**3. Conventional methods for heavy metal removal**

**1. Introduction**

70 Biosorption

**2. Toxicity of heavy metals**

Water plays an important role in the world economy. Majority (71%) of the Earth's surface is covered by water, but fresh water constitutes a miniscule fraction (3%) of the total. Water fit for human consumption is obtained from the fresh water bodies. Approximately, 70% of the fresh water goes to agriculture. This natural resource is becoming scarce at many places and its unavailability is a major social and economic concern [1]. Though access to safe drinking water has improved over the last few decades, it is estimated that five million deaths per year are caused due to consumption of polluted drinking water or drought. In many developing countries, 90% of all wastewater still goes untreated into the fresh water bodies making it unfit for human consumption, which either leads to scarcity or affects the human population [2]. The concern to protect fresh water bodies for a healthy population is a challenge in recent times. Industrialization to a larger degree is responsible for the contamination of environment especially water where lakes and rivers are overwhelmed with a large number of toxic substances. Heavy metals are reaching hazardous levels when compared with the other toxic substances [3]. Heavy metals are a unique group of naturally occurring compounds. Their continuous release leads to overconsumption and accumulation. As a result, people around the globe are exposed to adverse consequences of these heavy metals. Many industries (fertilizers, metallurgy, leather, aerospace, photography, mining, electroplating, pesticide, surface finishing, iron and steel, energy and fuel production, electrolysis, metal surface treating, electro-osmosis, and appliance manufacturing) discharge waste containing heavy metals either directly or indirectly into the water resources [4]. Toxic heavy metals, which are of concern, are chromium (Cr), lead (Pb), zinc (Zn), arsenic (As), copper (Cu), nickel (Ni), cobalt (Co), cadmium (Cd), mercury (Hg), and so on. As these metals are not biodegradable, they tend to accumulate in the living organisms and lead to various diseases and disorders which ultimately threaten human life. They can cause ill health, even when present in the range of parts per billion (ppb) [5]. Biosorption has emerged as an attractive option over conventional methods for the removal of heavy metal ions from effluents discharged from various industries which ultimately reach and pollute fresh water bodies. This chapter reports the toxicity of heavy metals, the advantages of biosorption, various biosorbents used for the removal of metal ions, effect of immobilization and modifications of biosorbents, various factors affecting the process of biosorption, different bioreactors used in biosorption, and the application of biosorption for the removal of metal ions from various wastewaters like industrial effluents and contaminated water resources. The recent advances, current status, and future of the process are discussed.

The pathway of exposure for heavy metals is mainly through inhalation, dermal contact, and ingestion. The individual metal exhibits its own specific signs of toxicity [6]. The severity of health effects is dependent on time and dose, the type of heavy metal, and its chemical form. The nature of effect may be toxic, mutagenic, neurotoxic, teratogenic, or carcinogenic [6]. Many studies reported that heavy metals affect cell organelles and interact with cell components causing cell damage and apoptosis. Even at a low level of exposure, they induce multiple organ damage. Heavy metals like nickel, copper, zinc, cadmium, chromium, lead, and mercury are major pollutants that affect the fresh water reservoirs due to the discharge of large amounts of metalcontaminated wastewater from industries. Because of their persistent, non-biodegradable, and toxic nature, they accumulate in the environment such as in the food chain and cause serious health disorders. Over the last few decades, many conventional treatment methods have been used for the removal of heavy metals from contaminated wastewaters. The commonly used methods include chemical precipitation, ultra-filtration, ion exchange, reverse osmosis, electro winning, and phytoremediation, and they are introduced briefly [10–14].

**Chemical precipitation** is the most widely used method for heavy metal removal from inorganic effluents. The conceptual mechanism involved is that the dissolved metal ions get precipitated by chemical reagents (precipitants) and result in the formation of metal hydroxides, sulfides, carbonates, and phosphates (insoluble solid particles) that can be simply separated by sedimentation or filtration.

**Ion exchange** is based on the reversible exchange of ions between solid and liquid phases. An ion exchanger is a solid resin capable of exchanging both cations and anions from an electrolytic solution and releases counter-ions of similar charge in a chemically equivalent amount.

**Membrane filtration** is capable of removing not only metal ions but also suspended solid and organic components. A membrane is a selective layer used to make contact between two homogenous phases with a porous or non-porous structure for the removal of pollutants of varied size.

**Ultrafiltration (UF)** is a permeable membrane separation process with pore sizes in the range of 0.1–0.001 micron which permeates water and low molecular weight solutes, while retaining the macromolecules, particles, and colloids that are larger in size. The removal of Cu (II), Zn (II), Ni (II), and Mn (II) from aqueous solutions was achieved by using ultrafiltration assisted with a copolymer of malic acid and acrylic acid attaining a removal efficiency of 98.8% by forming macromolecular structures with the polymers which are rejected by the membrane [15].

**Microfiltration (MF)** works with the same principle as ultrafiltration. The major difference between the two processes is that the solutes which are removed by MF are larger than those rejected by UF. Cross-flow microfiltration (CFMF) in yeast-based bioaccumulation process was used for the removal of metal ions from tap water artificially contaminated with Cu (II), Cd (II), Pb (II), and Cr (III). The method was efficient for the removal of metal ions with an efficiency of 31, 7, 63, and 71%, respectively [16].

