement industry is responsible for approximately five per cent of global CO2 emissions).

Green nanotechnology in environmental sustainability

We end this brief survey by citing three examples of how nanotechnology can specifically aid in environmental sustainability efforts from the aspects of pollution monitoring and possible remediation.

1.Real-time monitoring of air, water and soil quality has become a necessity in our present-day world to detect and quantify polluting sources. Advances in solid-state sensors and nano-fabrication technology have led to the development of many intelligent detection systems for this purpose. Examples include gas leak detectors, fire and toxic gas detectors, breath alcohol detectors, and the like. Nanosensors in the farm have prevented excessive use of fertilisers and pesticides, thereby reducing both environmental contamination and production cost. Recent innovations have seen the development of nanotin oxide and carbon nanotube (CNT) sensors to detect variant kinds of gas; CNT functionalised with palladium can detect hydrogen leaks in hydrogen fuel cells of electric cars.

2.An interesting serendipitous finding is that a sponge of entwined potassium manganese oxide nanowires can soak up to 20 times its weight in oil, while rejecting water with its water-repellent coating. The sponge can be reused after heating. The possibility that it can be produced in large quantities augurs well for its use to mop up and recover oil spillages in waterways.

3.An international team of researchers has advanced a futuristic solution to reducing toxic metal levels in wastewater and oceans through their development of nanobots. These nanobots have a graphene exterior to absorb the heavy metal, a nickel core that enables control of the nanobots' movement via a magnetic field, and an inner platinum coating that powers the bots forward via a chemical reaction with hydrogen peroxide. The nanobots can be reused for further sweeps.

-- BERNAMA

Academician Professor Emeritus Dato' Dr V G Kumar Das has offered his views solely in his private capacity and they do not in any way represent the views of the Academy of Sciences Malaysia.