Economic Viability: Desorption vs Disposal

After adsorption, the adsorbent can be desorbed and recycled until it favorably keeps the pollutant concentration in eluent within the permissible limit set up by the regulatory agencies. After this, the spent adsorbent can be repurpose for alternative uses like catalyst, production of ceramic, and use for removal of alternate contaminant or can be disposed.

The desorption of contaminants can be done by alkali or acid reagent, chelating agent, and salt (Vakili et al. 2019), and in the case of organic pollutants, by thermal, chemical, microwave, and other methods (San Miguel et al. 2001; Wang et al. 2010; Guilane and Hamdaoui 2016). Lata et al. (2015) observed that alkali was most efficient for removal of heavy metals from chemical-based adsorbent (Table 8.1).

The use of acid, alkali, chelating compound, or chemical as a desorbing agent is associated with the generation of waste (secondary pollution) in eluent laced with contaminants. So, it faces the same problem in disposal as the spent adsorbent due to environmental and economic reasons. However, there are some instances in which the metal laced with heavy metal can be recovered such as Cr with BaCl, (Zelmanov and Semiat 2011), Hg from EDTA-Hg complex as HgCl, or HgSO₄ (Jeon and Park

TABLE 8.1

Desirable Desorbing Agents for Various Adsorbents

References

Lata et al. (2015)

Lata et al. (2015)

Lata et al. (2015)

2005), and palladium as palladium chloride (Boricha et al. 2007). The summarized processes are as follows.

The adsorbent, i.e. Fe(III) oxide/hydroxide nanoparticle-based agglomerates separated by filtration followed by desorption with NaOH in the pH range of 9-10 (Zelmanov and Semiat 2011). The concentrated Cr solution was treated with BaCl₂, which led to the production of BaCrO₄ crystals, and the crystals were removed by filtration with a 0.45-pm filter paper. BaCrO₄ produced has a larger market value than BaCl₂. Similarly, mercury desorption from novel aminated chitosan bead with EDTA was separated as solid EDTA and metal (mercury) chloride or sulfate by using HC1/ H₂SO₄ (Jeon and Park 2005).

In another case, palladium (Boricha et al. 2007) adsorbed on the silica gel as palladium phthalocyanine was first thermally calcined in air to partially burn the organic moiety of the complex. Afterwards, 2M HC1 was added to calcined silica so that palladium dissolution takes place as H₂PdCl₄. The palladium was recovered as PdCl, from the filtrate (solution containing H₂PdCl₄) by adjusting it’s pH to 6 with the help of 0.1-0.5 M NaOH.

In addition, there are some examples of recycling available from the petroleum industry, battery sector, and metallurgical sector where metals can be reclaimed after their use. The spent catalyst can be recovered by hydrometallurgical methods (Akcil et al. 2015), water leaching (Kar et al. 2005; Biswas et al. 1985; Zeng and Cheng 2009; Le and Lee 2020), bioleaching (Yu et al. 2020), precipitation method (Wang and Chen 2019; Paudyal et al. 2020), and vacuum heat decomposition (Liu et al. 2018). Some examples from petroleum industries are as follows.

CoMo/A1₂O₃ was treated in sulfuric acid environment under solvothermal conditions followed by treatment with elemental sulfur or H₂S. This led to the precipitation of molybdenum and can be solidified by oxidation to molybdic acid and cobalt sulfides can be converted to cobalt sulfate followed by its extraction via ion exchange. In this way, separation and extraction of Al, Co, and Mo were successfully carried out by Hyatt (Hyatt 1987; Akcil et al. 2015).

The process of recovery of metals similar to the nutrient cycle can be achieved by biohydrometallurgical methods. The bacteria and fungi used for the metal solubilization in biohydrometallurgical methods are positively enhanced by the generation of the acidic media or oxidizing media during their growth in the medium (Akcil et al. 2015). Bioleaching can be achieved by Acidithiobacillus thiooxidans, Acidithiobacillus ferrooxidans (Yu et al. 2020), or Aspergillus nomius (Liu et al. 2018). The recent rise of LED (light emitting diode) has led to increased gallium, copper, and nickel waste in recent times. These metals can be bioleached with the help of Acidithiobacillus ferrooxidans (Pourhossein and Mousavi 2018). The bioleaching process has the advantages of low energy requirement and low operation and maintenance costs. However, the longer time period required for operation and the dependency on atmospheric conditions are certain limitations.