**Nanofiltration (NF)** is used for the separation of large molecules possible by small pores when they are within the molecular weight range from 300 to 500 Da with a pore diameter of 0.5–2 nm. A commercially available nanofiltration membrane NF270 was used for the removal of Cd (II), Mn (II), and Pb (II) with an efficiency of 99, 89, and 74%, respectively [17].

step is the adsorption of metal ions onto cells, which is quick and identical to biosorption, and the later step is slower which includes the transport of metal species inside the cells by active transport [25]. Unlike biosorption, it is an irreversible, complex process which depends on the metabolism of the cells. The process of bioaccumulation occurs by cultivating the biomass of a microorganism in the vicinity of the metal to be accumulated. Since the solution contains the growth medium, the organism begins its metabolic processes and activates the intracellular transport systems for the accumulation of the sorbate. However, the major limitation of the process is that the nutritive medium for growth of the microorganism contains organic carbon sources [26, 27]. Bioaccumulation is an active process which requires a living biosorbent and is mediated by the metabolism of the microorganism used. The process operates by cultivating the microbe in the presence of a metal ion which has to be removed. Part of the biosorbate accumulates inside the cell which enables the biomass to increase and bind greater amounts of metal ions. The organisms which are capable of resisting high loads of metal ions are best suited for accumulating metal species. They do not possess any mechanisms for hindering the accumulation of metal ions in large quantities [28]. They may possess special mechanisms for synthesizing special intracellular binding regions rich in thiol groups as a response to metal ions in their surviving environment. It was found that morphology and physiology of the cell changes upon increase in concentration of the metal ion to be accumulated [29]. Efficient bioaccumulation can be achieved by selecting the microbes that are screened from polluted environments [30]. *Pichia stipitis* yeast was capable of bio-accumulating Cu (II) and Cr (III) with the maximum uptake capacity of 15.85 and 9.10 mg/g, respectively, from aqueous solutions with an initial concentration of 100 ppm at pH 4.5 [31]. *Aspergillus niger* was capable of removing Cu (II) and Pb (II) with the maximum uptake capacity of 15.6 and 34.4 mg/g, respectively [32]. **Table 1** summarizes some more

Application of Biosorption for Removal of Heavy Metals from Wastewater

http://dx.doi.org/10.5772/intechopen.77315

73

examples of biosorbents used for metal bioaccumulation.

*Aspergillus flavus* Cu (II) 93.65

*Bacillus megaterium* 32

**Table 1.** Use of microorganisms for bioaccumulation of metal ions.

*Drepanomonas revolute Uronema nigricans Euplotes* sp.

the wet weight of the biosorbent.

a

**Biosorbent type Metal ion Uptake capacitya**

*Pichia guilliermondii* Cu (II) 20 [29] *Aspergillus niger* Pb (II) 172.25 [33]

*Bacillus circulans* Cr (VI) 34.5 [34]

*Saccharomyces cerevisiae* Cr (III) & (VI) 11.3, 3.3 [35]

Zn (II), Cd (II), Cu (II) 24.3, 0.37, 0.95 Zn (II), Cd (II), Cu (II) 71.5, 0.83, 0.25

Since the process of bioaccumulation is achieved with the living organisms, the uptake capacity was determined with

Zn (II), Cd (II), Cu (II) 22.1, 0.75, 0.2 [36]

 **(mg/g) Reference**

**Reverse osmosis (RO)** is a pressure-driven membrane separation process that forces the solution to pass through a semi-permeable membrane for the removal of heavy metals from various industries. Reverse osmosis was used for the removal of Cu (II), Ni (II), and Zn (II) by using a polyamide thin-film composite membrane TW30-1812-50 [18].

**Electrodialysis (ED)** is a novel liquid hybrid membrane separation process used for the separation of ionized species in the solution that passes through an ion exchange membrane when electric potential is applied or due to concentration gradient. The removal of heavy metal ions in groundwater in Korea was achieved by an ED system for the removal of arsenic, lead, manganese, and nitrate nitrogen with 73.9, 89.9, 98.9, and 95.1%, respectively [19].

**Photocatalysis** is used for the rapid and efficient destruction of environmental pollutants by using semiconductors which are non-toxic. This method is achieved by a five-step process: transfer, adsorption to the surface of the semiconductor, photocatalytic reactions at the surface, and finally decomposition and removal of the pollutants at the interface region. The heavy metals present in the pharmaceutical waste were photocatalytically degraded and removed by using selenium-doped ZnO nanocomposite semiconductor and the removal capacity was found to be 0.421 (Cu), 0.211 (Cr), 0.147 (Pb), and 0.097 (Cd) per 0.5 g of ZnO/Se nanocomposite [20].

Besides these conventional methods, techniques like coagulation/flocculation [21], electrocoagulation [22], electro-floatation [23], and electro-deposition [24] have been used for the removal of heavy metals from contaminated water resources. However, all the above-mentioned technologies are associated with various disadvantages like incomplete metal removal, generation of sludge, high reagent and energy requirements, and aggregation of metal precipitates and fouling of the membranes.
