Decontamination & Disposal of High Level Heavy Metals Waste
Anil R Patil, Senior Scientist
Jajati Nanda, Senior Scientist
Omprakash Pal, Principal Scientist
Analytical Chemistry Halliburton Technology Center Pune

Hydraulic fracturing process can require up to millions of gallons of water per well. Additionally, oil and gas wells can also produce significant quantities of produced/formation water. The produced water can contain substantial concentrations of elements, such as barium, strontium, calcium, magnesium, and chloride, etc. To reuse produced water for the purpose of fracturing, treatment of produced water to remove these elements is necessary. This paper discusses adsorption of water containing barium and strontium ions using bentonite and activated carbon as an adsorbent.

Produced water is a secondary product that oil and gas producers worldwide must address, both onshore and offshore. As part of the extraction process of recovering oil and gas from the reservoir, water is produced as a secondary product. On average, per barrel of oil produced in the world, approximately nine barrels of produced water will be pumped to the surface. This water must be handled and properly disposed. Produced water can contain water from the reservoir that has been injected into formation and can contain high amounts of heavy metals and different minerals1. The most commonly found scales within the oil industry are the carbonate and sulfate salts of calcium, barium, and strontium, which can be encountered from the reservoir all the way to the surface processing equipment2,3. Barium sulphate and strontium scale formation are often, noted, mainly in the tubing of producing wells, where this scale can seriously jeopardize oil production. The primary cause of sulfate scaling is the mixing of incompatible waters, such as sea water and formation water, for example4,5. The water requirements on a wellsite for injection or hydraulic fracturing are quite large, increasing daily. Additionally, it is often difficult to obtain fresh water. Presently, most research is focused on use of produced water by removing contaminants and reuse of the same water for fracturing purpose.

Removal of heavy metals from aqueous solutions can be achieved using different technological methods. These methods include chemical precipitation, ion exchange, membrane filtration, electro-deposition, and flotation. Some of these methods have disadvantages and limitations. Precipitation, for example, produces large amounts of sludge in solutions6; but, membrane filtration, ion exchange, electro-deposition, and filtration are costly7,8,9. Alternatively, adsorption can remove these metals efficiently at low cost10,11,12. Several solid materials can be employed as adsorbents. Activated carbon is considered an effective adsorbent because of its extensive porosity and large available surface area13,14. On account of higher surface area, cation exchange capacity, and adsorption affinity for organic and inorganic ions, bentonite (mainly montmorillonite) is the most promising candidate for use in decontamination and disposal of high-level heavy metal wastes15,16. In the discussed study, adsorption of Ba and Sr ions were carried out on the activated carbon and bentonite. Surface area of both adsorbents was largely responsible for the adsorption of both the metals.

Materials and Methods
Instrumentation

Elemental concentration was analyzed using a Thermo Inductively Coupled Plasma (ICP) optical emission spectrometer coupled with peristaltic pump and AS-93 plus auto sampler unit.

Chemicals
Ba and Sr standard were used in this study. The pH adjustments were carried out using 0.1N hydrochloric acid (HCl) and 0.1N sodium hydroxide (NaOH). All Ba and Sr solutions were prepared with ultra-pure water (specific resistivity of 18 M . cm).

Adsorbent
The commercial bentonite (NB) was obtained from a local supplier; activated carbon was also obtained from a local supplier. To enhance the adsorption capacity of the commercial bentonite material, a 50-g sample was washed several times with deionized water to remove any particles adhering to the surface, salt, and any other water-soluble contamination; it was then was oven-dried overnight at 60C under vacuum. The dried bentonite sample was then ground and sieved.

General Procedure
Adsorption of Ba and Sr with bentonite and activated carbon was carried out in a conical flask. 1000 mg/L Ba and Sr standard was stock solution; Ba and Sr solutions ranging between 10 and 100 mg/L were prepared by diluting the discussed stock solutions. 0.5 Gm of bentonite mixed with 50 mL of Ba (II) and Sr (II) solution with different concentrations (10 to 100 mg /L) were applied in the stirrer. A 200-rev/min stirring rate and 298K temperature in all experiments were chosen. The flasks were then kept in a constant temperature bath with stirring. After 6 hr when equilibrium was attained, the concentration of Ba (II) and Sr (II) remained in the solution. After being centrifuged, the concentration was analyzed by an ICP spectrometer. In this study, the effects of several factors, such as pH, temperature, concentration of bentonite, and activated carbon were tested.

