Effect of a Natural Cactus Based-Mucilage Dispersant on the Surface Tension and Droplet Size of Dispersed Crude Oil

The purpose of this work is to evaluate the effect of the cactus plant-based mucilage on the surface tension and droplet size of dispersed crude oil in oil in water emulsions. The mucilage is extracted from the cactus plant, Opuntia ficus-indica, commonly known as Nopal or Prickly pear cactus. The extraction process yields two types of mucilage: the non-gelling extract (NE) and the gelling extract (GE). Cactus mucilage extracts and conventional dispersants were evaluated under different conditions and concentrations, including two concentrations of oil (3, 6% v/v) and several dispersant to oil ratios (1:1,1:3, 1:20, 1:40) at room temperature. Synthetic seawater and surrogate oil from Britsh Petroleum (properties similar to the Macondo rig) were used. The results of the dispersion obtaned using mucilage extracts were compared with results using conventional dispersants (Corexit EC9500A). The average droplet size was smaller in the systems with cactus mucilage (1.5% NE generated 5 µm droplets) when compared with the systems using the conventional dispersant Corexit EC9500A (1.5% EC9500A formed 6.2 µm droplets). The surface tensions of Oil/Water emulsion (3, 6% v/v) were reduced from around 50 mN/m to 30 mN/m by adding the cactus mucilage extracts. A standard EPA toxicity test using Daphnia magna colonies was exposed to both NE and GE mucilage extracts in concentrations ranging from 0 to 5000 mg/L showed that mucilage can be considered non-toxic to the evaluated species with a median lethal concentration (LC₅₀) above 2000 mg/L. Cactus mucilage is an alternative technology to mitigate the damage that oil may cause to the aquatic ecosystem and minimize undesired effects associated with the use of synthetic dispersants in oil spills.