Decontamination of water polluted with oil through the use of tanned solid wastes

Decontamination of water polluted with oil through the use of tanned solid wastes

Amal Gammoun, Soufiane Tahiri, Abderrahman Albizane, Mohammed Azzi, and Miguel de la Guardia

Abstract:Sorption by natural organic substrates, inorganic materials or synthetic fibers is one of the most popular meth- ods used for the separation of oily wastes from contaminated water. In this work, the ability of chrome shavings (CS) and of buffing dusts of crust leather (BDCL) to remove motor oils and oily wastes from demineralised water and natural sea- water has been studied. Tannery solid wastes are formed mainly by proteins and have a highly organized structure in the form of fibers (F: 100 nm). These wastes have a high oil sorption capacity. Tanned solid wastes are capable of absorbing many times their weight in oil (6.5–7.6 and 12.8–14.5 g/g dry substrate, respectively, for ground CS and BDCL). The sorption capacity depends strongly of sorbent nature. The removal of oils from the water surface is a quasi-instantaneous process. After use, the saturated waste floats and can be removed in an efficient and easy manner. The results look fairly promising as to possibilities of using tanned wastes to remove oils from industrial effluents and from contaminated coastal areas.

Key words:tannery, solid wastes, chrome shavings, buffing dusts, oils, sorption.

Re´sume´ :L’absorption par des substrats organiques naturels, des mate´riaux inorganiques ou des fibres synthe´tiques est l’une des me´thodes les plus employe´es pour la se´paration des de´chets huileux de l’eau contamine´e. Dans ce travail, la ca- pacite´ des de´rayures de cuir chrome´ et des poussie`res de ponc¸age du cuir en stain, issues de la tannerie, a` enlever des huiles pour moteurs et des de´chets huileux de l’eau de´mine´ralise´e et de l’eau de mer naturelle a e´te´ e´tudie´e. Les de´chets solides de tannerie sont constitue´s essentiellement de prote´ines et ils ont une structure fibreuse tre`s organise´e (K: 100 nm). Ces de´chets ont une grande capacite´ d’absorption des huiles. Les de´chets solides de peau tanne´e peuvent absorber plusieurs fois leur poids en huile (6,5 – 7,6 et 12,8 – 14,5 g/g de substrat sec respectivement pour les de´rayures broye´es de cuir chrome´ et les poussie`res de ponc¸age). La capacite´ d’absorption de´pend fortement de la nature de l’absorbant.

L’e´limination des huiles de la surface de l’eau est un processus quasi-instantane´. Apre`s utilisation, les de´chets sature´s flottent et peuvent eˆtre facilement collecte´s. Les re´sultats permettront d’envisager la possibilite´ d’employer les de´chets de peau tanne´e pour enlever les huiles des effluents industriels et des zones coˆtie`res contamine´es.

Mots-cle´s :tannerie, de´chets solides, de´rayures de cuir chrome´, poussie`res de ponc¸age, huiles, absorption.

[Traduit par la Re´daction]

Introduction

Used or spent oils are toxic for the microflora used in the biological degradation of organic substances contained in ef- fluents. This can be explained by the formation of a layer on the water surface limiting energy, heat, moisture, and oxy- gen exchange between the water reservoir and the atmos- phere (Lee et al. 1999; Pushkarev et al. 1983; Haussard et al. 2001). Consequently, the efficiency of microorganisms for the degradation of organic matter in wastewater treat- ment will be decreased. The spilled oil also contributes to an undesirable taste and odor to drinking water and causes severe environmental damage. Contaminated water cannot

be used for municipal water supply, for industry, nor for ir- rigation (Blumer 1969). Contamination of coastal areas due to oil spills is a tragic event. Under favorable conditions, oil may continue to spread over the water surface and form a thin layer. As a result, marine life is seriously threatened, beaches are polluted and tourism and the fishing industry gravely suffer the effects.