Adsorption Isotherms
From the discussed batch adsorption experiments, the adsorbed amount (qe) of Ba and Sr (II) per unit of sorbent mass was calculated as follows:

qe=(Co-Ce)V/M (1)
Where Co is the initial Ba and Sr concentration, Ce is the concentration of Ba and Sr at equilibrium (mg/L), m is the clay mass (mg), and V is the solution volume (L).

Effect of pH
The influence of pH in the range of 2.5 to 12 was studied, keeping all other parameters constant (Ba and Sr concentration = 10 to 100 mg/L; stirring speed = 200 rev/ min; contact time = 6 hr, adsorbent concentration = 0.5 g, temp. = 25C). The pH of Ba and Sr solution was adjusted after adding the adsorbent by using a dilute of NaOH and HCl solutions. After equilibration (6 hr), samples were withdrawn for the analysis of Ba and Sr.

Effect of Temperature
Effect of three different temperatures (303, 320, and 333K) on the adsorption of Ba and Sr were also checked in a temperature bath. After attaining the respective temperature, adsorption studies were carried out in stirring conditions. Effect of Adsorbent Concentration The adsorption efficiency of Ba and Sr was studied at different adsorbent concentrations [0.1to 0.5 gm/50 mL Ba and Sr solution] at Ba and Sr solutions (10 and 100 mg/L), keeping stirring speed (200 rev/min), temperature (25C) and contact time (6 hr) constant.

Results and Discussion
Effect of Adsorbent Concentration: Bentonite and Activated Carbon Adsorption of Ba and Sr (II) on bentonite and activated carbon was studied at different bentonite concentrations (0.1, 0.3, and 0.5 g/50 mL, respectively.) The results showed that, with increasing the adsorbent concentration, the adsorption % of Ba and Sr (II) were increased (Figure 1). The increase to the adsorption percentage of Ba and Sr with bentonite and activated carbon concentrations can be explained by the increase to the adsorbent surface area and the availability of more adsorption sites17 . An increased surface area provides more sites to adsorb Ba and Sr on the surface of adsorbents. Figure 1 shows at 0.1 gm, adsorbent adsorption of Ba is 5 mg/gm, and that, of 0.5 gm, it is 14 mg/gm of adsorbent. In the case of Sr, also at 0.1gm, adsorbent adsorption of Ba is 3 mg/gm, and increased to 9 mg/gm if the adsorbent amount is increased to 0.5 gm.

Effect of pH
The pH of the aqueous solution is an important variable that controls cationic adsorption onto adsorbent surface. This is attributed to the change of adsorbent surface properties and the metal species with pH change. The plots of adsorbed amount versus pH of Ba and Sr (Figures 2 and 3) have inflection points at pH 8 where significant adsorption of Ba and Sr actually begins. With an increase of pH of the solution from 2.5 to 12.0, Ba adsorption increases from 6 mg/gm of activated carbon to 30 mg/gm, and Sr adsorption increases from 5 mg/gm of activated carbon to 22 mg/gm. Ba adsorption on bentonite increases from 7 to 21 mg/gm, and Sr from 3 to 20 mg/gm when pH increased from 2.5 to 12. It is known that the increase of pH decreases the competition between the protons and the metal ions for surface sites and results in increased uptake of metal ions by the bentonite.

The effect of pH on the adsorption of Ba and Sr on bentonite can be explained on the basis of aqua complex formation of the oxides present in the bentonite. A positive charge develops on the surface of the oxides of bentonite in an acidic medium as follows:

-------SiOH + H+              ------Si -----OH+ (2)

A lowering of Ba and Sr adsorption at low pH is caused by surface charge, thus developed is not suitable for Ba and Sr adsorption. At low pH values, the high hydrogen ion concentration at the interface (the hydrogen ions are more specifically adsorbed than Ba and Sr ions) repels the positively charged metal ions electrostatically and prevents their approach to the adsorbent surface18.

