Sorption of trace elements on xylain: An experimental study

International Journal of Coal Geology 150–151 (2015) 166–169

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Sorption of trace elements on xylain: An experimental study Eskenazy Greta Sofia University “St. Kliment Ohridski”, 1504 Sofia, Bulgaria

a r t i c l e

i n f o

Article history: Received 20 May 2015 Received in revised form 24 August 2015 Accepted 25 August 2015 Available online 28 August 2015 Keywords: Sorption Xylain Trace elements

a b s t r a c t Sorption and desorption of Sr, Mn, Co, Ni, Cu, and Zn on xylain from lignites of the Mariza-East coal basin (Bulgaria) were studied. Sorption behaviour for all the studied elements was found to be of similar pH dependence for all elements. The maximum quantities bound vary from 0.104 meq (Sr) to 0.167 meq for Cu. On treatment with a complex-forming agent such as tartaric acid, a high degree of extraction was achieved. However, appreciable amounts of elements remain tightly bound on the sorbent. When present simultaneously in solution, the quantity of every element sorbed, excluding Cu, is lower as compared with the quantities from the experiments for single cations. The cation-exchange mechanism of the sorption and formation of covalent links was determined. Such processes may be responsible for part of the organically-associated elements in coals in case they are influenced by solutions enriched in some trace elements. © 2015 Elsevier B.V. All rights reserved.

1. Introduction It is common knowledge that trace elements in coals are both inorganically (the greatest part of them) and organically associated (Dai et al., 2015; Finkelman, 1993; Liu et al., 2015; Riley et al., 2012; Swaine, 1990; Yudovich et al., 1985). For many years experimental studies tried to elucidate the elements' type of binding with the organic matter (OM) of coals. Experiments of sorption on humic and fulvic acids and on peat revealed their capacity to bind elements (Manskaya and Drozdova, 1964; Ong and Swanson, 1966; Szalay and Szilagyi, 1967; Rashid, 1974; Tipping and Hurley, 1992; Tipping, 2002.). In order to elucidate the probable mechanism of accumulation of trace elements on humic acids, peat, vitrain, and xylain, a series of experiments on adsorption of Ga, Be, Ti, In, and REE were performed by Eskenazy (1967, 1970, 1972, 1979, 1999). This study aims to compare the sorption on xylain of several divalent elements: lithophile (Sr), siderophile (Mn, Co, Ni) and sulphophile (Cu, Zn) from: a) solutions containing only a single cation, and b) solution containing all cations (Mn, Cu, Zn, Co, Ni, and Sr). 2. Material and methods 2.1. Sorbent characterization Xylain with 3.7% ash yield, (air-dried and ground to 100 μm) from the Mariza-East coalfield lignite (Bulgaria) was used as a sorbent. Its elemental composition, on a dry and ash free basis, contains: 63.4% C, 6.2% H, 28.0% O, and 0.7% N. The content of OH− and COOH+ groups is 2.15 meq/g and 2.33 meq/g, respectively. Its specific surface area is 12.0 m2/g, determined by the Brunauer Emmett–Teller method (BET). The sorbent was analysed for 39 trace elements by inductively coupled

http://dx.doi.org/10.1016/j.coal.2015.08.013 0166-5162/© 2015 Elsevier B.V. All rights reserved.

plasma atomic emission spectroscopy (ICP-AES). The wet chemical analysis showed that major elements in the xylain sample include 3.54% S, 0.58% Ca, 0.63% Fe, 0.15% Mg, and 0.07% Si (on ash basis, 500 °C ashing temperature). Electron microprobe analysis showed that these elements are evenly distributed in the organic matter. A small quantity of quartz, corresponding to the silica content was detected by X-ray powder diffraction. 2.2. Experimental The following experiments were carried out: (1) Effect of pH on sorption at constant initial concentration was studied over a pH range of 2–6 (Fig. 1). (2) Construction of sorption isotherms (Table 1 and Fig. 2). Aqueous solutions for every element were prepared by respective p.a. salts. Aliquots of these stock solutions were evaporated to dryness, dissolved in 50 ml water and adjusted to pH 5.5. Portions of 0.3 g xylain were added to every solution and occasionally stirred at room temperature for seven days. The sorbents were filtered and washed with distilled water up to 100 ml. The concentrations of Cu, Zn, Sr, Ni, Co, and Mn in the filtrates were measured by atomic absorption spectroscopy (AAS). The amount of the element sorbed was calculated by the difference between the initial and equilibrium concentrations. In duplicate experiments after sorption and washing with water, the contents of Ca (II) and Mg (II) in the filtrates were measured by AAS. (3) Time dependency of the sorption. For Co and Ni, the sorption in duplicate experiments was evaluated after 24 h (Table 1).

