Coals as sorbents for the removal and reduction of hexavalent chromium from aqueous waste streams

Fuel 81 (2002) 691±698

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Coals as sorbents for the removal and reduction of hexavalent chromium from aqueous waste streams q J. Lakatos a, S.D. Brown b, C.E. Snape c,* a

Department of Analytical Chemistry, University of Miskolc, Miskolc H-3515 Hungary Centre For Thermal Studies, School of Applied Sciences, University of Hudders®eld, Hudders®eld HD1 3DH, UK c School of Chemical, Environmental and Mining Engineering (SChEME), University of Nottingham, University Park, Nottingham NG7 2RD, UK b

Received 19 December 2000; revised 10 July 2001; accepted 11 July 2001; available online 29 October 2001

Abstract The aim of this study is to demonstrate the potential of coals as a low-cost reactive barrier material for environmental protection applications, with the ability to prevent leaching of toxic Cr(VI) and other transition metals. Depending upon the type of ion and the surface functionalities, the uptake can involve ion sorption, ion exchange, chelation and redox mechanisms with the surface functionalities being considered as partners in electron transfer processes. The capacity for Cr(VI) uptake of low rank coals and oxidized bituminous coals has been found to lie within the range 0.2±0.6 mM g 21. Air oxidation of bituminous coals can increase their Cr(VI) removal capacities. The effect of air oxidation of coals on uptake capacity was more pronounced for Cr(VI) than Cr(III), but less than for Hg(II) and the other ions (Ca 21, Ba 21, Zn 21, Cd 2) investigated. As previously found for Hg(II), redox mechanisms play an important role in Cr(VI) uptake, with sorption of the resultant Cr(III) being aided by the functionalities arising from oxidation of the coal surface. In acidic media, much of the resultant Cr(III) is exchanged back into solution by hydrogen ions, but some of the sorbed chromium is irreversibly bound to the coal. The reduction of Cr(VI) alone is often considered a satisfactory solution in view of Cr(III) being essentially non-toxic. q 2001 Elsevier Science Ltd. All rights reserved. Keywords: Coal; Chromium removal; Redox capacity; Water puri®cation

1. Introduction The minimization of the environmental impact of transition and heavy metals in aquatic system and leachates requires the application of different chemical process: precipitation, ion-exchange, adsorption, electrochemical and membrane ®ltration. Finding the cost effective methods requires further investigation in the ®eld of natural sorbents, industrial and agricultural wastes or by products. The environmental and biological effects of chromium are dependant upon its oxidation state. Cr(III) is essentially non toxic and Cr(III) complexes (picolinate) are applied to human food supplements, and these are also bio-available for plants [1]. Cr(VI) is poisonous to most living organisms and has found application in wood preservation [2]. However, some organisms, namely Bacillus QC 1±2, Escherichia coli ATCC 33456 and sulphate reducing * Corresponding author. Tel.: 144-115-951-4166; fax: 144-115-9514115. E-mail address: [email protected] (C.E. Snape). q Published ®rst on the web via Fuel®rst.comÐhttp://www.fuel®rst.com

bacteria can tolerate and reduce hexavalent chromium, providing a biological route to eliminate Cr(VI) [3±6]. Two types of chemical treatments are currently used for Cr(VI) removal: the ®rst type remove Cr(VI) anions directly while the second type rely on the reduction of Cr(VI) to Cr(III). Zero valent iron, copper and sul®des are applicable only for reduction [7±12]. Often the reducing agent can act as a sorbent for the resultant Cr(III) ion. Activated carbon, modi®ed clays and zeolites were found to be effective for both the reduction and removal of Cr(VI) [13±17]. Additionally, a number of naturally occurring organic materials including agricultural wastes and biomass, peat, and coals have also been investigated for this purpose [18±36]. Phenolic oxygen bound carbon is considered to be the main reaction partner of Cr(VI) when it is reduced by soil humic acid, peat and activated carbon surfaces [14,21,37±40]. Since coal has a large number of these functional groups, they should also be able to provide a means for reducing Cr(VI). This study aims to determine the characteristics of Cr(VI) removal by a wide range of coals. Our goal is also to gain an understanding of how the Cr(VI) coal interaction operates

