Mechanism of modification of bitumen zeta potential by sodium citrate: Interactions between bitumen metal content and ligands

Sodium citrate (Na(3)Cit) has been proved to significantly increase the negative charges on bitumen surfaces, which enhances bitumen liberation and prevents slime coating in the bitumen recovery process. To understand the in-depth theory of Na(3)Cit modifying the bitumen zeta potential, different assumptions of Na(3)Cit interacting with bitumen were proposed and experimentally verified in this study. Accordingly, chemicals with similar properties were selected and compared with Na(3)Cit to confirm the mechanisms. We found that adding Ethylenediaminetetraacetic acid (EDTA) and phosphate (PO43-), which act like citrate as a ligand, resulted in a more negatively charged bitumen surface at pH 8.5 compared with sodium chloride (NaCl). However, even though the sulfate ion (SO42-) carries multiple negative charges, increasing the concentration of sodium sulfate (Na2SO4) led to less negatively charged bitumen surfaces at the same pH, a trend similar to NaCl. Meanwhile, Na(3)Cit and EDTA promoted the release of metal cations from bitumen to the aqueous solution. However, the concentration of metal ions released by sodium phosphate dibasic (Na2HPO4) was much smaller than expected. Adsorption of Na(3)Cit, EDTA, and Na2HPO4 on the bitumen surface was observed through mainly outer-sphere complexation and minor inner-sphere complexation. Our study revealed that ligands such as Na(3)Cit could remove metal ions from bitumen and adsorb onto the bitumen surface, which were the dominant reasons for modifying the zeta potential at the bitumen/water interface.

Automated summary

Sodium citrate has been found to increase the negative charges on bitumen surfaces, leading to enhanced bitumen liberation and prevention of slime coating. Similar chemicals like EDTA and phosphate also increase the negative charges on bitumen surfaces, while sodium sulfate has the opposite effect. Ligands such as sodium citrate can remove metal ions from bitumen and adsorb onto the bitumen surface, modifying the zeta potential at the bitumen/water interface.