3.1 Removal of Inorganic Ions, TOC and TSS
The primary inorganic components of the untreated produced water include cations: Na (49.2 g/L), Ca (10.0 g/L), Mg (1.70 g/L), K (0.31 g/L), and Fe (0.56 g/L), and an anion: Cl (90.8 g/L). Na and Cl comprised about 92% by weight of the total inorganic ions determined by ICP-MS. These inorganic ions were effectively removed by filtration through the coal column (Fig. 1, Supplementary Table 1). Ten filtration-regeneration cycles were conducted and showed 60–80% removal of individual ions with an average removal of 73%. Interestingly, the percent overall removal of all ions was similar, ranging between 64% and 76%, suggesting there was no specific removal selectivity of these ions. An average of about 0.12 g of TDS was removed per g coal for each filtration-regeneration cycle, although there is residual water in the pores of the packed column after washing. There may be dilution effects from the residual water that need further investigation.
The TOC concentration of the original produced water was 42 mg/L. During filtration through the coal column, 71–92% of the TOC was removed. The removal efficiency of the initial cycle (Cycle-1) was the lowest (71%), whereas the mean value of the remaining 9 cycles was 90%. The removal capacity of organic components was significantly improved after coal regenerations. The TSS was decreased from 233 mg/L in the raw produced water to a negligible level (below detection) in the filtrate.
Figure 2 shows SEM and EDS images of the coal samples directly after filtration and also after regeneration with the acid wash. The EDS analysis (Fig. 2b) clearly shows adsorption/deposition of major inorganic ions on the surface of coal particles. After regeneration with the acid wash, most of the cations were removed (Fig. 2d), thus confirming how the treatment cycles worked. Ions were retained in the coal through filtration and subsequently removed by acid wash, thereby regenerating the coal medium for further filtration cycles. After regeneration with the acid wash, Cl dominated the surface of coal (Fig. 2d) because diluted HCl solution was used as the washing agent.
3.2 Results of 13C NMR analysis
The raw coal, washed coal before filtration (Before filtration), and coal after filtration (After Filtration) were subjected to NMR analysis to identify functional groups and investigate the changes of these functional groups. As shown in Fig. 3, many oxygen-containing functional groups, including alcohol, carboxyl, carbonyl, phenol, ester, and ether were identified in all samples with different intensity (Kim et al. 2013). Other non-oxygen-containing functional groups including aliphatic and aromatic C-H groups such as methyl, methylene, and methyne were also identified. This is in line with other studies suggesting low rank coal contains hydroxyl-, methoxy-, and/or methyl-substituted benzene rings, carboxylic acids, and aliphatic linkers line –CH2CH2– group (Liu et al. 2013; Liu et al. 2019).
In general, the intensity of the three samples decreased in the order Before filtration > raw coal > After filtration. Before filtration coal was raw coal that had been washed in preparation for use in the column. The washing process removed ions within the coal. In contrast, the After filtration sample contained ions that were removed from the produced water and were bound/adsorbed onto the coal. Because the electron distribution of 13C can be affected by factors such as binding partners, the binding of ions can significantly reduce the response in NMR analysis. This may explain the intensity differences among coal samples of before filtration, raw and after filtration. In particular, the differences in intensity of the phenolic and carboxylic functional groups were prominent, suggesting their involvement in the removal of ions from the produced water.
3.3 Effects of Solvent Extraction on Coal and Solvents
To investigate the mechanisms of ion removal by coal filtration, PRB coal was solvent-extracted with tetrahydrofuran (THF), methanol, or N-methyl-2-pyrrodidone (NMP). The total ion removal capacity of the coal (as determined from a filtration cycle without subsequent acid washing) was significantly decreased by solvent extraction, to 26%, 23%, and 16% removal for THF, methanol, and NMP extractions, respectively.