0016-2361/02/$ - see front matter q 2001 Elsevier Science Ltd. All rights reserved. PII: S 0016-236 1(01)00159-4

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Table 1 Characteristics of the coals studied Coal a

Visonta (Hungary) Skye peat (UK) a Borsod (Hungary) a Mequinenza (Spain) a Nograd (Hungary) a Hambach (Germany) Can (Turkey) a Tatabanya (Hungary) a Dorog (Hungary) a North-Dakota (USA) Tuncbilek (Turkey) a Daw Mill (UK) Gedling (UK) Mecsek (Hungary) a Gascoigne Wood (UK) Pittsburg #8 (USA) Pont of Ayr (UK) Taff Merthyr (UK) Gedling oxidised Taff Merthyr oxidised a

Ash (db wt%)

C (dmmf wt%)

H (dmmf wt%)

N (dmmf wt%)

O (dmmf wt%)

S (db wt%)

Sorg. (db wt%)

40.8 2.49 35.1 13.0 43.4 4.3 5.8 17.7 49.7 2.6 15.0 4.69 2.2 19.5 21.2 9.0 10.1 4.03 0.1 6.2

56.7 58 65.3 65.8 67 67.5 68.8 70.5 65 74.5 76.7 81.3 81.6 82.6 84.5 84.9 87.2 92.4 69.5 76.1

5.49 6.61 5.6 5.4 5.3 4.4 5.0 5.1 5.7 4.9 5.4 4.8 5.2 4.8 4.9 5.4 5.8 4.2 3.11 2.63

0.23 1.51 0.84 0.8 1.3 0.03 1.58 0.91 0.77 1.17 2.5 1.3 1.7 1.65 1.8 1.7 1.6 1.5 1.62 1.64

36.5 33.8 25.5 17.9 26.4 27.2 20.7 23.5 18.6 19.3 14.3 11.5 10.3 9.3 7.7 6.9 4.6 1.2 24.8 18.7

1.08 traces 2.75 10.3 traces 0.8 3.9 3.5 4.35 0.8 1.04 1.50 0.98 1.60 1.42 2.17 1.69 0.72 0.99 0.9

± ± ± 9.9 ± 0.73 ± ± ± 0.63 ± 1.12 0.89 ± 0.76 0.81 0.63 0.67 ± ±

0.1 M HCl treated coals.

with the view of using coals for the reduction and removal of Cr(VI) from waste streams and leachates. 2. Experimental The characteristics of the coals and peat used in this study are summarised in Table 1. The UK and USA coals were from the CRE and Argonne Premium Coal Banks, respectively. Before use, these coals were stored in deep freeze and were ground under nitrogen. The Hungarian coals were averaged samples from their respective basins. These coals were stored at ambient temperature. Air oxidised samples were prepared from a high volatile (Gedling) and a volatile bituminous coal (Taff±Merthyr) by heating in a muf¯e furnace. The temperature and the duration of treatment were 2508(C, 28 h and 3008C 24 h, respectively. An activated carbon (Norit C Extra USP) was also used for purposes of comparison. The biomass samples were wood and agricultural by products ground to a particle size range of between 75 and 215 mm, except the cotton bagasse which consisted of ,6 mm pieces. Prior to metal ion loading the coals were treated with 0.1 M hydrochloric acid (1:50 solid liquid ratio, 24 h, magnetic stirring) to remove exchangeable cations and carbonates, washed with distilled water and then dried at 333 K under vacuum. Batchwise ion exchange experiments were conducted in 0.1 M acetic acid±sodium acetate (1:1) buffer and 0.005 and 0.05 M sulfuric acid media. Reagent grade chemicals were used. In most cases, a 1:40 solid to liquid mass ratio was used. The concentrations, solution pH and contact times used for the experiments are speci®ed in

the ®gures. The slurries were agitated once a day. The phases were separated by ®ltration. The ion concentrations in the solutions were determined by ¯ame atomic absorption spectrometry and the Cr(VI) remaining was determined by the standard colorimetric 1,5-diphenylcarbazide method. Surface areas of the as-received and oxidised samples of Gedling and Taff±Merthyr coals determined by CO2 at 273 K, and calculated by the Dubinin±Radushkevich method.

