Granular Activated Carbon for Soluble Oil in Groundwater
The leakage of crude oil and its products may occur during the process of extraction, refining, transportation, and storage, and contaminate the groundwater through the vadose zone. The pollutants exist in the form of floating oil, dispersed oil, emulsified oil, soluble oil, solid oil, etc. Among them, soluble oil is mainly composed of short-chain hydrocarbons and low molecular weight polycyclic aromatic hydrocarbons (PAHs), which are difficult to remove by regular methods and affect human health, especially the carcinogenic PAHs.
The commonly used oil pollution remediation technologies for groundwater include adsorption, permeable reactive barriers (PRBs), microbial bioremediation, phytoremediation, chemical oxidation, electrodynamic remediation. The adsorption technology uses the porosity and large specific surface area of the adsorbents such as activated carbon, activated clay, biological materials, to adsorb soluble oils and other organics in water, so as to achieve the separation of water and oil.
Activated carbon is rich in surface functional groups, has highly developed pores, strong adsorption capacity for organic matters that are hard to biodegrade. Activated carbon can be regenerated and reused. And modification can further improve its adsorption efficiency to oils and phenols. Besides, activated carbon combined with zerovalent iron, zeolite, vermiculite, sand, and microorganisms as the reactive media for PRBs can remove heavy metals, phenols, BTEX, etc.
Comparison of activated carbon made from different raw materials
Surface characteristics
The specific surface area is one of the most important factors affecting the adsorption performance of activated carbon. Test results show that with the same particle size (40-60 mesh), the specific surface area of coconut shell activated carbon is much larger than those of nutshell and coal-based activated carbons. The latter two activated carbons have a similar surface area, pore volume, and average pore size. And coal-based activated carbon is slightly larger in the parameters. According to the SEM images, coconut shell activated carbon has a honeycomb-like, regularly arranged pore structure, which is conducive to the adsorption process. Nutshell activated carbon has an uneven pore structure and some small pores are difficult to adsorb macromolecular substances. The coal-based activated carbon has a larger pore size. The coconut shell activated carbon has the highest carbonization degree, lowest polarity, and the highest hydrophobicity followed by nutshell, then coal-based activated carbon.
Adsorption kinetics
Comparing the three kinds of granular activated carbon with the same particle size for diesel and crude oil, the adsorption of coconut shell activated carbon for diesel increases rapidly in the first 120 min, then slows down as the content of soluble oil decreases. The coconut shell activated carbon reaches equilibrium at 300 min, with a 123.4 mg/g adsorption amount. In the first 360 min, the performance of coal-based activated carbon is better than that of nutshell activated carbon, the adsorption kinetics curves of the two after 360 min are close, and they both reach equilibrium at 540 min, with an adsorption amount of 104.14 mg/g and 102.33 mg/g, respectively. Coconut shell activated carbon also has the best adsorption performance for crude oil among the three kinds, and reaches equilibrium at 300 min, with 26.81 mg/g. The adsorption equilibrium time and amount of coal-based and nutshell activated carbon are 60 min, 21.52 mg/g, and 240 min, 21.52 mg/g, respectively.
According to the results, among the three 40-60 mesh granular activated carbons, coconut shell activated carbon has the best adsorption effect on soluble diesel and crude oil in water. The iodine number of coconut shell, nutshell, coal-based activated carbon is larger than 1100 mg/g, 850 mg/g, 800 mg/g, respectively, which is proportional to the adsorption performance. And with the same particle size, the surface area of coconut shell activated carbon is the highest, followed by that of coal-based which is slightly higher than that of nutshell.
Comparison of coconut shell activated carbon with different particle sizes
Comparing the adsorption kinetics curves of coconut shell activated carbon with different particle sizes (10-20 mesh, 20-40 mesh, 40-60 mesh), for the adsorption of diesel, those with 10-20 mesh and 20-40 mesh can reach equilibrium within 60 min, but have much lower adsorption amount around 29 mg/g than the 100.88 mg/g of that with 40-60 mesh. The equilibrium amount for crude oil of 10-20, 20-40, and 40-60 mesh is 17.64 mg/g, 18.69 mg/g, 26.45 mg/g.