In an alkaline medium, the oxide surface of the adsorbent becomes negatively charged, as shown in Equations (3) and (4), favoring the adsorption of Ba and Sr.

---SiOH+OH-               ---- SiO-+H2O (3)

---SiO-+M                           ---Si---O---M (4)

Increasing pH was reported to increase the adsorption of metal ions from kaolinite suspensions19. Gutierrez and Fuentes20 studied the adsorption behavior of Sr, Cs, and Co by Ca-montmorillonite, and showed that Co adsorption increases above the pH of precipitation of Co (OH)2. Mekhemer21 showed the precipitation of Co(OH)2 has been observed during the adsorption experiment; therefore, the drastic increase in cobalt removal above pH = 6 was attributed to the precipitation of cobalt ions as insoluble Co(OH)2, rather than the adsorption on the negative surface charges of bentonite. In the present study, no precipitation was observed at higher pH; adsorption increases when pH is increased.

Adsorption Isotherm
Batch equilibrium studies of Ba and Sr with the adsorbents were conducted at different temperatures. The adsorption capacity of the adsorbents can be empirically correlated by Langmuir type isotherm. Initial adsorption data (i.e., q [Ba and Sr adsorbed amount moles/kg] and C [equilibrium concentration moles/dm3]) were used to plot a 1/q vs 1/c plot, which is a linear line. From the linear line slope, intercepts were calculated. Qmax and K were calculated by using slope and intercept, as shown in Equations (5) and (6).

Qmax= 1/ intercept (5)

K= 1/ (slope*qmax) (6)

The values obtained qmax and K were used to calculate the q predicted. Equation (7) was used to calculate the q predicted.

q/qmax= KC/(1+KC) (7)

Where q is the loading of the adsorbents expressed as moles of Ba and Sr adsorbed per kg of adsorbent, qmax is maximum loading capacity, K is the equilibrium constant and is a quantitative measure of interaction between the Ba and Sr and adsorbents, and C is equilibrium concentration moles/dm3. The lines shown in Figures 4 and 5 are the curve obtained by fitting the experimental Ba adsorption data on bentonite and activated carbon, respectively, into Langmuir adsorption isotherm at three different temperatures (i.e., at 303, 320, and 333 K). It was observed that, by increasing the temperature, the adsorption of Ba increased, indicating that the heat of adsorption was positive (endothermic). Qmax for Ba on bentonite and activated carbon was 0.079 and 0.192 moles/kg, respectively (Table 1). Sr adsorption was also increased as the temperature increased. At 303K, qmax for Sr was 0.024 moles/kg, and increased to 0.05 moles/kg at 33K. Maximum loading capacity (i.e., qmax for Sr) on bentonite and activated carbon was 0.050 and 0.1 moles/kg respectively at 333K (Table 2 and Figures 6 and 7).

Adsorption Thermodynamics
The thermodynamic parameters of the adsorption (i.e., the standard enthalpy H), Gibbs free energy G and entropy S were calculated using the following equations:

G = -RT ln KL (6)

ln KL = S/ R H /RT (7)

Where R is the general gas constant (kJ .mol-1. K-1), KL = Langmuir adsorption constant and T is the temperature (K). H and S values can be obtained from the slope and intercept of the Vant Hoff plots of ln KL (from the Langmuir isotherm) versus 1/T22,23. The results of these thermodynamic calculations are shown in Table 3. The negative value for the Gibbs free energy for Ba and Sr (II) adsorption shows that the adsorption process is spontaneous and that the degree of spontaneity of the reaction increases with increasing temperature. The overall adsorption process of Ba and Sr is endothermic (activated carbon: Ba, H = 1.53 kJ mol-1, Sr H = 1.39 kJ mol-1 and bentonite: Ba, H = 5.01 kJ mol-1, Sr H = 2.87 kJ mol-1). This result explains why the Ba and Sr (II) adsorption capacity of bentonite and activated carbon increases with increasing temperature. Table 3 also shows that the S value was positive, indicating that the metal ions near the surface of the adsorbent was more ordered than in the subsequent adsorbed.