3. Results and discussion 3.1. Mechanisms of Cr(VI) uptake As discussed earlier, there are two principal means by which coals and ions can interact with each other in aqueous solutions. The ®rst and most common way involves the sorption, ion exchange and chelation of metal ions [41± 47]. These interactions are characterised by the absence of change in either the coal structure in terms of the functional groups present or the valency of the metal. The other means by which coal and ions interact involves electron transfer or redox reactions [37±40,47±51], which will result in the coal structure being oxidized and a metal ion of a lower oxidation state. The interaction of Cr(VI) with one of the bituminous coals (Gedling), one of the lignites (Can) and the activated carbon (Norit C) was investigated at different pHs: pH < 5 in 0.1 M sodium acetate±acetic acid buffer, pH < 2 and pH < 1 in 0.005 and 0.05 M sulphuric acid, respectively. For the conditions used here, Cr(VI) initially exists in

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Fig. 1. Time dependence of chromium removal by Norit C activated carbon, Can lignite and Gedling high volatile bituminous coal in 0.1 mol dm 23 1:1 sodium acetate: acetic acid buffer solution (pH < 5). Initial Cr(VI) concentration: 50 mmol dm 23, solid to liquid ratio 1:40.

Fig. 3. Time dependence of chromium removal by Norit C activated carbon, Can lignite and Gedling high volatile bituminous coal in 0.005 mol dm 23 sulphuric acid solution (pH < 2). Initial Cr(VI) concentration: 50 mmol dm 23,solid to liquid mass ratio 1:40.

solution as the HCrO42 ion whilst Cr(III) at pH 1 occurs as Cr 31 and at pH 5 as the Cr(OH) 21 cation [22,52]. The total concentration of chromium and the concentration of Cr(III) remaining in solution were determined as a function of time (Figs. 1±3).

Figs. 1±3 show the contact time required between the phases to achieve complete reaction. The uptake of Cr(VI) is initially rapid then continues at a much slower rate. The process does not appear to achieve equilibrium over the time interval used of 13 days for the experiments. Since the coal structure must be altered if the reaction is to occur by a redox mechanism, higher loadings can be expected for the resultant Cr(III) from the new oxygen functionalities that form. However, the ®nal equilibrium depending upon the concentration of exchangeable cations in solution (dominated by H 1 here) dictates that some of the Cr(III) formed has to be released back to the bulk solution. This process should clearly become more effective at lower pH as demonstrated for the peat sample in Fig. 4. The result that small amounts of Cr(III) are released back into solution at pH < 5 is particularly intriguing (Fig. 1). Increased levels of Cr(III) desorption were found to occur only at pH < 1 (Fig. 2) but the concentration of the desorbed Cr(III) is far less than expected if Cr(III) loading is reversible (c.f. Fig. 4). These results can be explained if the sorption of the in situ formed Cr(III) is considered to be irreversible or the desorption is very slow [53]. This is obvious if the high stability and the inert character of Cr(III) complexes in aquatic solution is considered. At pH < 1 (0.05 M sulphuric acid), not only the removal of chromium was great, but also a considerable amount of the reduced chromium was released back into solution (the Cr(III) concentration being the difference between the total and Cr(VI) concentration, Fig. 2). This result also proves clearly that two different processes have to be distinguished:

Fig. 2. Time dependence of chromium removal by Norit C activated carbon, Can lignite and Gedling high volatile bituminous coal in 0.05 mol dm 23 sulphuric acid solution (pH < 1). Initial Cr(VI) concentration: 50 mmol dm 23, solid to liquid ratio 1:40.

