Pollution Prevention Technology for the Removal of Gold(Ⅲ) from Aqueous Solutions by Medicago sativa(Alfalfa)

摘要

The allure of gold and other precious metals to the mining industry has increased within the last decade primarily as a result of their high metal prices. As mining technology has increased, many old and abandoned mines have been reopened. Technologies such as heap leaching have established themselves as economical methods for the recovery of precious metals from low grade mineral deposits. Amalgamation, cyanidation, and thiourea leaching are some of the methodologies that are used to separate gold from their ores. These gold recovery processes require the use of hazardous chemicals, such as cyanide, which pose a serious threat to the public's health. Due to these health concerns, there is a need for the development of an environmentally friendly alternative for the recovery of precious elements. It has been known for quite some time that plants have the unique ability to uptake gold from the soils and accumulate gold in their tissues. As early as 1900, gold was detected in plant tissues as a method of fire ashing was used to obtain gold beads from hardwood trees. It has also been suggested that black gold deposits might be due to the dissolution of fine gold particles in water by humic substances derived from decaying vegetable matter. Dissanayake and coworkers found gold and platinum accumulated in natural deposits of peat and algal biomats. The peat was found to contain gold in the colloidal phase where platinum was bound on the humic material. In addition, Gardea-Torresdey and coworkers used modified biomass to construct a biosensor for the detection of gold in solution. Therefore gold mineralization by biomaterials from aqueous solutions may be an alternative to the use of hazardous chemicals. Medicago sativa (Alfalfa) has been found to tolerate heavy metals contaminated soils. Gardea-Torresdey et al., have shown that alfalfa is a potential source for removal and recovery of heavy metal ions. Batch laboratory experiments have determined that alfalfa possess the ability to bind various heavy metal ions from aqueous solutions. In addition, considerable amounts of the bound metal ions were recovered from the reusable silica-immobilized biomaterial. This biorecovery process for heavy metals could also be utilized for the removal and recovery of gold(Ⅲ) from aqueous solution. Therefore, the development of a new recovery method using alfalfa biomass to collect gold(Ⅲ) from industrial waste sources might not only reduce the risk of hazardous chemical use, but it may also be a more cost-effective method as well. The objective of this study was to investigate the ability of alfalfa roots and shoots to bind gold(Ⅲ). Batch laboratory pH profile, time dependency and capacity experiments were performed to determine the binding characteristics of the Malone shoots and roots to gold(Ⅲ). Batch recovery experiments were carried out to determine the amount of gold that could be recovered after being bound by the alfalfa shoots and roots. In addition, column experiments were performed with silica immobilized Malone alfalfa shoots to determine the extraction and recovery ability of gold(Ⅲ) under flow conditions. These studies may be useful in the development of an innovative method for gold removal and recovery from mining leachate and smelting waste waters through phytofiltration.