To overcome these, there are other methods available such as the water leaching method for extraction of vanadium and molybdenum with the use of temperature, salt, and water only (Biswas et al. 1985; Zeng and Cheng 2009; Kar et al. 2005; Le and Lee 2020), and the use of organic acids such as gluconic acid and lactic acid for leaching (Roshanfar et al. 2019) and oxalic acid for precipitation (Verma et al. 2019).

In the oxalic acid leaching method for battery waste (LiCoO,), the leaching efficiency of greater than 99% and 96% for Li and Co can be achieved, respectively (Verma et al. 2019; Sohn et al. 2006). The lithium ion can be leached into aqueous phase and cobalt can be precipitated out as cobalt oxalate in this method.

In addition to the chemical, biological, and solvothermal methods, there are electrochemical methods, which are costly but more environmentally friendly. One example is the removal of chromium with a polyaniline-resin composite (resin is a porous polymer XAD-4 adsorbent). The recovery of chromium achieved is nearly 62.9% in the first cycle, followed by 91.63% and 90.33% in the second and third cycles, respectively (Ding et al. 2020). In this case, there is negligible generation of secondary waste as compared to the other methods. Some examples of desirable methods and chemicals for the recovery of metals and anions are presented in Table 8.2.

Industries which have expertise in removing metal from batteries and used catalyst are operational at the moment, for example, Eramet, Treibacher Industrie AG, Moxba-Metrex, GS EcoMetal Co. Ltd., Taiyo Koko, Full Yield Industry Co. Ltd. (Akcil et al. 2015), and Tata Chemicals. The spent adsorbents can be sent to industries, thus avoiding the need for additional separate infrastructure.

The success of the adsorption process for upliftment in standard of living depends on the localized conditions. It also depends on the performance cost and appropriateness (Lata et al. 2015), so in countries with low income, it is not possible to use the chemical adsorbents, and thus, bio-sorbents are preferred in those countries. In the case of biosorption of heavy metal, it is best suggested to look for its conversion to biochar and, if possible, use it for the adsorption of alternate pollutant, and after its use, apply it on the soil to improve the soil fertility. However, we should be aware of the limitation regarding the maximum permissible limit of heavy metals or pollutants by the environmental regulatory agency of the region.

So, the applicability of the adsorbent and desorption is based on the economics of the area and the rarity of the material to be used for adsorption. The another reason to avoid chemical adsorbent in countries with low income is that precursor for the chemical adsorbents have to be extracted it from its ore by mining. The mining industry is already marred by environmental damage to biodiversity, and open cast mining is the most preferable mode of mining, which has a severe impact on the local ecosystem.

Figure 8.3 suggests that, if the cost of replacing the adsorbent is high {(cost of fresh adsorbent-i-economic loss to biodiversity during mining of material for fresh adsorbent)-(economic cost of desorption and extraction-(-economic value of end product)} and the adsorbent post adsorption cannot find alternative uses, then it is better to desorb the adsorbent for reuse and treat the effluent or eluent (for metal extraction) generated during desorption using various processes. The process can be followed by precipitation of the metal extracted of metal in the spent adsorbent. The spent chemical adsorbent serves as a source of raw material for the production of goods (Federal Ministry for the Environment 2016). This helps in achieving the circular economy.

If extraction is not possible, then it is suitable to dispose the pollutant by adding binders (such as cement) before its final disposal. Arsenic-laden waste disposal is likely carried out by stabilization of solidification, followed by disposal of treated waste. In the absence of proper guidelines, the adsorptive filter media and regenerative waste are dumped into the small brick-lined pits (Mondal and Garg 2017; Sullivan et al. 2010; Ali et al. 2003). The pits need to be tested for TCLP (toxicity characteristic leaching procedure). A column leaching test on the arsenic waste was conducted, and the results showed that leaching was nonsignificant and nonhazard-ous as per the guidelines of the USEPA (Ali et al. 2003).

The adsorbent can also be made up of Mg and Ca (in addition to adsorbent based on carbon) to make the adsorption process sustainable. These elements (Mg and Ca) can serve as nutrients in the soil after its use as adsorbent. So, focus must be on an efficient process and adsorbent must be made up of a material with large abundance in nature. In the case of spent adsorbent that is applicable in alternative uses such as soil fertilizer and abundant to replace, it is economically and environmentally friendly to put it to alternative use rather than desorption.