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Fig. 4. Isotherms of Cr(III) sorption by Skye peat at two different pH's: pH < 5 (0.1 M sodium acetate±acetic acid 1:1 buffer), pH < 1 (0.05 M sulphuric acid). Solid to liquid mass ratio 1:40, contact time 15 days.

Fig. 5. Comparison of Cr(VI) removal at pH ˆ 1 and pH ˆ 5 as a function of coal rank. Initial Cr(VI) concentration: 5 mmol dm 23, Solid to liquid mass ratio 1:40, contact time 12 days.

the electron transfer reaction and the loading of the different chromium species to the surface [40]. In the buffer and the 0.005 M sulphuric acid media, no direct evidence of the electron transfer reaction has been obtained so far, as Cr(III) was not released back into solution and no attempt has been made to date to determine the chromium form in the solid phase. The relative extent of the uptake of Cr(VI) as a function of pH (see Figs. 1 and 2) can also be considered as a clear indication of the electron transfer reaction. The redox potential of the Cr(VI)/Cr(III) system depends upon pH; at pH < 1, E < 1.3 V and at pH < 5, E < 0.68 V. Taking into account that the ease of oxidation of the surface functionalities is likely to vary considerably, it is obvious that improving the redox potential of the oxidant will extend the oxidation towards the more resistant functionalities. This means that the redox capacity of coal depends on the pH. Additional to this key point, the redox reaction also consumes protons [14,22]. If the system does not have a suitable buffer capacity, the pH will continuously increase with a concomitant decrease in the redox potential of the Cr(VI)/Cr(III) system until the reaction stops. The 0.1 M acetic acid±sodium acetate buffer at pH 5 can supply the protons for the redox reaction, but the low concentration of protons (10 25 M) results in a low redox potential but a high ion-exchange capacity (Figs. 1 and 4). The 0.05 M sulphuric acid medium used contains the same number of protons available for the redox reaction as the buffer but clearly, the pH is lower and this will result in a higher redox potential. The redox reaction is more ef®cient, but the ion exchange equilibrium is shifted strongly in the direction

of desorption (Figs. 2 and 4). In the 0.005 M sulphuric acid medium (Fig. 3), there is one order of magnitude fewer protons available than in the previous case. As a result, the amount of reduced chromium remains small, despite the initial redox potential being higher than at pH < 5. These experiments prove that the extents to which oxidation and chromium reduction occur are governed by pH. The type and the number of accessible surface functionalities change from solid to solid with the result that the concentration of Cr(VI) remaining the solution will also differ when a different solid is used. To summarise thus far, the in¯uence of pH on the redox potential and the ion sorption capacity can explain why less Cr(VI) was reduced in the buffer than in the sulphuric acid medium (compare Figs. 1 and 2) and why more Cr(III) was released back in acidic medium (see Fig. 4). Although further experiments involving the speciation of the sorbed Cr by XPS and EXAFS are needed to determine the relative contributions of redox and anion sorption processes in Cr(VI) removal, the data presented support the hypothesis that redox mechanisms mainly resulting in exchangeable Cr(III) are the major pathway for Cr(VI) uptake. Indeed, the reduction of Cr(VI) alone is often considered a satisfactory solution in view of Cr(III) being considered as non-toxic. A comparison of the removal of Cr(VI) at pH < 1 and pH < 5 as a function of coal rank is presented in Fig. 5 which shows that removal was more effective in acidic medium than in the acetate buffer for the wide range of coals. The peat is an exception. However, to adequately explain the differences in the extent of desorption of

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3.2. Trends in Cr(VI) uptake with coal rank

Fig. 6. Dependence of Cr(VI) removal on the total oxygen content of coals. Initial Cr(VI) concentration: 5 mmol dm 23, solid to liquid mass ratio 1:40, contact time 12 days.

