The removal of highly stable dispersed oil produced during oil-recovery processes is challenging, especially in offshore operations. The use of magnetic nanoparticles (MNPs) to remove the dispersed oil from produced water is a promising way to overcome the difficulties faced by current treatment technologies. The MNPs can be also regenerated and reused, minimizing the generation of hazardous waste. The authors investigated not only the optimal operating conditions, such as MNP concentration and salinity, but also the mechanisms of MNP/oil attachment and magnetic separation.
The main advantage of the use of MNPs to remove dispersed oil droplets from produced water is their quick response to move in a desired direction with application of an external magnetic field. Similar high-gradient magnetic-separation technology has been commonly applied in other industries for many years to separate magnetic minerals. It is important that the magnetic force must be greater than other competing forces, such as the drag force and gravitational force, to accomplish high recovery of magnetic particles.
The authors modified a previously developed mathematical model for the magnetic separation of nanoparticles to estimate total time and velocity of magnetic separation of MNPs attached on oil droplets. The mathematical model is described in detail in the complete paper, as are the materials and the experimental method used by the authors. A schematic of oil-droplet removal with MNPs is shown in Fig. 1.
MNP Characterization. Hydrodynamic sizes of bare MNPs and free-amine-functionalized MNPs (A-MNPs) measured with dynamic light scattering (DLS) were 51 and 66 nm, respectively.
Transmission-electron-microscopy image observation showed the small degree of aggregation of MNPs and A-MNPs, whose individual particle diameter was around 10 nm. MNPs, having a large ratio of surface area to volume, tend to agglomerate to minimize high surface energy by strong magnetic dipole-dipole attractions along with Van der Waals force.
X-ray diffraction patterns of MNPs and A-MNPs followed the characteristics of magnetite. This showed that synthesized iron oxide (Fe3O4) nanoparticles were magnetite and that the polymer surface coating did not change the crystallinity of Fe3O4 nanoparticles. For both MNPs and A-MNPs, high saturation magnetization values of approximately 90 emu/g Fe3O4 were obtained and the Langevin curves showed the absence of hysteresis, indicating that they are superparamagnetic nanoparticles.
Oil-Droplet Removal. Initial oil content of 0.25 wt% oil/water (O/W) emulsion was reacted in various Fe concentrations of A-MNPs and polyacrylic acid (PAA)-coated MNPs (PAA-MNPs). A-MNPs, having the opposite surface charge to oil droplets, were successfully attracted to and attached to negatively charged oil droplets, resulting in neutralized surface charge and making MNP-attached oil droplets stick together and form larger MNP/oil-droplet aggregates. This allows efficient magnetic separation of MNP-attached oil droplets from water. A larger total acid number (TAN) corresponds with greater acidity of crude oil. TAN 4.5 oil, having the greatest acidity among three tested crude oils, will require the greatest amount of A-MNPs to have the same level of removal as two other lower-TAN oils. Reacted with the same amount of A-MNPs, the percent removal of TAN-4.5 O/W emulsion was slightly lower than the other two. However, this amount of A-MNPs added to three TAN-differing oils was sufficient to remove almost all oil from water.
The negatively charged PAA-MNPs were also reacted with TAN-2.9 and -4.5 oil to investigate if the electrostatic interaction is indeed the major reaction mechanism to remove oil droplets from water. The authors confirmed that the negatively charged PAA-MNPs (-38 mV) and oil droplets repel each other, causing oil droplets to remain in water even after magnetic separation. In contrast with PAA-MNPs, clear water resulted after magnetic separation with A-MNPs, whose surface charge is positive with zeta potential of +26 mV. This result confirmed that the major reaction mechanism to separate oil droplets from water is electrostatic attraction, which is consistent with previous work.
The effects of Fe concentration of A-MNPs on oil-droplet removal were investigated as well. It took almost 3 hours to remove oil droplets to near-zero from water by use of 0.438 g/L of Fe. It is assumed that the amount of terminal amine groups is proportional to Fe concentration, resulting in greater density of surface charge on MNPs.
