# Precipitant Effects on Aggregates Structure of Asphaltene and Their Implications for Groundwater Remediation

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## Abstract

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## 1. Introduction

_{f}= 1.7 ± 0.2 for DLA [18,19] while Huang et al. and Seifried et al. reported much larger values of d

_{f}for crude oil in the DLA regime [8,15]. Moreover, Hoepfner et al. concluded that no clear trend was observed for the fractal dimensions of asphaltenes aggregates as a function of precipitant (n-heptane) concentration [17]. Therefore, a high-resolution experimental study, which allows a direct analysis of aggregate structure, is strongly needed to better understand the subtle differences in fractal dimensions at different precipitant concentrations.

## 2. Experiments

#### 2.1. Materials

#### 2.2. Experiments

#### 2.3. Image Analysis

_{g}is the radius of gyration. The radius of gyration can be determined by:

## 3. Results and Discussion

_{f}) were computed by the following scaling law using the area and the radius of gyration: $A\propto {R}_{g}{}^{{d}_{f}}$ (Figure 3). More details on the goodness of fit are shown in Table 1. For asphaltene aggregates formed at different precipitant concentrations, the sizes of the aggregates R

_{g}were in a wide range of 10–1000 µm, suggesting these aggregates are highly polydisperse. The higher precipitant concentration resulted in a slight increase of initial particle size from 1 to 4 µm, indicating a faster aggregation rate in a short time. The aggregates for all three experiments described above have a fractal scaling region that spanned over two orders of magnitude in length. However, there is a change in the scaling behavior at R

_{g}= 30 µm for all precipitant concentrations, suggesting that the structure of fractal clusters changes as they transition from small to large clusters. For small clusters, regardless of the precipitant concentration, the fractal dimensions of asphaltene aggregates are nearly identical (≈2), indicating a more compact and dense structure. The compact structure of small clusters observed in this study is consistent with previously reported results [17]. The compact structure observed for small clusters can be attributed to cluster restructuring often seen at smaller length scales [17]. Small aggregate clusters were also observed at a precipitant concentration of 57 vol%, which could imply that some asphaltenes are very stable which can only form small fractal aggregates even at high precipitant concentrations. For large clusters, the fractal dimensions were determined to be 1.66 ± 0.17, 1.73 ± 0.17, and 1.83 ± 0.28 for precipitant concentrations at 52 vol%, 55 vol%, and 57 vol%, respectively. A slightly higher fractal dimension with increasing precipitant concentrations indicates that aggregates are less amorphous with more compacted structures. Approaching the precipitation onset point (52 vol% and 55 vol%), the fractal dimensions determined in this study are in good agreement with the findings from the DLA in classic colloids and previous asphaltene aggregation studies [17,22]. Although the mechanisms/factors associated with the observation of compact aggregates structure at higher precipitant concentration are not immediately clear, it is likely due to increased interaggregate attractions (e.g., London dispersion force) or the removal of a steric stabilization barrier [17]. Nevertheless, the fractal dimensions based on image analysis used in this study fall in the range of the d

_{f}values reported in the literature for asphaltene aggregates [15,17]. It is worth noting that the large stitched images used our analysis are two-dimensional projected areas of actual three-dimensional aggregates. Thus, some of the details of the original aggregate structure were obscured in the projection of the resultant profile. The d

_{f}determined here is two-dimensional and therefore intrinsically smaller than d

_{f}determined from three-dimensional based measurement such as light scattering or settling velocity. Thus, the differences between the d

_{f}measured by image analysis and the d

_{f}determined based on the latter techniques should be taken into consideration.

## 4. Conclusions and Environmental Implications

## Author Contributions

## Funding

## Conflicts of Interest

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**Figure 1.**Large-stitched images of asphaltene aggregates at the end of each experiment. (

**A**–

**C**) represent bright-field images of aggregates formed by adding 52 vol%, 55 vol%, and 57 vol% heptane to model oil, respectively. (

**D**–

**F**) represent the corresponding binary images after applying Ostwald’s global thresholding criterion. The scale bar is 50 μm.