Sorption by natural organic substrates (wood, cotton fi- bers, bark, wool, milkweed, etc.) or inorganic materials (ac- tivated carbon, charcoal, exfoliated graphite, flyash, etc.) or synthetic fibers (acetylated cellulose, polypropylene, poly- ethylene terephtalate, polyhexamethylene adipamide, etc.) is

A. Gammoun and M. Azzi.Faculte´ des Sciences, Universite´ Hassan II– Aı¨n Chock, Casablanca, Maroc.

S. Tahiri.¹Faculte´ des Sciences El Jadida, De´partement de Chimie, Universite´ Chouaib Doukkali, B.P.20, El Jadida, Maroc.

A. Albizane.Faculte´ des Sciences et Techniques, Universite´ Hassan II, Mohammedia, Maroc.

M. de laGuardia.Departamento de Quimica Analitica, Edificio de Investigacion, Universidad de Valencia, Spain.

Written discussion of this article is welcomed and will be received by the Editor until 31 January 2008.

1Corresponding author (e-mail: t_soufiane@yahoo.fr).

one of the most popular methods used for the separation of oily wastes from contaminated water (Fanta et al. 1986;

Schrader 1991; Choi and Cloud 1992; Choi et al. 1993; An- thony 1994; Lee et al. 1999; Mackay and Gschwend 2000;

Toyoda and Inagaki 2000; Haussard et al. 2001). The sorp- tion capacity of different materials employed is the amount of oil retained by 1g of dry substrate. According to Gregg and Sing (1967) the sorption capacity in a vast range of sol- ids depends on the surface area and pores. Tanned solid wastes generated by the leather industry are very porous and are a good candidate for sorption material.

The production of chromium-containing solid wastes from the tanning process has been recognized as a problem for many years. Tanned solid wastes have been disposed of in landfills. Increased local restrictions on land disposal, recent increases in the costs of land disposal and decreases in the number of disposal sites have combined to spur research into alternative treatments for this material. Many scientific groups have oriented their research to find a process to re- cycle and reuse these tanned wastes (Tahiri et al. 2001a, 2001b, 2003, 2004; Cabeza et al. 1998; Taylor et al. 1998;

Brown et al. 1996; ANRED and CTC 1982).

Our research has been designed to aid the leather industry in the development of alternatives to disposal for solid tan- nery waste. The aim of this work is to study the possibility of using tanned solid wastes as a sorbent in an oil spill cleanup. Two wastes were tested as sorbents: chrome shav- ings and buffing dusts of crust leather, being employed syn- thetic oil–water mixtures to evaluate the efficiency of the tanned solids assayed.

Materials and methods Sorbents

Tanned wastes products assayed in the present study were collected from a Moroccan tannery located in Mohammedia.

Chrome shavings

The small pieces of leather shaved off when the thickness of wet blues is rendered uniform by a bladed cylinder. The wet blue is the wet chrome tanned leather, without dressing.

The molecular structure of wet blue and chrome shavings is presented in Fig. 1.

Buffing dusts

In our case, these wastes were generated after treating the surface of crust leather (without dyes) by abrasion. The crust leather is, in general, the leather having undergone dressing operations (wet or mechanical) but not yet finished. Wet leather dressing includes neutralization, retanning and fat liquoring. These operations are carried out after tanning to give certain properties to the leather.

Tested oils

Three oils are used for the experiments to study their sorption on wastes. The samples used are: diesel motor oil, premium motor oil, and used motor oil (oily wastes or oil from draining). Oil samples have relatively similar densities:

0.885, 0.881, and 0.894, respectively. The viscosity at 20

is 0.337, 0.294, and 0.281 Pas, respectively.

Tanned wastes characteristics

Bulk density and specific surface area of chrome shavings and buffing dusts were determined. The tested wastes were analysed for: ash, chromium oxide, fat, dermal substance, and pH of soluble matter. The physicochemical characteris- tics of wastes were analysed using standard methods (Dutel 1984).

Microscopic observations of the structure and morphology of the aforementioned products wastes were made using a Scanning Electron Microscope (HITACHI S-4500).

Determination of the sorption capacity of tanned solid wastes

Raw and dry tanned wastes were used for sorption of oils in a pure form. To obtain dry substrate, solid wastes were

Table 1.Characteristics of tanned wastes.

