Current World Environment

Introduction

Soil pollution is anordinary side-effect of several indus­tries. The oil industries is mainly responsible for soil contamination due toactions related to crude oil extraction, refineries and transfer, underground crude oil storage tanks and the wastewaters.¹

Polycyclic aromatic hydrocarbons (PAHs) are one of such above mentioned contaminants. They are hydrocarbons-organic compounds containing only carbon and hydrogen, consist of two or more benzene rings.² United States Environmental Protection Agency (USEPA) named 16 different compounds of PAHs as priority pollutants according to their carcinogenicity and mutagenicity at very low concentrations.^3,4

Stabilization/Solidification (S/S) method has been widely used for treatment of heavy metals⁵ and other contaminants in hazardous wastes, industrial sludges, power plant residues, municipal ashes, nuclear wastes, and contaminated soils and debris before final disposal since 20 years ago.⁵ USEPA documented S/S method as the Best Demonstrated Available Technology (BDAT) for the land disposal of the majority toxic and hazardous substance.^6-8 Furthermore, it is one of the most regular remediation processes used at Superfund sites in the U.S. (about 24% of the sites applied this method between 1982 and 2002),^8,9 and it has been known as one of the most cost effective techniques.^10,11 The stabilization stands for a process that is convert contaminants into forms that are much less mobile, soluble, and toxic.^8,11 Additionally, solidification encapsulated the contaminants within a monolithic solid with high structural integrity ^8,11.

Since organic compounds interfere with cement hydration process, S/S technology could be run into difficulties when trying to immobilize organic contaminants.^12,13 A possible process for improving the efficiency of S/S for organic contaminants is using binders as an additive like fly ash, modified clay, activated carbon.¹⁴ Microsilica (MS) is a by-product of the smelting process of silicon metal and ferrosilicon alloy production.¹⁵ It’s amorphous structure, high SiO₂ content, and large surface area makes it reactive with calcium hydroxide produced by cement hydration.^15-17

The aim of this research was to analyse the effect of adding MS on leaching behaviour of high oily PAHs contaminated soil solidified and stabilized by Portland cement.

Materials and Methods

Sampling and preparation

Soil sampling was carried out from different parts of petrochemical industry (Rey Petrochemical Industry) located in the South-East of Tehran, Iran (Figure 1).

Figure 1: PAHs contaminated soil sample collected from Rey Petrochemical Industry Click here to View figure

Following collection of the samples, they were dried at room temperature for 24 hours. Subsequently soil samples moved into 1000 ml glass jars and then placed in fridge (with the temperature of 4 °C). American Society for Testing and Materials (ASTM) methods were performed to characterize the properties of soil samples[18](Table 1).

Table 1: Characteristics of soil samples in Ray Petrochemical Industry

Binders

PC is most frequently used and widely studied binder. S/S with cement is widely understood and simple, is easily available and results in a stable product.¹⁹ The PC applied in this study was obtained from Tehran Cement Factory. Other binders (activated carbon, fly ash, MS, modified clay, lime, etc.) can be partially added to PC, which can enhance or negatively affect the properties of cement in S/S.^19-21

The properties of MS are determined by the procedures it has been undergone during the manufacturing, also based on this its colour could vary from light to dark gray. The surface area of MS varies between 15,000 to 35,000 m²/kg and its average diameter is smaller than 1 μm and it can come in different forms of slurry, powder and condensed.^22,23 Numerous researchers have been conducted during past years to evaluate the properties of concrete by adding MS to the cement^24-26 Table 2 contains the Comparative Physical properties and chemical compositions of MS and PC which have been used in this study.

Table 2: Compared chemical compositions and physical properties of PC and MS

Its effects are related to the strength, ductility, air void content,freeze-thaw durability, permeability, shrinkage, creep rate, specific heat, abrasion resistance, coefficient of thermal expansion (CTE), chemical attack resistance, bonding strength with reinforcing steel, defect dynamics, thermal conductivity, and degree of fibre dispersion in mixes containing short microfibers.^26-29

Preparation of specimens

The various mixes and their designations in presence, absence and different ratios of MS used in this study are shown in Table 3. Specimens were divided in two groups:

• C25 with constant Portland cement percent of 25
• C35 with constant Portland cement percent of 35

Samples were transferred into the molds which have been made due to ASTM D 1633:00 method A with the Height to diameter ratio equals 1.15.¹⁸