**Figure 2.**Morphology parameters and criteria used for identification of single particle, small and large clusters. (

**A**,

**E**,

**I**) demonstrate the definition of Solidity 2, Eccentricity, and Solidity Ellipse, respectively. Specifically, Solidity 2 is the ratio of filled area to bounding box; $Eccentricity=\sqrt{1-\left({b}^{2}/{a}^{2}\right)}$, where $a$ and $b$ are the semi-major and semi-minor axes of the best fit ellipse respectively; Solidity ellipse is the ratio of filled area to area of the best fit ellipse. (

**B**–

**D**,

**F**–

**H**,

**J**–

**L**) represent plots of Solidity 2, Eccentricity, and Solidity Ellipse as a function of Filled Area (in pixel) of aggregates formed respectively, upon addition of 52 vol% (red squares), 55 vol% (blue squares), and 57 vol% (black squares) n-heptane. Small clusters were identified under the following criterion: Solidity 2 < 0.9 & Solidity 2 > 0.7 & Eccentricity < 0.5. Large clusters were identified under the following criterion: Eccentricity > 0.7 or Solidity_ellipse < 0.9. Single particles were identified under the following criterion: Solidity 2 < 0.9 & Solidity 2 > 0.7 & Eccentricity < 0.5 & Filled Area < 2000 (in pixel).

**Figure 3.**Log-log plots of average area A as a function of the radius of gyration ${R}_{g}$ for asphaltene aggregates formed under different precipitant concentrations. (

**A**–

**C**) represents aggregation induced by adding 52 vol%, 55 vol%, and 57 vol% heptane to model oil, respectively. The solid dark blue and black lines represent the best fit line for small clusters and large clusters, respectively. The solid red lines represent the prediction bounds for the fitted functions and the bounds reflect 95% confidence intervals. Single particles were excluded from the regression.

**Figure 4.**Cluster size distribution of aggregates formed at different precipitant concentrations. (

**A**–

**C**) represents aggregation induced by adding 52 vol%, 55 vol%, and 57 vol% heptane into model oil, respectively. Cluster size distributions for each experiment are normalized by their respective mean values to facilitate direct comparison. The red lines are the particle size distribution for Smoluchowski kinetics.

**Figure 5.**Schematic of the proposed asphaltene aggregates structure effects on settling velocity and wettability of aquifer surface, and associated implications for in situ remediation of sites contaminated with heavy oils.

**Table 1.**Estimation of fractal dimension by least square fitting for power law of ${R}_{g}{}^{{d}_{f}}$.

Experiments | Cluster | Goodness of Fit | |||||
---|---|---|---|---|---|---|---|

Best fit d_{f} | R^{2} | SSE | DFE | Adjusted R^{2} | RMSE | ||

1 | Large | 1.66 ± 0.17 | 0.96 | 5.77 | 203 | 0.96 | 0.17 |

Small | 1.99 ± 0.01 | 0.99 | 0.009 | 82 | 0.99 | 0.01 | |

2 | Large | 1.73 ± 0.17 | 0.97 | 4.88 | 163 | 0.97 | 0.17 |

Small | 2.00 ± 0.01 | 0.99 | 0.006 | 34 | 0.99 | 0.013 | |

3 | Large | 1.83 ± 0.28 | 0.96 | 10.87 | 138 | 0.96 | 0.28 |

Small | 2.00 ± 0.01 | 0.99 | 0.004 | 28 | 0.99 | 0.012 |

^{2}: R-squared; and RMSE: root mean square error.

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**MDPI and ACS Style**

Hammond, C.B.; Wang, D.; Wu, L. Precipitant Effects on Aggregates Structure of Asphaltene and Their Implications for Groundwater Remediation. *Water* **2020**, *12*, 2116.
https://doi.org/10.3390/w12082116

**AMA Style**

Hammond CB, Wang D, Wu L. Precipitant Effects on Aggregates Structure of Asphaltene and Their Implications for Groundwater Remediation. *Water*. 2020; 12(8):2116.
https://doi.org/10.3390/w12082116

**Chicago/Turabian Style**

Hammond, Christian B., Dengjun Wang, and Lei Wu. 2020. "Precipitant Effects on Aggregates Structure of Asphaltene and Their Implications for Groundwater Remediation" *Water* 12, no. 8: 2116.
https://doi.org/10.3390/w12082116