Characteristics Chrome shavings Buffing dusts

Average moisture (%) 22.33 13.23

Ash (%)^a 12.92 12.17

Chromium oxide Cr₂O₃(%)^a 4.40 4.85

Fat (%)^a 2.50 7.77

Dermal substance (%)^a 78.64 78.00

pH of soluble matters^b 3.56 4.25

Bulk density (gcm^–3) 0.10 0.05

Specific surface area (m²/g) 1.12 12.11

aBased on dry weight.

b5 g of waste was stirred in 100 mL of distilled water.

dried at 100

material and fibers with 2–5 mm of length were used. It is difficult to define a constant granulometry for raw chrome shavings because these wastes have no regular fiber size. In a 100 mL beaker, 80 mL of oil was placed and 1 g of sorb- ent was immersed in the bath for 4 h. Material was then re- covered and left to drip for 1 h and the amount of oil sorbed was determined by subtracting the initial sorbent weight from the total weight of the wetted sorbent. Tanned wastes sorption capacity (SC, weight of pollutant picked by one gram of sorbent) was thus quantified. All of the sorption ex- periments were carried out at room temperature (20 ± 2

Sorption study in a water bath

This study will examine the feasibility of using tanned solid wastes for sorption of oils in polluted water. Deminer- alised water and natural seawater were used. For each oil and waste, a mass of oil (5.5 g of commercial oils and 7.5 g of oily wastes) was added to a 1000 mL beaker con- taining 500 mL of water. To this mixture, sorbent material was added. The effect of the sorbent mass on oil removal efficiency was studied using different mass ranging from 1

to 3 g. Tests were carried out, respectively, without and with stirring. In the case of stirring, the beakers were placed on a shaker, and then shaken for 10 min at 60 r/min.

At the end of the experiments, the solid–liquid phases were separated and the sorbent was left to drip for 1 h. The amount of residual oil in water and that retained by the tanned wastes were determined. Residual oil in water was extracted with petroleum ether. The solvent extractor was evaporated and the residual oil was weighed. The amount of oil sorbed on wastes was calculated by subtracting the amount of residual oil in water from the initial mass of oil added to beakers containing water.

Results and discussion

Physicochemical characters of wastes

The chrome shavings and buffing dusts, used as raw mate- rials in this study, were analysed for pH, ash, fat, proteins and chromium oxide and the results are presented in Table 1. As can be seen, the analysis indicates that these wastes have an important percentage of proteins (78.64%–78.00%). The amount of chromium oxide is about 4.40%–4.85%. The

Table 2.Oils sorption capacities of tanned wastes for pure oils.

Oil type Waste type

Oil/waste (g/g)

Raw waste Dry waste Diesel motor oil Ground chrome shavings 5.74 7.00

Chrome shavings 4.06 3.73

Buffing dusts of crust leather 13.00 14.43 Premium motor oil Ground chrome shavings 6.48 6.54

Chrome shavings 2.60 3.81

Buffing dusts of crust leather 12.91 14.55

Used oil Ground chrome shavings 5.28 7.60

Chrome shavings 3.43 3.82

Buffing dusts of crust leather 12.08 12.80

buffing dusts have a significantly higher amount of fat (7.77%) than chrome shavings (1.57%); this can be ex- plained by the use of greases in the fat-liquoring process.

The bulk density of chrome shavings and buffing dust are 0.1 and 0.05 gcm

, respectively. This low density indi- cates that these wastes occupy a large volume as compared to other solid wastes. This causes problems in the handling of high amounts of tanned wastes generated in the leather industry. The specific surface area of chrome shavings and of buffing dusts is 1.12 and 12.11 m

/g, respectively.

Scanning electron micrographs of chrome shavings and buffing dusts

The scanning electron microscopic method is a good tech- nique for showing structure and morphology of solids. The resulting micrographs (Fig. 2) reveal that the studied wastes have a highly organized structure in the form of fibers (F = 100 nm). The fibers seem to be more separated in the case

of buffing dusts. The chrome shavings fibers are more paral- lel and very tight to each other.

