Aggregation of Sediment and Bacteria with Mucilage from the Opuntia ficus-indica Cactus

Flocculants are commonly used in industrial settings where solid-liquid separations are desired including industrial and municipal wastewater management and potable water production facilities. Conventional flocculants include inorganic metal salts and synthetic organic polymers. The cost, availability, and harmful effects of the non-biodegradable nature of these flocculants have led to the widespread study of natural flocculants. Current natural flocculants being studied include polysaccharides cultivated from microbial extracellular matrix products and plant based materials. In this study, the mucilage of Opuntia ficus-indica cactus was evaluated as a natural flocculant for sediments and bacteria. The O. ficus-indica cactus is also known as the nopal or prickly pear and is commonly used as a food source in Mexico and Latin America. Using simple extraction techniques non-gelling (NE) and gelling (GE) mucilage extracts were isolated from fresh cactus pads. Column tests were used to evaluate the flocculation and removal of suspended sediment and bacteria caused by mucilage addition. Throughout this work the mucilage\u27s ability as a flocculant was evaluated by varying mucilage type and concentration, suspended contaminant type and concentration, and cation type and concentration. Many of the results are explained in terms of the morphology and chemical composition of the GE and NE mucilage extracts. The extracts consist primarily of polysaccharides and differences in physical structure between mucilage types were seen using atomic force microscopy and transition electron microscopy. A variety of suspended particles were used to evaluate the mucilage as a flocculant including kaolin, acid-washed kaolin, and bacteria. The bacteria employed in this study include Bacillus cereus, Escherichia coli HB101 K12, and fully attenuated Bacillus anthracis Sterne strain. The addition of monovalent (Na+ and K+), divalent (Ca2+ and Mg2+), and trivalent (Al3+ and Fe3+) cations was studied alone and in combination with mucilage. In cation studies Ca2+ had the most profound effect on flocculation efficiency; therefore its efficacy was further explored. Mucilage was most effective with dosages between 5 and 50 mg/L for the contaminants studied. Using these optimal concentrations, 20 to 200 L of water could be treated with only 1 g of mucilage powder. Based on the extraction method used in this work, 1 g of mucilage can be obtained from a cactus pad weighing approximately 250 g (wet weight). Mucilage remained an effective flocculant over a wide range of suspended contaminant concentrations showing that mucilage is a versatile flocculant that can be tailored for a variety of applications. Overall, this work shows that the O. ficus-indica cactus is an effective flocculant for suspended sediments and bacteria. The cactus\u27 low cost, abundance, and current use in many areas make it an attractive alternative to traditionally used flocculants. Additionally, this work builds upon existing knowledge pertaining to natural flocculants and could offer insight into their general behaviors for water treatment applications