Novel Biomaterials

Heavy metals are important among the toxic pollutants encountered in various ecosystems of the environment. The dissolved metals (particularly heavy metals) escaping into the environment pose a serious health hazard. These metals have been classified as priority pollutants by the US Environmental Protection Agency. Heavy metal pollution in the aquatic system has become a serious threat today and of great environmental concern as they are non-biodegradable and thus persistent. They accumulate in living tissues throughout the food chain which has humans at its top, multiplying the danger. Thus, it is necessary to control presence of heavy metals in the environment (Fig. 1).

Several efforts have been made to detoxify the effect of metals once they are administered in the human body. Chelation is considered the best method used so far. Medicinal treatment of acute and chronic metal toxicity is provided by chelating agents. Chelation is one of the chemical functions that take place in the bodies of almost all living organisms. It is a process by which plants and animals utilize inorganic metals. Chlorophyll, the green matter of plants, is a chelate of magnesium. Hemoglobin, cytochrome C, catalase, and peroxidase are chelators of iron. A host of other metallo-enzymes could be used as examples involving chemical processes. Many of the successful drugs used in the treatment of disease are dependent on chelation processes for their effective therapeutic properties. Chelating agents are organic compounds capable of linking together metal ions to form complex ring-like structure called chelates. Chelate is derived from a Greek word meaning the claws of a lobster and somehow the chelators act in this way. Chelators form complex with the respective (toxic) ion. These complexes reveal a lower toxicity and are more easily eliminated from the body. This chapter focuses on the chemistry of these chelating agents and their pharmacological and toxicological properties. The beneficial and adverse effects including their limitations are briefly mentioned along with the recent developments to ameliorate the problems.

The constantly increasing degree of industrialization and rising standards of living are strongly impacting on the use of available water sources. Controlling heavy metal discharges and removing toxic heavy metals from aqueous solutions have become a challenge for the twenty-first century. The commonly used procedures for removing metal ions from aqueous streams include distillation, ion exchange, reverse osmosis, electrodialysis, precipitation, coagulation, flocculation, and nanofiltration. The basic principle, procedural details, and commercially available instrumentation based on the above phenomenon are described in brief below.

The currently practiced technologies for removal of pollutants from industrial effluents appear to be inadequate, often creating secondary problems with metal-bearing sludge, which are extremely difficult to dispose of. The process cannot precipitate metals to low levels of solubility unless additional treatment reagents are employed, the use of which may significantly add to the volume of sludge. Most authorities consider such treatments to be best performed in specialized environments by trained personnel. Further, research findings have clearly raised strong doubts about the advisability of the use of synthetic coagulants used for metal removal. Their usage leads to several serious demerits concerned with the central nervous system of human beings. For electroplating and finishing industries, the cost involved in the treatment of effluent produced is sometimes prohibitively expensive, especially for the smaller installations, and far outweighs the advantages of recycling and regenerating materials. Present treatment strategies require costly chemical and physical operations and involve a high degree of maintenance and supervision. While wastewater treatments by ion exchange resins are both effective and convenient, they are too expensive to be used by the developing countries and their availability is limited to developed nations of the world. Government closely monitors their handling and disposal, in most instances. Processes like reverse osmosis include many of the small organic molecules that are precursors of trihalomethane generated during chlorine disinfection. The Environmental Protection Agency (EPA) strictly limits the allowable amounts of these compounds in drinking water based on the possibility that chronic exposure to them could cause cancer.

The prime requisite of agriculture is soil which serves as a reservoir of nutrients and water for the crops. Unfortunately, all the soil available on this planet is not arable, fertile, and it remains agriculturally unproductive. Land is mainly contaminated with heavy metals like Zn, Pb, Cr, Ni, and Cd. Metal-rich mine tailings, metal smelting, electroplating, gas exhausts, fuel production, downwash from power lines, intensive agriculture, and sludge dumping are the most important human activities that contaminate soils with large quantities of toxic metals. Metals are non-biodegradable and have long biological half-life. Remediation of soil and water metals has been found to be a difficult and expensive goal. The remediation of heavy metal-contaminated soils can be achieved through various physical and chemical conventional remediation measures. Due to ever-growing number of toxic metal-contaminated sites, the commonly used methods dealing with heavy metal pollution are either extremely costly processes or simple isolation of the contaminated sites. The remediation of large volumes of contaminated sites by conventional technologies has been proved to be very expensive. It has been estimated that the cost of conventional remediation of heavy metal-contaminated sites in the USA alone would exceed $7 billion.

Unfortunately, the science particularly chemistry, despite numerous contributions to the well-being and progress of humanity, has been blamed for the present ills of the world. In fact, it is not chemistry or science or technology but our past mistakes of increasing only the production without considering the simultaneous generation of large amounts of side products or waste which have underlined us as the culprit. Basically unscientific and careless rapid urbanization, industrialization, and agriculturalization are major threats to the environment. It is not the need of poor but the greed of rich persons, which has been the main cause of environmental degradation of the world.

The complex structure of plant materials and microorganisms implies that there are many ways for the metal to be taken by the biosorbent. Numerous chemical groups have been suggested to contribute to biosorption metal binding by either whole organisms or molecules. These groups comprise hydroxyl, carbonyl, carboxyl, sulfhydryl, thioether, sulfonate, amine, amino, imidazole, phosphonate, and phosphodiester. The importance of any given group for biosorption of certain metals by plant biomass depends on factors such as number of sites in the biosorbent material, the accessibility of the sites, the chemical state of the sites (availability), and affinity between site and metal (Volesky et al. 1999). Adsorption and desorption studies invariably yield important information on the mechanism of metal biosorption. This knowledge is essential for understanding of the biosorption process and it serves as a basis for quantitative stoichiometric considerations, which constitute the foundation for mathematical modeling of the process (Yang and Volesky 2000).