Determination of oil sorption capacity

The tests were conducted on oils in their pure form. Ta- ble 2 groups the sorption capacity (SC) of ground chrome shavings (GCS), raw chrome shavings (RCS) and buffing dusts of crust leather (BDCL). The sorption capacity was calculated for raw and dry wastes. The SC of these wastes is very strong: about 6.5–7.6, 3.7–3.8, and 12.8–14.5 g/g dry substrate, respectively, for GCS, RCS, and BDCL. As can be seen, sorption capacity depends strongly of the nature of sorbent. The results indicate that the investigated oil sam- ples showed relatively similar sorption trends since they have relatively similar densities and similar viscosities. For example, in the case of dry ground chrome shavings the sorption capacity is about 7.00, 6.54, and 7.60 g/g for diesel motor oil, premium motor oil, and used motor oil, respec-

Waste type Oil type

Oil (g)

Waste (g)

Oil in waste (%)

Water in waste (g/g)

Chrome shavings Diesel motor oil 5.55^a 1.00 96.58 0.15

5.54^b 1.07 99.53 0.18

5.59 2.01 100 1.29

5.50 2.05 100 1.24

5.57 3.03 100 1.95

5.59 3.08 100 2.09

Premium motor oil 5.58 1.02 98.62 0.54

5.63 1.08 98.49 0.34

5.54 2.06 100 1.33

5.51 2.08 100 1.48

5.58 3.10 100 2.98

5.51 3.09 100 3.14

Used oil 7.54 1.01 99.12 0.02

7.54 1.02 92.08 0.02

7.54 2.06 100 1.11

7.56 2.07 100 1.59

7.58 3.04 100 1.67

7.58 3.04 100 1.91

Buffing dusts of crust leather Diesel motor oil 15.16 1.03 98.54 0.04

15.13 1.04 99.94 0.08

15.43 2.01 100 1.01

15.21 2.04 100 1.34

15.14 3.02 100 0.87

15.00 3.08 100 0.95

Premium motor oil 15.34 1.04 98.65 0.13

15.01 1.01 99.35 0.08

15.17 2.06 100 0.56

15.03 2.04 100 0.17

15.33 3.05 100 1.19

15.02 3.02 100 1.64

Used oil 16.07 1.10 99.20 0.06

16.09 1.06 95.67 0.04

16.02 2.02 100 0.51

16.09 2.03 100 0.56

16.18 3.02 100 0.84

16.05 3.02 100 0.98

aWithout stirring.

bWith stirring 60 r/min.

tively. Moisture content relatively decreased the waste’s ability to absorb oils. Buffing dust showed better sorption properties than chrome shavings for all investigated oils.

This is due to the morphology of wastes: the fibers seem to be more separated in the case of BDCL and consequently allow the penetration of a greater amount of oils into the sorbent. On the other hand, the density of BDCL (0.05 gcm

) is lower than that of chrome shavings (0.1 gcm

). BDCL occupy a large volume as compared to chrome shavings and consequently a large amount of oils could be sorbed by dusts of tanned hides.

For chrome shavings, reduction in fibre size contributed to the change in sorption capacity. When the wastes are ground up (2–5 mm) the retention capacity increases.

Sorption study in a water bath

Tables 3 and 4 show the variation of removal efficiency (RE) value of the tanned solid assayed in water using 1, 2,

and 3 g of sorbent/500 mL. A study was carried out for demineralised water and natural seawater, without stirring and using a stirring velocity of 60 r/min. Results show that the removal efficiency value is very significant. Tanned solid wastes have a high selective affinity for oils in aque- ous medium. The removal efficiency of the oils from a syn- thetic oil–water mixture exceeds 90% (ranging from 90 to 100%). Observation shows that oils are removed only in the zones that are in contact with the sorbent. Thus, the removal efficiency (RE) of oil is a direct function of the surface ex- posed to the oil–water interface. No significant difference between the sorption in demineralised water and that in nat- ural seawater bath was observed.