Solid coal, both before and after extraction, and liquid solvent after extraction were examined by FTIR analysis. Although there are difficulties in using FTIR to analyze heterogeneous materials like coal with respect to sample preparation, band assignments, and baseline correction (Solomon and Carangelo 1982), a number of functional groups and changes due to solvent extraction were identified in this study. Figure 4 shows the FTIR spectra of coal and liquid from solvent extraction. The peaks were identified according to the literature (Sigma-Aldrich ; Xie 2015). The coal structure was modified by solvent extraction to different extents (Fig. 4a). Specifically, coal extracted with NMP exhibited the greatest structural changes with respect to functional groups, followed by methanol and THF. This is consistent with the ion removal capacity where NMP extraction showed the greatest decrease in capacity, followed by methanol and then THF which showed the smallest decrease in ion removal capacity.
The liquid solvents after extraction contained compounds with oxygen- and nitrogen-containing functional groups (Fig. 4b), consistent with the changes in the solid coal functional groups. The intensity of N-H stretching for solvent-extracted coal was significantly reduced, whereas peaks of N-H stretching existed in all the liquid extracts. The intensity of the N-H peaks of the coal followed the same tendency of ion removal capacity, with less ion removal capacity corresponding to decreased intensity of the N-H peaks. This was also true for other functional groups such carboxyl and phenolic-hydroxyl groups, suggesting that these functional groups may be involved in ion removal. These results are most evident in the NMP treatment where the intensity of these peaks in the extracted coal were significantly reduced while the NMP after extraction had a strong presence of these peaks. NMP has been shown to facilitate the extraction of hydroxyl-containing moieties from bituminous coal (Sun et al. 2014). These results indicate that the extraction of these functional groups significantly impaired the ion removal capacity of the coal, largely related the removal capacity to the number of these oxygen- and nitrogen-containing moieties.
3.4 Composition of coal extracts
Volatile and small-molecular compounds in the extracts were characterized by GC/MS to investigate the impact of extractable components on the ion removal. As shown in Fig. 5, the group components detected in the extracts mainly included alkanes, alkenes, arenes, alcohols, phenols, ketones, carboxylic acids, and esters. The relative abundances of oxygen-containing compounds (i.e., alcohols, phenols, ketones, carboxylic acids, and esters) compared to all extracted compounds were 47.2%, 86.4%, and 82.0% for THF, methanol, and NMP extracts, respectively. Oxygen-containing functional groups, especially carboxyl and hydroxyl, in these compounds may interact with metal cations to form cation-bridging linkages (Liu et al. 2016; Mathews and Chaffee 2012), which are beneficial for ion removal. Extraction of these oxygen-containing compounds appears to have significantly reduced the ion removal capacity of the coal.
The detected compounds in the methanol and NMP extracts were dominated by esters with 76.6% and 52.9% relative abundance, respectively (Fig. 5), while the hydroxyl-containing compounds (like carboxylic acids, alcohols, and phenols) had a relatively low abundance. As polar solvents, methanol and NMP have proven to be effective for extracting hydroxyl-containing compounds from coals (Liu et al. 2016; Sun et al. 2014). Because GC/MS is only sensitive for volatile and less polar compounds, the GC/MS detectable compounds account for only a fraction of the compounds in the extracts. According to FTIR analysis, NMP extract may also contain many hydroxyl-containing compounds, especially carboxylic acids with low carbon numbers, which were not detected by GC/MS.
4. Proposed Mechanisms Of Ion Removal
The structure of a coal macromolecule may be visualized as a condensed aromatic carbon-atom lattice surrounded by a typical “fringe” formed by functional side groups. The left panel of Fig. 6 is a hypothetical model of coal structure (Malumbazo 2011). It is a heterogeneous mixture composed of a macromolecule network with varying degrees of cross-linking (Smith et al. 2013). Modified lignin, as well as cellulose and melanoidin-type materials, are considered to be the ‘backbone’ of this macromolecule network. The cross-linkage of lower rank coal, including subbituminous coal, is dominated by alkyl and aryl ether groups with oxygen functional groups. The chemical heterogeneity of coal decreases from low rank coal to high rank while the aromaticity increases, suggesting that lower rank coals (lignite and subbituminous) have more complex chemical structures than high-rank coals (bituminous coal and anthracite) because the low rank coal contains several distinct classes of constituents (Hofrichter and Fakoussa 2001; Wang et al. 2015). Carboxyl and hydroxyl groups, among others, are the main oxygen-containing functional groups in coal structure which are present in low-rank coal (Xie 2015). Phenolic hydroxyl groups are the main form, but some alcoholic hydroxyl groups also exist.