E. Greta / International Journal of Coal Geology 150–151 (2015) 166–169

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Fig. 1. Effect of pH on sorption for Sr and Mn.

(4) Desorption with 5% tartaric acid. The sorbents after washing with water, were treated with 100 ml 5% tartaric acid solution (pH 1.4)

Table 1 Sorption and desorption of Me (II) on xylain from the Mariza-East lignite deposit. Concentration of Me (II), μg/50 ml

Me (II) sorbed

Me (II) desorbeda

Initial

Equilibrium μg

Mn (II) 1000 2000 3000 5000 10,000

85 417 1012 2318 6510

915 0.033 1583 0.058 1995 0.072 2682 0.098 3490 127

92.5 79.2 66.5 53.6 34.9

94.1 80.8 83.2 62.8 75.6

Cu (II) 1000 2000 3000 5000 10,000

62 212 414 1070 4550

938 1788 2586 3930 5450

0.0295 0.056 0.081 0.124 0.171

93.8 89.4 86.2 78.6 54.5

82.6 83.6 78.5 78.2 58.9

Zn (II) 1000 2000 3000 5000 10,000

39 235 618 1770 5760

961 1765 2382 3230 4240

0.029 0.054 0.073 0.099 0.129

96.1 88.3 79.4 64.6 4.41

86.1 86.1 85.2 77.8 66.6

Co (II) 1000 89 2000 324 2000 (24 h)b 427 3000 868 5000 2230 10,000 6050

911 1676 1573 2132 2770 3950

0.03 0.057 0.053 0.072 0.094 0.134

91.1 83.8 76.7 71.6 55.4 39.5

87.8 83.8 80.4 85.8 76.5 66.1

Ni (II) 1000 28 2000 239 b 2000 (24 h) 410 3000 722 5000 1980 10,000 6310

972 1761 1590 2278 3020 3690

0.033 0.060 0.054 0.077 0.103 0.126

97.2 88.1 79.5 75.9 60.4 36.9

79.7 84.9 83.3 76.4 73.5 68.6

Sr (II) 1000 2000 3000 5000 10,000

922 1611 2123 2905 4559

0.021 0.037 0.049 0.066 0.104

92.3 80.6 70.9 58.1 45.6

91 89.8 87.4 83.2 66.5

a b

78 389 872 2095 5441

Desorbed with 5% tartaric acid. Sorption for 24 h.

mgeqv

% of initial concn % of total sorbed

for 24 h. The concentration of the respective element was assayed in centrifuged aliquots (Table 2). (5) Sorption on xylain from 50 ml solution containing 1000 μg of each element (Sr, Mn, Cu, Zn, Co, and Ni) at the same conditions (Table 2).

3. Results and discussion The results obtained showed a capacity of xylain to bind different divalent cations from solutions in approximately equal quantities. Concerning the type of binding, the most probable mechanisms could be cation-exchange and complexation via carboxylic and phenolic groups of the sorbent. Such presumption is supported by the following relations. The sorption was found to be pH dependent and similar for all elements — it is very low up to pH 3 beyond which it gradually increases and reaches saturation in the interval of pH 4–5.5. Examples are shown in Fig. 1. The pH effect on the ability of humic substances to bind cations is well known (Tipping, 2002). The same is true for low rank coals. For example, the cation exchange capacities of Mn, Cu, Zn, Co, and Ni for several low-rank Australian coals were dependent on solution pH and reached maximum at pH 5 (Lafferty and Hobday, 1990a). In this interval, the carboxylic and phenolic functional groups of the sorbent undergo dissociation in solution to form a negatively charged site upon the coal surface. The process of binding is slightly time dependent. Parallel experiments for Co (II), and Ni (II) with an initial concentration of 2000 μg/ 50 ml showed that during 24 h appreciable amounts were bound. After seven days, the quantity sorbed was about 1.1 times higher (Table 1). Cation quantities sorbed at the same initial concentration are very similar (Table 1). For example, at an initial concentration of 3000 μg/ 50 ml, the quantities attached vary from 0.072 to 0.081 mgeqv/0.3-g xylain for all elements, excluding Sr. This indicates that the elements occupied one and the same places on the sorbent. The isotherms of all Me (II) are similar (Fig. 2). With increasing initial concentration, the slope of the isotherms changes so as to form a platform parallel with the abscissa, suggesting an almost complete occupation of the adsorption sites of xylain. Some amounts of Ca (II) and Mg (II) were displaced from xylain after sorption — 0.124 meq Ca and 0.09 meq Mg. In parallel blanks of distilled water with 0.3 g-xylain, no Ca or Mg was observed to have been displaced from the sorbent. This indicates that the same type of active sites (carboxylic and phenolic groups) is responsible for the binding of the studied elements.

