Recently, reduction of water consumption by industry has been an important challenge. Also an important challenge is the purification of industrial waste waters containing heavy metals including lead. Enhanced ultra-filtration with polymers is a feasible method to remove metal ions from diluted wastewater streams. Biomacromolecules, biodegradable polymers as biomaterials have an important role in a wide range of industrial fields such as water treatment. For separation of toxic heavy metal ions, including lead ions, several natural polymers have been investigated. The most valuable properties of these biopolymers are their biocompatibility, biodegradability, and flocculating activity for metal ions.
Flocculation is a process that enhance agglomeration or collection of smaller floc particles into larger, more easily sedimentable particles through gentle stirring by hydraulic or mechanical means. The addition of flocculating agents may promote the formation of flocs. Flocculating agents that are usually used for water treatment can be categorized into three major groups, namely, inorganic flocculants, organic synthetic polymer flocculants, and natural biopolymer flocculants. Among these flocculants, organic synthetic polymers are widely used, because they are inexpensive and effective. However, the use of these flocculants may give rise to environmental and health problems, because some of them are not completely biodegradable, and the intermediate products of their degradation are toxic for humans. To solve these problems, biodegradable flocculants have been investigated to minimize the risk for the environment and humans.
Flocculation of polyelectrolytes in the presence of bivalent ions is an important process, and is widely used in water treatment technologies. For separation of toxic heavy metal ions, natural poly-γ-glutamic acid (γ-PGA) and other natural polymers have been investigated. The aggregate size distribution in flocculants was studied and it was found that it mainly depended on the pH and concentration of the electrolytes.
Poly-γ-glutamic acid consists of repetitive glutamic acid units connected by amide linkages between α-amino and γ-carboxylic acid functional groups. The secondary structure of PGA has been described as α-helix in a water solution. The naturally produced γ-PGA usually contains nearly equal amounts of D- and L-units and the molecular weight of the polymer is usually high (up to 1 million). The ratio of the two optical isomers can partially be controlled by technological means so that polymers with varying degree of stereoregularity can be made. γ-PGA is different from other proteins, in that glutamate is polymerized via the γ-amide linkages, and thus is synthesized in a ribosome-independent manner.
γ-PGA is a water soluble, biodegradable, edible and nontoxic polyanion for the environment and humans. Therefore, the potential applications of γ-PGA and its derivatives have been in the limelight in the past few years in a wide range of industrial fields such as cosmetics, food, medicine, and water treatment.
The γ-PGA as a polyacid has appropriate flocculating activity, high affinity and binds proportional toxic heavy metal ion, such as lead ions. Therefore, the biopolymer may be useful for treating wastewater.