Biosorption promises to fulfill the requirements, which are competitive, effective, and economically viable. Efforts have been made to use different forms of inexpensive plant materials for the abatement of toxic metals from the aqueous media. Biosorbents, explored so far in removing toxic metals from water bodies, have been listed in Table 1.

Over the past few years, intensifying research into metal biosorption elucidated the principles of this effective metal removal phenomenon. Biosorption can be cost-effective, particularly in environmental applications where low cost of the metal removal process is most desirable. Some efficient natural biosorbents have been identified that require little modification in their preparation. It is particularly in ecological aspects where biosorption can make a difference due to its anticipated low cost. The application aspect is what makes the research and development work in this novel area exciting and worthwhile. While the biosorption process could be used even with a relatively low degree of understanding of its metal-binding mechanisms, better understanding will make for its more effective and optimized applications. If the biosorption processes were to be used as an alternative in the wastewater treatment scheme, the regeneration of the biosorbent may be crucially important for keeping the process cost down and to open the possibility of recovering the metals extracted from liquid phase. For this process it is desirable to desorb the sorbed metals and to regenerate the biosorbent material for another cycle of application.

The biosorption experiments involve the following major steps:

Percentage removal for metal ions is calculated in terms of sorption efficiency of the respective biosorbents under various experimental conditions, viz., biomaterial dosage, contact time, metal concentration, optimum particle size, volume, and pH. The data are to be handled with appropriate statistical treatment and tabulated. The concentration of the removed metals may be represented in terms of μmol, μg, and ppm. The representative tables exhibiting the sorption efficiency of particular biosorbents used for the decontamination of toxic metals from water bodies are given below. The influence of each variable is taken into account for its effect on the sorption phenomenon.

The distribution of metal ions between the biosorbent and the metal solution, when the system is at equilibrium, is of paramount importance in determining the maximum adsorption capacity of the biosorbent toward the metal ions. The adsorption data are to be analyzed in the light of various isotherm models like Freundlich and Langmuir adsorption models. The Freundlich isotherm model proposes a monolayer sorption with a heterogeneous energetic distribution of active sites, accompanied by interactions between adsorbed molecules. The Langmuir isotherm model suggests that uptake occurs on a homogenous surface by monolayer sorption. In addition, the model assumes uniform energies of adsorption onto the surface and no transmigration of the adsorbate.

Regeneration of metal-treated biosorbent is an important aspect of cost-effectiveness of wastewater treatment. In general, desorption behavior of metals from biomaterials is usually carried out by using an appropriate stripping agent. Desorption behavior of metal ions from biosorbents is to be observed after eluting with different stripping agents (soft acid, hard acid, base, and distilled water).

Various important techniques used for the characterization of metal–biomaterial interaction responsible for sorption phenomenon are Brunauer–Emmett–Teller technique, Fourier transform infrared spectroscopy, scanning elecron microscopic technique, and X-ray diffraction. The salient features of some of them are explained here.

Various mechanisms for the sorption of metal ions onto biomaterials have been discussed in the literature based on the presence of different functional groups like polyphenols, carbohydrates, polypeptide hydroxyl groups, sulfonic acid groups, nitro, carboxyl acid groups, and proteinaceous amino acids. Among the above functional groups, proteinaceous amino acids have been found to play an important role in metal sorption mechanism. The proteinaceous amino acids have a variety of structurally related pH-dependent properties of generating appropriate atmosphere for attracting the cationic and anionic species of metal ions simultaneously. This chapter is devoted to provide experimental proof that carboxyl and amino ligands present in the biomaterial are responsible for metal binding.

Biomaterials have been found to be associated with drawbacks related to less sorption efficiency and stability, restricting their commercial use (Pagnanelli et al. 2000; Scowronski et al. 2001). Sincere efforts toward structural modifications on the biomaterials leading to the enhancement of binding capacity and selectivity are, therefore, in great demand. A special emphasis is to be paid on green chemical modifications resulting in tailored biomaterials improving its sorption efficiency and environmental stability and thus making it liable for its commercial use as simple, fast, economical, eco-friendly green technologies. Attention has been paid by various research groups (Tsezos 1985; Saito et al. 1991; Gardea-Torresdey et al. 1998) to increase the sorption capacity of biomaterials for abatement of different metal ions. Klimmek and Stan (2001) reported that the maximum sorption capacities of the alga Lyngbya taylorii could be increased significantly after phosphorylation of the biomass. Bai and Abraham (2002) noted that the sorption ability of the fungus Rhizopus nigricans for Cr(VI) is also improved after the introduction of carboxyl and amino groups. The pretreatment of biomass with surfactants and cationic polyelectrolyte and deacetylation treatment of amino groups of chitin are favorable for abatement of metal ions (Tan and Cheng 2003). Pretreatment methods using different kinds of modifying agents such as base solution, organic acid solutions, and oxidizing agents have been used for the purpose of removing soluble organic compounds. Increase in efficiency of metal adsorption has been recently performed by many researches (Taty-Costodes et al. 2003; Gupta et al. 2003; Min et al. 2004; Acar and Eren 2006; Abia et al. 2006; Wankasi et al. 2006; Hannafiah et al. 2006). These chemical modifications in general improved the adsorption capacity of biomaterials probably due to higher number of active binding sites after modification, better ion exchange properties, and formation of new functional groups that favor metal uptake. The semi-synthetic biomaterials are deemed to be good candidates for their commercial use as biomaterials for removing toxic metals from wastewater with high adsorption efficiency.