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Fig. 2. Sorption isotherms of Me (II) on xylain: 1 — initially sorbed and 2 — after desorption with tartaric acid.

On treatment with a complex-forming agent such as tartaric acid, a high degree of extraction of the elements was achieved. Despite the low pH (1.4) of the tartaric acid solution, appreciable amounts remain tightly bound with the sorbent, suggesting that chemical complexion was involved. It should be pointed that Co (II) and Ni (II) quantities sorbed were lower in experiments for 24 h than after seven days. However, the quantities after desorption with tartaric acid are equal in both experiments (Table 1), indicating saturation of the active sorbent sites.

When present simultaneously in solution, the quantity sorbed of every element, excluding Cu, is lower as compared with the quantities obtained from the experiments for single cations (Table 2). The order of decreasing sorption is Cu N Ni N Zn N Co ~ Mn N Sr. The same order of sorption was established for low rank brown coals — the cation exchange capacity in a mixture of Mn, Cu, Zn, Co, and Ni was lower for a particular cation than in a single metal solution (Lafferty and Hobday, 1990a,b). This is the result of all cations in solution competing

E. Greta / International Journal of Coal Geology 150–151 (2015) 166–169

References

Table 2 Sorption on xylain of Me (II) present simultaneously in the initial solution. Concentration of Me (II), meq/0.3 g xylain Me (II)

Initial

Sorbed on 0.3 g xylain

% of initial concentration

Mn Cu Zn Co Ni Sr Σ

0.036 0.031 0.031 0.034 0.034 0.031

0.016 0.031 0.018 0.015 0.021 0.007 0.108

0.033a 0.029a 0.029a 0.031a 0.033a 0.021a 0.176a

a

43.0 98.6 57.4 44.2 61.6 30.7

169

92.5a 93.8a 96.1a 91.1a 97.2a 92.3a

Data from the experiments for every single element.

for a limited number of adsorption sites, as shown for other systems (Kalis et al., 2006; Loewenstein et al., 1999). The highest quantity sorbed was for Cu (II), 98.6% of its initial concentration (Tables 1 and 2). This is in agreement with the experiments of Rashid (1974) in which from solutions containing Cu, Co, Mn, Ni, and Zn, the highest content bound with peat humic acids was found to be for Cu. The same was established for humic substances: binding strength increases in the order Mn b Co b Ni ~ Zn b Cu (Tipping and Hurley, 1992). The high stability of Cu complexes with humic and fulvic acids was confirmed by numerous investigations (Yudovich and Ketris, 2005). 4. Some geochemical considerations The xylain used as a sorbent contains some amounts of the elements studied: 39 ppm Mn, 44 ppm Sr, 3.7 ppm Cu, 4.7 ppm Zn, b 4 ppm Co, and 7.7 ppm Ni. Experiments on powdered xylain at laboratory conditions show that at an interval of pH from 3 to 6, additional amounts of these elements present in solution were bound, part of them irreversible. Hence, the sorptive capacity of xylain was not saturated and more elements may be bound from solutions. Such interaction may take place syngenetically, diagenetically or epigenetically to the coalforming process, suggesting infiltration of the coal beds by solutions enriched in some elements. Such a mechanism was proposed by Seredin (1996) for REE element-bearing (300–1000 ppm) coals from the Russian Far East deposits — “the domination of sorbed REE mode of occurrence suggests that these elements came to the coal accumulation basins in a dissolved form”. The REY enrichment of some coals in southern China was as well “attributed to leaching of the REY-bearing minerals in the partings by hydrothermally solutions and then adsorption of the REY by the underlying organic matter” (Dai et al., 2013). In selected Indiana coals of enhanced Ge content, Mastalerz and Drobniak (2012) suggested that “organic matter was an effective scavenger of Ge from the circulating fluids”. The same case is also applicable for some high Ge coals from both China and Far East Russia (Dai et al., 2012; Du et al., 2009; Qi et al., 2007; Seredin and Finkelman, 2008; Seredin et al., 2006). It is proposed that the sorption of cations on xylain in laboratory conditions is a model of a process which may take place, especially in lignite coal beds, and may result in accumulation of enhanced contents of organically associated elements. With increasing coal rank, the elements bound to the carboxylic and phenolic groups, are more or less released. On the other hand, the loss of functional groups diminishes the capacity of coals of higher rank for binding elements from solutions.

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