Utilization of Phytoremediation ByProducts

Phytoextraction involves growing plants in heavy metal contaminated soil until the metal concentration declines to tolerable levels. The metal removed can be determined by measuring the metal concentration in the plant, multiplying it by the biomass produced, and comparing this with the reduction in the metal concentration in the soil. In commercial phytoextraction, the next step is to dispose of the contaminated plant material. After harvesting, the plant is removed from the field, and this leads to the accumulation of a large amount of hazardous biomass. This hazardous biomass should be stored or disposed of properly so that it does not create any risk to the ecosystem. Biomass is solar energy stored as plant mass; it is also considered to be the combustible organic matter from the plant. It contains C, H, and O, the elements found in oxygenated hydrocarbons. Biomass (especially wood) can be represented by the chemical formula CH144O066 (Iyer et al. 2002); its main constituents are lignin, hemicellulose, cellulose, minerals, and ash. It contains high levels of moisture and volatile matter, has a low bulk density, and calorific value. The proportions of these constituents vary from species to species. The dry weight of Brassica juncea for induced phytoextraction of Pb amounts to 6 tonnes/hectare, with 10,000-15,000 mg kg-1 of metal in dry weight (Blaylock et al. 1997). This huge amount of waste is a problem and thus the volume must be reduced (Blaylock and Huang 2000).

Composting and compaction has been proposed as a postharvest biomass treatment by some researchers (Raskin et al. 1997; Kumar et al. 1995; Garbisu and Alkorta 2001). Leaching tests for the composted material showed that soluble organic compounds enhanced metal (Pb) solubility (Hetland et al. 2001). Reduction in dry weight of contaminated plant biomass is advantageous, as it will lower the cost of transportation (Blaylock and Huang 2000). One very promising way to utilize the biomass produced by phytoremediation in an integrated manner is to use a thermochemical conversion process. If phytoextraction could be combined with biomass generation and commercially utilized as an energy source, then it can be turned into a profit-making operation, and the remaining ash could be used as bio-ore (Brooks et al. 1998). This is the basic principle of phytomining. Nicks and Chambers (1994) reported a second potential use for hyperaccumulator plants for economic gain in the mining industry. This operation, termed phytomining, includes the generation of revenue by extracting saleable heavy metals produced by the plant biomass ash, also known as bio-ore. Combustion and gasification are the most important routes to the organized generation of electrical and thermal energy. Recovery of the energy in biomass by burning or gasification could help make phytoextraction more cost-effective. Thermochemical energy conversion best suits the phytoextraction biomass residue because it cannot be utilized in any other way as fodder and fertilizers. Combustion is a crude method of burning the biomass, but it can be done under controlled conditions, allowing the volume of biomass to be reduced to 2-5% of the original mass, and then the ash can be disposed of properly (Raskin et al. 1997; Bridgwater et al. 1999). It is not wise to burn the metal-bearing hazardous waste in the open, as the gases and particulates released into the environment may be detrimental; also, while the volume is reduced, the heat produced in the process is wasted using this approach.

Gasification is the process by which biomass material can be subjected to a series of chemical changes to yield clean and combustible gas at high thermal efficiencies. This mixture of gases is termed producer gas and/or pyro gas, and it can be combusted to generate thermal and electrical energy. The gasification of biomass in a gasifier is a complex process; it involves drying, heating, thermal decomposition (pyrolysis) and gasification, as well as combustion chemical reactions that occur simultaneously (Iyer et al. 2002).

Hetland et al. (2001) reported the possibility of co-firing plant biomass with coal. Results suggested that ashing reduced the mass of lead-contaminated plant material by over 90% and partitioned lead into ash. It may be possible to recycle the metal residue from the ash, but so far the cost or feasibility of such a process has not been estimated (Raskin et al. 1997). Future experiments should concentrate on the development of a combustion system and methods to recycle different metals from the ash. The process destroys organic matter, releasing metals as oxides. The liberated metals remain in the slag, and modern flue gas cleaning technology ensures effective capture of the metal-containing dust. Considering the other disposal technologies, this method is environmentally friendly.

Bridgwater et al. (1999) reported that pyrolysis is a novel municipal waste treatment technique that could also be used for contaminated plant material. Pyrolysis decomposes material under anaerobic conditions; there is no emission to the air. The final products are pyrolytic fluid oil and coke; heavy metals remain in the coke, which could be used in a smelter. Koppolua et al. (2003) reported that 99% of the metal recovered in the product stream was concentrated in the char formed by pyrolyzing the synthetic hyperaccumulator biomass used in the pilot scale reactor. The metal component was concentrated 3.2- to 6-fold in the char compared to the feed. The fates of the metals in various feeds during pyrolysis have been studied and addressed in the literature in different contexts, but results on the pyrolysis of phytoextraction plant biomass are limited. Helsen et al. (1997) conducted low-temperature pyrolysis experiments with chromated copper arsenate-treated wood, and it was concluded that most of the metal was retained in the pyrolysis residue. The influence of metal ions on the pyrolysis of wood has been studied extensively by many researchers (Pan and Richards 1990; Richards and Zheng 1991). The high cost of installation and operation could be limiting factors on such a treatment if used solely for plant disposal. To avoid this, the plant material could be processed in existing facilities together with municipal waste.

Singh and Ghosh (2003) worked on high-biomass species, as they showed positive results in screening (germination) studies. The schematic shown in Fig. 19.6 describes the work performed by these authors into relation to phytoextraction. Their results showed that more Cd, Cr, and Pb was phytoextracted by Ipomoea carnea, Datura innoxia, and Phragmytes karka than by Brassica juncea and Brassica campestris (known to be indicator species) (Henry 2000; Ghosh and Singh 2005). The study, conducted with 10-200 mg kg-1 of Cd, Cr, and Pb (separately), indicated that I. carnea was more effective at extracting these species from soil than B. juncea. Among the five species, B. juncea accumulated maximum Cd but I. carnea followed by D. innoxia and P. karka were the most suitable species for phytoextracting cadmium, provided the whole plant or aboveground biomass was harvested. In a relatively short time, I. carnea produced more than five times more biomass than B. juncea. It was more effective at translocating Cr from soil to plant shoots. P. karka showed a much greater tolerance of chromium than other plants, though its uptake was low. Ipomoea extracted the most lead at 200 mg kg-1; Datura and Phragmytes were the best extractors at 100 mg kg-1, whereas the Brassica species were the best at 50 mg Pb kg-1 soil. Brassica species were difficult to cultivate, as they required pesticides to protect them from army moths, and they also cannot grow throughout the year. High-biomass species do not have these limitations and show higher potential, and their extraction capacities can be increased through the use of chelators or soil additives.

Continue reading here: Genetic Engineering to Improve Phytoremediation

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