Negatively charged, micron-size oil droplets repel each other, keeping them highly stable. However, the positively charged A-MNPs successfully overcome the stabilization energy barrier between oil droplets by neutralizing their charge, making them stick together and grow into flocs, which then can be magnetically separated from water. The proper contact time in the rapid-mix chamber during the coagulation process in the water treatment is typically 1 to 3 minutes to produce larger particles, while insufficient mixing results in incomplete particle growth. For this study, larger aggregates of MNP-attached oil droplets were formed within 1 minute after mixing them on an orbital shaker table. Microscope observations indicate that the oil-droplet-removal process by use of MNPs occurs very rapidly.
Determination of Magnetic Separation Velocity and Time of Free A-MNPs Without Oil. The progress of magnetic separation of MNPs by itself was monitored continuously with a digital camera every 10 minutes until the level of the interface between the clear water and the particles was stationary. The complete water and MNP separation took 36 hours for 0.438 g/L Fe, 60 hours for 0.875 g/L Fe, and 72 hours for 1.75 g/L of Fe. The interface between water and MNPs began to appear at the top and bottom of the left side after a half-hour, and it was clearly seen 2 hours after magnetic separation was initiated. Most of the A-MNPs were collected within 12 hours.
The magnetic field was induced by a permanent magnet. With the magnetic force, it will take 2.7 hours to collect 66 nm of A-MNPs from the left (the furthest from a magnet, z=1) to the right (the closest to a magnet, z=0) in the model system. As particles near the magnet, the velocity of the particles grows faster because of the stronger magnetic field. The estimated total collection time is in the range of 0.2 to 20 hours, depending on the particle size. The simulation results show some deviation from the experimental results mainly because the electrostatic force is not considered in this model. The greater the Fe concentrations of A-MNPs, the greater the electrostatic force that the magnetic field has to overcome.
Determination of Magnetic Separation Velocity and Time of MNP-Attached Oil Droplet. These calculations are more accurate than those for free A-MNP because the charges of particles are neutralized or reduced, resulting in negligible electrostatic forces. The magnetic separation of MNP-attached oil droplets was monitored with an automatic camera. When 0.875 g/L Fe of A-MNPs was reacted with 0.25 wt% oil of TAN-2.9 O/W emulsion for 15 minutes, MNP-attached oil droplets were collected almost instantly and clear water was obtained. There were no particles at the top of the magnet. Because particles grow larger, the gravitational force also has an effect on particles.
Interestingly, when 1.75 g/L Fe of A-MNPs was reacted with O/W emulsion, the magnetic separation time depended on the reaction time. This might be caused by free MNPs not attached to oil droplets. However, as the reaction time increases, the probability of collision increases between free MNPs and between free MNPs and MNP-attached oil droplets. As a result, the particles might become more aggregated and magnetic separation time shortens. After A-MNPs were reacted with oil droplets for 5 minutes, it took approximately 10 hours to collect all particles magnetically. When A-MNPs were reacted with oil droplets for 120 minutes, it took approximately only 5 minutes for complete magnetic separation. This result suggests that the concentration ratio of MNPs and oil and the mixing time could be the most important variables to obtain quick and effective oil-droplet separation from water.
Positively charged, in-house-synthesized MNPs effectively removed negatively charged oil droplets by as much as 99.5% after 5 minutes of reaction between MNPs and oil. The less negatively charged oil droplets were easier to remove from water than the more negatively charged ones.
On the basis of microscope observations, the electrostatic attraction between negatively charged oil and positively charged MNPs controls the attachment of MNPs to oil droplets; the subsequent aggregation of the neutralized MNP-attached oil droplets plays a critical role in accelerated and efficient oil-droplet removal and magnetic separation. Particle aggregation occurred within 1 minute. The particles with an average hydrodynamic size of 66 nm were significantly aggregated. Thus, the total magnetic separation time was dramatically reduced to as little as 1 second.
Model calculations of magnetic-separation velocity show that the velocity of free A-MNPs increases by 1 to 3 orders of magnitude as the particles near the magnet, depending on the particle size. The smaller the particles, the greater the effect of the magnetic field on the velocity.
The application of MNPs for the oil-and-water-separation process is a promising technology because it can be designed as a compact system that generates less-hazardous sludge. The MNP-based method of oilfield fluids separation reported here is a promising, potentially green technology. The authors’ work has provided understanding of the underlying mechanisms and critical parameters for optimization of the process.