The smaller the particle size of activated carbon, the better the adsorption effect on soluble oil in water. This is because the pore structure of different particle sizes is not exactly the same. Activated carbon with smaller particles sizes has faster adsorption kinetics and adsorbate diffusion rates.
According to the BET results, 40-60 mesh coconut shell activated carbon has the largest specific surface area. Although the specific surface area of 10-20 mesh coconut shell activated carbon is slightly larger than that of 20-40 mesh, due to its larger particle size, it has fewer total adsorption sites, and a weaker adsorption effect.
Adsorption isotherm
When the equilibrium concentration of soluble diesel in groundwater increases from 1.37 mg/L to 44.68 mg/L, the equilibrium adsorption capacity of activated carbon increases from 25.87 mg/g to 143.15 mg/g; When that of soluble crude oil increases from 1.23 mg/L to 1.86 mg/L, the equilibrium adsorption capacity of activated carbon increases from 0.31 mg/g to 9.63 mg/g.
From the SEM images of 40-60 mesh coconut shell activated carbon after adsorbing diesel and crude oil, it can be seen that a dense film with many bulges is formed on the activated carbon surface. This is because small molecular substances can pass through macropores and enter into mesopores and micropores, while large molecular substances cannot enter mesopores and micropores, but adhere to the opening.
Factors influencing the activated carbon adsorption
The dosage of activated carbon
As the dosage of coconut shell activated carbon increases from 0.02 g to 0.10 g, 0.10 g to 0.12 g, the adsorption capacity for soluble diesel increases from 55.28 mg/g to 100.83 mg/g, then to 100.99 mg/g. While for soluble crude oil, the capacity decreases from 20.37 mg/g to 5.56 mg/g, but the adsorption rate shows an upward trend. With a dosage of 0.06 g, the crude oil adsorption rate reaches 97.33%. Then the rate changes little from 0.06 g to 0.10 g. This is because for the soluble oils in groundwater, the concentration of diesel is much higher than that of crude oil. As the dosage increases, the adsorption sites also increase, so more diesel is adsorbed. As the content of soluble diesel in water decreases, the adosrption rate slows down.
Since the concentration of crude oil is low, a small dosage of activated carbon is enough to adsorb basically all the content at the beginning. When the dosage of activated carbon continues to increase, the increment of crude oil adsorption content is much lower than that of activated carbon content.
Therefore, the optimal dosages of 40-60 mesh coconut shell activated carbon for soluble diesel and crude oil with concentrations of 101 mg/L and 5.67 mg/L are 0.10 g and 0.06 g, respectively.
The content of sodium chloride
With the increase of sodium chloride content, the adsorption capacities of activated carbon for soluble diesel and crude oil both change little. So it can be considered that the content of sodium chloride has little influence on the adsorption.
The pH value of groundwater
Activated carbon has the largest adsorption capacity (82.72 mg/g) for soluble diesel under the neutral condition, and the adsorption effect under the acidic condition is better than that under the alkaline condition. For soluble crude oil, the optimum pH value for adsorption is 8, with which the adsorption capacity is 12.96 mg/g, and the adsorption effect under the alkaline condition is better than that under the acidic condition.
This difference is related to the surface properties of activated carbon and the diverse composition of soluble oil in water. Under the acidic condition, H+ easily attaches to the activated carbon surface, and the electrostatic attraction between the positively charged carbon surface and the soluble oil is favorable for enhanced adsorption. Whereas under the alkaline condition, there are many negative charges on the carbon surface, resulting in the enhanced electrostatic repulsion between the carbon surface and soluble oil. The OH– at the same adsorption sites as the soluble oil also affects the oil adsorption capacity.
Temperature
As the temperature increases from 7.5 °C to 25 °C, the adsorption capacities of activated carbon for soluble diesel and crude oil gradually increase from 82.75 mg/g to 88.28 mg/g, from 10.76 mg/g to 17.72 mg/g, respectively. When the temperature increases to 30 °C, the adsorption capacities of diesel and crude oil decrease to 88.04 mg/g, 13.23 mg/g, respectively. This is because, with the increase of temperature, the collision between activated carbon and soluble oil molecules intensifies, which is beneficial to the adsorption. When the temperature is higher than 25 °C, the viscosity of the oil decreases, and the adsorption capacity of activated carbon also decreases.