claimed to reduce the error to less than 3% with this arrangement of AI tools. It is found that ANN‐PSO and ANN‐GA models have proven to be a perfect choice for demonstrating and enhancing Se (IV) removal by the adsorption and reduction apparatuses.
The authors also studied the Publication Agencies mapping for all the most influential research papers discussed here. Figure 1.4 shows Representative Highest Influential Publications for selenium removal technological published worldwide. It can be clearly seen that the highest number of highly cited research is published by Springer, making up 25% of all publications. The next 20% of contributions comes from Elsevier Science BV in Amsterdam. The other well‐regarded publications range from 1 to 10% of total contributions.
The major objective of this section is to present a modality that helps researchers to visualize “outlines” in the said research field to “perceive effects” that may then be unexplored, by recognizing “gaps” in the research field and “limitations” to issues under examination. The authors emphasize that the fundamental outcome of this study is identifying probable innovative areas of study and the constraints.
To add to the discussion discussed so far, some technologies indicating comprehensive treatments, such as desalination and brine management solutions, are observed. A robust ultra‐high recovery reverse osmosis system has been proposed by several researchers for distant commercial set‐ups enabling reverse osmosis to confiscate scaling ions and thereby causing maximum recovery and smooth operations. There are suggestions that low‐slung temperature evaporator‐crystallizers be used. Analysis results indicate that countries are using different treatment technologies, including reduction techniques, phytoremediation, bioremediation, coagulation‐flocculation, electro‐coagulation (EC), electrochemical methods, adsorption, co‐precipitation, electro kinetics, membrane technology, and chemical precipitation. However, the order of use of particular methods varies from country to country.
Figure 1.4 Representative most influential publications for selenium removal technology published worldwide.
Word dynamics analysis was carried out by key words, title, and abstract on the papers published by Asian countries. India and China emerged as the top five Asian countries. The pattern of word dynamics indicated that Se removal using phytoremediation is emerging as main technology, followed by bioremediation and UASB reactor. Adsorption, reduction, and sorption emerge as the dominant methods for Se removal technology in China.
1.2 Selenium Reduction Technologies Used in India
Phytoremediation is a technology in which selenium get accumulated in plant parts and then plants can be harvested and incinerated. Phytovolatilization, which is also a type of phytoremediation, is more effective, in which a few plant species which can tolerate selenium can be grown on selenium‐contaminated sites and then can volatilize less toxic forms or non‐toxic forms of it in the environment (Dhillon and Dhillon 2003). This technique efficiently remediates soils contaminated with selenium as well as being an eco‐friendly technology and using fewer resources.
In Punjab, mustard is found to be the best accumulator of selenium. It is may be because of the high concentration of sulfur in mustard and the resemblance of selenium to sulfur. In addition to mustard, other plants, including onion, garlic, broccoli, Brazilian nuts, and mushroom, also show a strong affinity to Se (Sharma et al. 2015). In soil systems, many remediation methods can be applied for selenium management, such as gypsum application in selenium‐contaminated farms, which has been found to be efficient in its reduction levels (Dhillon and Dhillon 2000). Application of organic matter such as press mud and poultry manure has shown to reduce Se levels in rice, wheat, and maize up to 97% in Se‐contaminated farms (Dhillon et al. 2010). In situ bioremediation can be achieved by bacterial cultures belonging to b‐Proteobacteria and Bacilli class, which are fairly selenium‐tolerant microorganisms (Ghosh et al. 2008; Prakash et al. 2010).
Figure 1.5 Upper panel: word dynamics for India. Lower panel: word dynamics for China.
Biofortification, in which harvested plant parts are decomposed in agricultural soil which can be used further for the enrichment of food products with Se (Bañuelos et al. 2015), is another method.
Among the other technologies being used for the remediation of selenium‐contaminated waters are: ion exchange, reverse osmosis, nanofiltration, solar ponds, chemical reduction with iron, microalgal–bacterial treatment, alumina adsorption, Fe+3 coagulation/filtration, lime softening, and ferrihydrite adsorption (El‐Shafey 2007; Luo et al. 2008). Use of waste wheat bran can be an eco‐friendly technology in a continuous up‐flow fixed‐bed column system for biosorption of selenium species in aqueous solution (Hasan et al. 2010). One of the efficient methods of selenium removal from waste water can be use of double‐layered synthetic hydroxide materials (Zn/Al, Mg/Al, and Zn/Fe) as an adsorbent (Mandal et al. 2009).
Another technology demonstrates photoreductive removal of selenium (IV) using spherical binary oxide photo catalysts under visible light. As a range of scavengers, EDTA (ethylene diamine tetra acetic acid) and formic acid are found to be the most suitable for the reduction reaction, and of these two, formic acid is found best for reduction of selenium – the catalyst used for the process is TiO2, which is non‐corrosive, non‐toxic, and has high photoactivity, high photostability, and an economical nature. It has been reported that catalyst can be used repeatedly at least five times with marginal change in the activity (Aman et al. 2011).
1.3 Selenium Reduction Technologies Used in China
Nanoscale zero‐valent iron (nZVI) is the most widely applied nanomaterial as an adsorbent for groundwater and hazardous waste treatment. It is also effective for selenium treatment and removal. Batch experiments have been conducted and show that nano‐ZVI has approximately a removal rate three times or higher than those of micro‐scale iron, nanoscale iron oxides, Fe(OH)3, nanoscale TiO2, and activated alumina for selenium removal (Ling et al. 2015). ZVI or nano‐ZVI is effective for SeVI removal from wastewater by reducing to more adsorptive SeIV and/or to insoluble Se species (i.e. Se0, SeI, and SeII). A key role is known to be played by dissolved Fe2+ in the reductive removal of selenate by ZVI. There are two major roles for Fe2+: (i) it participates in selenate reduction directly as partial electron donor with a Fe2+:Se stoichiometry of ~1:1, and (ii) helping in the transformation of the passive layer on iron grain and corrosion products to magnetite, favoring electron transfer and thus enhancing selenate removal. ZVI was the main electron donor. In a ZVI‐SeVI‐Fe2+ system, sequential reduction of selenate is reported where elemental Se was the dominant reductive product. Selenate reduction by ZVI assisted by Fe2+ has been identified as a sustainable treatment method for wastewater contaminated with selenate (Tang et al. 2014).
It was observed in the trials that the removal efficiency of Se (VI) by ZVI was only 4.8% within 120 minutes, although a much higher removal efficiency (62.1%) was obtained by nZVI. Owing to its smaller particle size (approximately 100 nm), nZVI has a higher capacity for removal of Se (VI) than ZVI (approximately 150 m). The smaller size of the iron particles suggests a greater surface area that gives more stable Se (VI) reaction sites. In comparison, the removal efficiency of Se (VI) by nZVI/Al‐bent was 95.7%, which was far higher than that of nZVI (62.1%). It suggests that there is a synergistic impact