The removal of cations from produced water by coal filtration is proposed to be through ion exchange (Fig. 6a). The carboxyl and hydroxyl groups may act as ion exchanger sites by exchanging protons (H+) for other cations (Na+, K+, Ca2+, Mg2+, etc.) allowing these cations to bind to the negatively charged hydroxyl and carboxyl functional groups. This hypothesis is supported by the pH measurements of the raw produced water before filtration (6.83) and the treated water (as low as 2.28), indicating that protons were released by the coal, presumably due to cation exchange. Additionally, NMR analysis of before and after filtration coal samples also showed significant intensity reduction of carboxyl and hydroxyl groups, further supporting the cation exchange hypothesis. The solvent extraction results provide addition support for the hypothesis. Solvent extraction removed carboxyl and hydroxyl functional groups, as illustrated by the FTIR analyses where the functional groups were removed from the coal and present in the extracts, resulting in significantly impaired ion removal capacity.
Proposed mechanisms for Cl− removal are less clear. One possible removal mechanism is adsorption of Cl− due to the delocalization of electrons within molecules to form resonance structures (Fig. 6b). This resonance effect or mesomeric effect or electron-donating effect occurs between a lone pair of electrons and a pi bond or two pi bonds next to each other (Dewick 2006). For example, the lone electron pair in the oxygen of the phenolic hydroxyl group may be donated to form a double bond and leaving a positive charge. The donation stabilizes the structure of the non-ionized acid such as phenols or derivatives. The donating effect passes the electrons on to the pi bonds along the ring to produce a negative charge of the para- or ortho-carbon in the same aryl ring. These charged molecules could then bond with both negative and positive ions in the produced water to remove them. The resonance effect could be positive or negative. In the positive resonance effect, -OH, -SH, -OR, and -SR can increase the electron density of the stabilizing ring while -NO2, -S = O, and -C = O could decrease the electron density of the stabilizing ring in the negative resonance effect (Dewick 2006). The removal of these functional groups, as occurred with solvent extraction, would reduce the ion removal capacity of coal.
Alternatively, the ion removal of any particular ion may not be attributed to any single functional group. Other possible mechanisms to produce charges in coal include changes of electron density by binding alkali metal ions (which are dominant species in produced water) with the functional groups in aromatic structures and electrostatic induction (Xie 2015). In recent decades, ion-π interactions have been recognized and found to be widely exist as a form of general noncovalent bonding (Dougherty 1996; Ma and Dougherty 1997; Schottel et al. 2008). Ion-π interactions happen not only in aromatic systems, but are also well documented in other simple π systems such as ethylene and acetylene. Studies show that highly solvated cations can be sequestered by such binding force in aromatic-containing structures (Ma and Dougherty 1997), while anion-π interactions happen in electron deficient aromatic systems (Schottel et al. 2008). The anion-π interaction combines effects of electrostatic and anion-induced polarization, with the former correlated to permanent quadruple moment, Qzz and the latter to molecular polarizability (Quiñonero et al. 2004; Schottel et al. 2008). Aromatic molecules with lower absolute values of Qzz could bind to both anion and cation which might account for some of the ion removal in our system (Schottel et al. 2008). The ion-π interaction binding energy is estimated to be 20–50 kJ/mol which is energetically favorable and is comparable to the binding energy of hydrogen bonds. In the ion-π theory, the inductive effect, rather than the resonance effect, facilitates binding(Ma and Dougherty 1997; Schottel et al. 2008). Although the mechanisms of anion removal remain hypothetical, based on the results presented herein, coal may be considered a “pseudo-amphoteric” exchanger that has the capability of removing both cations and anions simultaneously and effectively.