The high oil affinity of tested sorbents is mainly due to the diffusion phenomenon. The pollutant penetrates into the sorbent by capillary action and then fibers form aggregates that can be removed easily from the water surface. The sorp- tion of water is negligible and clearly lower than the oil

Waste type Oil type

Oil (g)

Waste (g)

Oil in waste (%)

Water in waste (g/g)

Chrome shavings Diesel motor oil 5.58^a 1.00 96.05 0.13

5.52^b 1.09 98.63 1.02

5.50 2.01 100 1.98

5.51 2.03 100 1.99

5.50 3.05 100 2.55

5.66 3.03 100 1.94

Premium motor oil 5.58 1.02 96.90 0.26

5.51 1.09 98.46 1.52

5.54 2.07 100 1.17

5.54 2.13 100 1.98

5.51 3.06 100 1.99

5.51 3.04 100 2.06

Used oil 7.64 1.04 88.64 0.09

7.57 1.02 83.10 0.96

7.53 2.03 100 0.97

7.57 2.07 100 1.41

7.57 3.04 100 1.24

7.51 3.05 100 1.98

Buffing dusts of crust leather Diesel motor oil 15.26 1.06 98.50 0.16

15.07 1.08 99.30 2.53

15.15 2.05 100 0.88

15.07 2.02 100 0.98

15.13 3.05 100 0.82

15.00 3.07 100 1.43

Premium motor oil 15.24 1.08 99.14 0.15

15.08 1.06 98.96 0.17

15.07 2.03 100 0.82

15.21 2.02 100 1.35

15.10 3.05 100 0.88

15.02 3.03 100 2.03

Used oil 16.24 1.01 98.13 0.06

16.35 1.06 99.78 1.76

16.01 2.00 100 0.93

16.10 2.08 100 0.90

16.10 3.01 100 0.61

16.04 3.09 100 1.48

aWithout stirring.

bWith stirring 60 r/min.

sorption. The sorption of water depends on the amount of solid wastes on the surface. Results of experiments showed that an excess of sorbent is not good because it can cause the retention of water on the sites not impregnated by the oil. To avoid this problem the oil sorption capacity values must be taken into consideration. Only the necessary amount of waste sorbent required to sorb the oil should be used and dispersed on the water surface.

Tests revealed that the removal of oils from the water sur- face is a quasi-instantaneous process. When wastes are added to a beaker containing polluted water, oils are sorbed immediately by the sorbent.

To show if there is any possibility of a chromium release into the water medium, 0.5 g of waste and 100 mL of demin- eralised water were placed in a rotatory agitator (250 r/min) for 4 h. After this period, the solid and the liquid phases were separated by gradual vacuum filtration and the liquid was analysed. Results obtained revealed that chromium con- centration in the filtrate is about 0.65 mg/L in the case of chrome shavings. The release of chromium in the case of buffing dusts is very low (0.06 mg/L).

The removal efficiency (RE) of oils is not affected by stirring velocity of 60 r/min. Consequently, the tanned wastes could be used efficiently in the field in the case of tragic events due to the contamination of coastal areas with oil spills.

Removal of saturated wastes from water

Observations show that saturated wastes float. The materi- als remain on the water surface forever because the wastes fibers become more hydrophobic than the raw wastes. The removal of saturated wastes from water is easy and rapid. A simple foaming with a sieve allows collection and evacua- tion of solid waste from the water surface.

Conclusions

The results obtained prove the utility of using tanned solid wastes for resolving an environmental problem. We have demonstrated that oil can be efficiently removed from water by sorption on tanned solid wastes. Extension of this work for the treatment of an industrial effluent containing hydro- carbons or oils will be the objective of future studies. The use of natural waste materials for the removal of pollutants from effluents prior to their treatment could decrease the cost of operation and upgrade the quality of the treated ef- fluents. The manufacture of sorption filter using wet-blue shavings for industrial wastewater is one of the objectives of our future studies.

Sorption of oils on wastes can increase the calorific value of these materials. Consequently, these wastes can be easily treated by incineration. The ashes obtained after thermal treatment of wastes have a significant amount of chromium oxide and could be used in several industries, for example, the cement industry.

Acknowledgments

Authors acknowledge the financial support of the ‘‘Projet Conjoint de Recherche PCI Maroco-Espagnole’’, Project N8 128/P/03 and 128/04/R/M.

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