Cr(III) as a function of pH, the different chemical forms of Cr(III) have to be considered [22,52]. In addition to the pH effect, it has to be taken into consideration that the sorption of Cr(III) may not be reversible.

The oxidation of organic materials by Cr(VI) is practised widely in organic and analytical chemistry, for instance it is used for the determination of the humic acid or organic content of soil and chemical oxygen demand in water. In these cases the ®nal goal is to produce CO2 from the carbon compounds. Oxidation by Cr(VI) has also been applied to the study of coal structure [54±56]. In these studies, a more strongly acidic medium than the one here was used, besides the carbons connected to oxygen functionalities, aliphatic carbons were also oxidised, and the poly-aromatic structure was degraded. As a result of the oxidation a range of organic acids were recovered. At ambient temperature, using low Cr(VI) concentrations and dilute or week acids, the most probable reaction is the oxidation of carbons bound to oxygen functionalities [21,40]. As a result, the rank dependence of the Cr(VI) reduction can be related to the change in the oxygen functionalities. Fig. 6 shows the correlation of Cr(VI) uptake with the total oxygen content of the coals investigated. The slope of the curve indicates that as a minimum, approximately 2% of the carbons bound to oxygen take part in the interaction. Furthermore, a better correlation would intuitively be expected if the carboxyl oxygen is subtracted from the total, as it is inert to oxidation. Nevertheless, the results indicate that the accessibility of Cr(VI) to the participating functionalities must be considered as the main factor controlling Cr(VI) uptake. Oxidation by Cr(VI) is likely to give rise to new surface carboxyl functionalities on the coals. The increase in the concentration of potentially accessible carboxyl sites formed by the redox mechanisms can clearly be demonstrated by the increase in the barium ion sorption capacities. These were determined in 0.1 mol dm 23 1:1 sodium acetate: acetic acid buffer solution (pH < 5). The initial barium concentration being 50 mmol dm 23, using a solid to liquid ratio of:1:40 and a contact time of 12 days. First, the coals were reacted with Cr(VI), washed and dried at ,350 K in vacuum. As shown in Fig. 7, oxidation resulted in large enhancements of the barium sorption only in the bituminous coals. 3.3. Comparisons with other ions and substrates

Fig. 7. Comparison of barium sorption on non oxidised and Cr(VI) oxidised samples of Skye peat, Mequinenza lignite, Gedling high volatile bituminous coal and Taff±Merthyr low volatile bituminous coal. The chromium was not removed prior to the barium sorption measurements. Oxidation was carried out using 50 mM Cr(VI) solution in 0.1 M 1:1 sodium acetate:acetic acid buffer solution (pH < 5) and in 0.05 M sulphuric acid solution (pH < 1) for 3 days.

A comparison of redox and and non-redox interactions has been considered by ®rst comparing Cr(VI) and Cr(III) removal with respect to coal rank, and secondly, by comparing Cr(VI) removal to that of alkaline earth, transition and heavy metal ions (Ca 21, Ba 21, Zn 21, Cd 21, Hg 21). The variation of Cr(VI) and Cr(III) removal as a function of coal rank are similar (Fig. 8), but due to the redox interactions throughout the rank range, the extent of Cr(VI) removal is higher than Cr(III). A comparison of the removal of Cr(VI) with the other ions (Fig. 9) indicates that the low rank coals and oxidised bituminous coals have comparable capacities. For the bituminous coals the Cr(VI) removal capacities are

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Fig. 8. Rank dependence of Cr(III) and Cr(VI) removal by coals. Initial Cr(III) and Cr(VI) concentration: 5 mmol dm 23, solid to liquid mass ratio 1:40, contact time 12 days, pH < 5 (0.1 M sodium acetate±acetic acid buffer, 1:1). Fig. 10. Comparison of Cr(III) and Cr(VI) removal by coals and biomass. Initial Cr(III) and Cr(VI) concentration: 5 mmol dm 23, solid to liquid mass ratio 1:40, contact time 12 days, pH < 5 (0.1 M sodium acetate±acetic acid buffer, 1:1).