Testing method

The United States Environmental Protection Agency (USEPA)developed toxicity characteristic leaching procedure (TCLP) to simulate the worst possible situation for disposal of municipal solid waste and hazardous waste in a landfill and has been widely applied as a regulatory test to evaluate contamination levels.^30,31 During last decade, the TCLP has been used extensively to test the leachability of contaminations in soils to evaluate the efficiency of contaminant immobilization.^32,33

For conducting leaching of the crushed wastes, Method 1311 of SW-846 of the USEPA (1992) Toxicity Characteristics Leaching Procedure was used.^34,35 Cylinders samples were crushed to less than 9.5 mm in size After 28 days of curing. Acetic acid-water solution with the pH of 2.88 was used as a extraction fluid and the liquid-to-solid ratio of it was 20:1. it was added to the crushed specimens at room temperature and were rolled thoroughly for 18 hr in extraction bottles made of stainless-steel at 28 to 30 rpm.A 0.45 μm membrane filter was used to remove suspended solid from the leachate and finally gas chromatograph with flame-ionization detector (GC-FID) and split/splitless injector were used to analyse the PAHs.³⁵

Results ad Discussion

Leaching Behaviour

PAHs Concentrations in TCLP leachate were evaluated by GC apparatus. For analyzing the efficiency MS in S/S process, a control sample of contaminated soil without any added MS was used, and the contaminants’ concentrations in leachates of solidified samples were compared to that of control samples as an index (100%). Wherever this amount is closer to zero indicates the higher solidification and stabilization prohibiting performance. Two factors of the S/S process effectiveness were investigated: (1) concentration of contaminants; and (2) effect of using MS in leachate behaviour.Table 4 and Table 5 show the results of leachateconcentrations and percentages. These observations indicated that addition of MS played a significant role in decreasing the amount of leachate in S/S specimens compared to the untreated contaminated soil.

Table 3: Mixing Ratio

In group C25 with 4 percent MS (C25MS4), B[a]P with 22.47% was the specimen with the least leachate percentage. Whilst, in C25 specimens with the 8% MS, the least leaching percent belonged to B[a] A with amount of 6.06%. results showed that with increasing the MS content to 8% a reduction of almost 50% in average leachate percentage would be observed. Confirming the above mentioned results the 36.54 percent TPAH leachate in C25MS4 decreased to 14.66 in C25MS8.

Data from group C35 showed differences to of group C25. Although 4 percent content of MS resulted in decrease in amount of leaching, with increasing MS to 8 percent not only no improvements were observed but also there was a noticeable increase in leaching percent. This occurrence probably caused due to the fact that using MS increases the water demand of samples for hydration and using more MS defected the cement hydration process in samples.^24,36

Table 4: Relative concentration of PAHs leaching in S/S samples

Table 5: Relative Leaching percentage of PAHs

The relation between the contaminant’s release and the MS proportion was found to have a logarithmic pattern, which is shown for different cement mixes in Figures 2 and 3. High correlation coefficients (r2 > 0.94) indicate that a slight increase of the Micro Silica significantly reduces the concentration contaminants in leaching.

Figure 2: logarithmic relationship between leachate and percent of Micro Silica for Naphtalin and chrysen Click here to View figure

In addition, equations extracted from Figure 2and 3illustrated that increasing in cement percent in S/S samples enhanced the efficiency of removal. However, Ln coefficients in the equations demonstrated that this improvement could be considered insignificant compared to the effect of MS in leaching behaviour of solidified and stabilized samples.

Figure 3: relationship between leachate and percent of Micro Silica for Benzo[a]Anthracene and Benzo[a]Pyrene Click here to View figure

Conclusion

All together it seems that stabilization/solidification is a useful method for decreasing leaching from contaminated sites. S/S offers technological advantages over the alternative remedial options for contaminated soils and sediments. This method offers reduction and prevention of further movements and mobility of hazardous wastes and contaminants in the environment by physical means. While the solidification contains the pollutant in a limited space, stabilization converts the toxic compound into a less toxic one. Conclusions of this study are highlighted below:

• The least contaminated leachate was related to B[a]A paste S/S by 25% cement and 8% of MS with the 6.06%
• In group C 25 increasing in MS percentage from 4% to 8% resulted in almost 50% reduction in contaminant leachability of contaminated soil.
• The TPAH leachate of the specimens S/S by 25% cement and 8% MS were lower than all of other S/S combinations
• In group C35 adding more MS posed adverse effect on leachability of contaminants in S/S samples
• Analysis of diagrams extracted from leaching data illustrated that soil S/S by 35% cement and different portions of MS had better results in immobilization of PAHs
• Equations extracted from results suggested that adding MS had more influence in S/S of the contaminated soil compared to the cement itself.

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