Fig. 9. Comparison of removal capacities for Cr(VI) and other ions for the initial and oxidised bituminous coals. Initial Cr(VI) concentration: 50 mmol dm 23, solid to liquid mass ratio 1:40, contact time 12 days, pH < 5 (0.1 M sodium acetate±acetic acid buffer, 1:1).

higher than for the other ions investigated, where simple cation sorption is occurring. The extent of Cr(VI) uptake by coals achieved in this investigation (0.2±0.6 mmol g 21, 10±30 mg g 21) compares well with those obtained for barks and tannin rich materials, chitosan and seafood processing wastes, biomass, xanthate, clays, zeolites, ¯y ash etc. [18]. Fig. 10 compares the Cr(III)±Cr(VI) interaction characteristics of the coals, peat and biomass. Generally, the biomass samples did not show the clear differences between Cr(IV) and Cr(III) removal displayed by the coals. However, the Cr(VI) removal capacity of biomass was the same order of magnitude as that of the coals. Therefore, in addition to coal, peat and biomass provide relatively cheap alternatives for transition and heavy metal ion removal [18,20±36]. As the reduction of Cr(VI) mostly takes place with the carbons bound to ±OH functionalities, the pre-treatment of coals to increase the concentrations of electron donor sites can improve the redox capacity of the coal. To this end oxidised samples of bituminous coals were prepared as described earlier. Oxidation under the conditions used here can have two effects; it can modify the type and the number of the functionalities on the surface (see the enhancement of the total oxygen content in Table 1) and also increase the surface area available for reaction by improving the pore structure [45]. The enhancement of the

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bituminous coals give comparable capacities. For the bituminous coal, the Cr(VI) removal capacities are higher than for the other ions investigated, where simple cation sorption is occurring. 3. The extent of Cr(VI) uptake by coals achieved in this investigation (0.2±0.6 mmol g 21, 10±30 mg g 21) compares well with those obtained for alternative naturally occurring materials. Acknowledgements One of the authors (J.L.) is greatly indebted to the Hungarian Science Foundation for providing a grant (OTKA T-031959.). The authors thank the European Coal and Steel Community for ®nancial support (Contract No. PR070). References

Fig. 11. Effect of air oxidation on Cr(III) and Cr(VI) removal for bituminous coals. Initial Cr(III) and Cr(VI) concentration: 5 mmol dm 23, solid to liquid mass ratio 1:40, contact time 6 days, pH < 5 (0.1 M sodium acetate± acetic acid buffer, 1:1).

surface area of Gedling and Taff±Merthyr coals was 10 m 2 g 21 (6.2%), and 57 m 2 g 21 (43%), respectively. Fig. 11 indicates the pre-oxidation improves both the Cr(III) and Cr(VI) removal capacities of the bituminous coals. However the extent and the ratio of the Cr(III)±Cr(VI) removal capacity enhancement is highly dependant on coal type. Our on-going research will address the multi component system and dynamic character of the Cr(VI)±coal interaction as a function of particle size and ¯ow rate since these experimental variables play important roles in function of reactive permeable barriers. 4. Conclusions 1. This investigation has demonstrated that the reduction of chromium(IV) is governed by the concentration and the extent of electron donor functionalities in the coals, which varies with pH. Lower pHs exhibit larger redox potentials for the Cr(III)/Cr(VI) system, and give rise to larger redox capacities of the coals. pH also affects Cr(III) desorption which was found to be partially irreversible or very slow. 2. The variations in Cr(VI) and Cr(III) removal as a function of coal rank were similar, but due to the redox effects accross the rank range the extent of Cr(VI) removal is higher than Cr(III). A comparison of the removal of Cr(VI) with the other ions (Ca 21, Ba 21, Zn 21, Cd 21, Hg 21) indicates that the low rank coals and oxidised

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