Keywords

1 Introduction

Heavy metals (HMs) are defined as metallic element with higher density and have toxicity at lower concentration. Heavy metals belong to the group of metal or metalloids with atomic density higher than 4 g cm−3 or five times or more greater than water (Kumar et al. 2015). However, in biochemistry HMs are defined as metallic elements with Lewis acid behavior, i.e., electron pair acceptor. About 53 chemical elements are considered as heavy metal, and most of these HMs are found as natural constituent of the earth crust and soil. From environmental perspective, any metals or metalloid that poses potential harmful effects to the living organism even at lower concentration can be termed as heavy metals. Most of these heavy metals (Zn, Cu, Mn, Ni, and Co) have vital function in plants, while other metals such as Cd, Pb, Hg, and Cr are found to cause toxic effects in biological system. The combined influence of urbanization, industrialization, and chemical consumption in agrarian practices has raised the heavy metal concentration up to the toxic level; hence its remediation is now considered as global concern. HM contamination in water is one of the most critical environmental problems, which include Cd, Cr, Cu, Hg, and Zn as common contaminants (Pathak et al. 2019; Ahmad et al. 2018; Kothari et al. 2012).

Heavy metal contamination in wastewater depends on industrial processing and is stated an as anthropogenic activity. HMs are classified as conservative pollutants, and their accumulation in the environment causes various negative impacts such as inhibition of photosynthesis and seed germination, decreased enzymatic activity, reduction of chlorophyll production, etc. In order to prevent the negative impacts of heavy metals, adequate treatment of wastewater is desired prior to its disposal or discharge in receiving water bodies. Chemical and biological treatment methods are available for HM removal, but biological methods are preferred because of limitation and drawbacks in chemical treatments. Bioremediation is a key process that utilizes microbes to tackle heavy metal pollution (Pathak et al. 2015). Biological processes to remove heavy metals are well explored by various researchers as a part of phycoremediation. However, remediation of heavy metals via algae gained substantial attention due to its effectiveness and feasibility in implementation. Algae offer potential solution for treatment of industrial wastewater containing heavy metals in a natural way. Algal-based remediation can be termed as phycoremediation, which not only resolves the challenges associated with conventional treatment methods but is also considered as an economically viable and environment-friendly treatment option (Ungureanu et al. 2015). Phycoremediation involves the natural ability of alga to uptake the nutrient, accumulate the heavy metals, and degrade the organic contaminant via symbiotic interaction with aerobic bacteria. Algae resemble the pigment of higher plants with higher photosynthetic efficiency; hence algae released greater extent of oxygen in aquatic system and induce the aerobic degradation of organic compounds (Majumder et al. 2015). Alga is found to have the ability to utilize waste as nutritional source, and it reduces the pollutants through metabolic and enzymatic processes. The xenobiotics and heavy metal pollution can be detoxified, transformed, and volatilized through the algal metabolic pathways (Gautam et al. 2015). Therefore, biological method employing algae has various advantages such as (i) minimum capital and operating cost compared to physicochemical/oxidation process (Mane et al. 2011), (ii) true destruction of organic and inorganic pollutants (Parameswari et al. 2010), (iii) oxidation of wide range of organic compounds, (iv) removal of reduced inorganic compound, i.e., sulfides and ammonia (Praepilas and Pakawadee 2011), etc.

Biosorption is the dominant mechanism in uptake of heavy metals either by active algal biomass (AAB) or passive algal biomass (PAB) and found as a cost-effective solution to eliminate HMs from industrial effluent. In case of passive algal biomass, biosorption doesn’t involve in metabolic pathway; however, it entirely depends on interaction between the biomass and metal ion; hence it resembles with the binding of metal ions through ion-exchange resins. Contrary to the ion-exchange resins, biosorption involves various steps such as chelation, partial adsorption, complexation, micro-precipitation, etc. On the other hand, biosorption in active algal biomass is carried out through energy-mediated transport of metal ions through the cell membrane. The ability of metal sorption through various organisms has been widely reviewed by researchers and concluded that PAB have massive potential to bind metal ions from very low concentration in the external solution. It has also been reported that biosorption is significant to remove toxic metal and at the same time, recovery of valuable metals such as gold, silver, and radionuclides is also possible. Regardless of various advantages with algal-based metal uptake, several drawbacks (low biomass generation, cost-effective biomass production less effective to remediate various industrial wastewaters) are also associated with phycoremediation. Therefore, phycoremediation coupled with effective cultivation system (solar-driven open pond or close photobioreactor) requires extensive investigation (Kothari et al. 2017; Ahmad et al. 2017). The present review deals with the various aspects of HM removal as well as factors affecting the metal removal efficiency.

2 Hazardous Effects of Heavy Metals

Heavy metals are considered to have hazardous effect on the flora and fauna. Key source of heavy metals involving anthropogenic activities such as extraction, excavation, etc. is depicted in Fig. 3.1. Nonpoint source pollution such as haphazard disposal and release of industrial and domestic wastewater in aquatic ecosystem, with even trace concentration, threatens the aquatic flora and fauna. Even trace concentrations of heavy metal pose significant problems to flora and fauna given in Table 3.1. Heavy metals don’t participate in the metabolism of the body, but it is accumulated through different mechanisms which include bioaccumulation, bioconcentration, and biomagnifications illustrated in Fig. 3.2. Organisms exposed in heavy metals tend to have protective mechanism against heavy metal toxicity. The HM concentration beyond a threshold limit causes direct toxicity to the aquatic flora and fauna.

Fig. 3.1
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Source of HMs in the environment

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Table 3.1 Maximum contaminant level (MCL) standards for the most hazardous HMs
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Fig. 3.2
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Effect of HMs on living organism

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2.1 Biosorption of HM Ions Using Algae

Biosorption process involves sorption of material in contact via biopolymer or biomaterial. It is found effective in detoxifying heavy metals in lower concentration even with less biomass supplementation with no additional nutrient requirement. Presence of organic ligands or the functional groups (carboxyl, hydroxyl, sulfate, phosphate, and amine group) in structural components of algal cell makes it as a potential biosorbent.

Moreover, studies have shown that inactive biomass may be even more effective than active (living) algal cell for removal of heavy metals (Gautam et al. 2015). Active algal biomass-based heavy metal removal is based on the efficacy of algal growth in heavy metal containing aqueous solution, which may pose toxic effects to the algal cells resulting in variation in heavy metal removal capacity. Heavy metal uptake by active algal biomass is more complicated than the inactive biomass as metals are absorbed and involved in intracellular pathway of living algal cells (Misbah et al. 2014). In contrast, PAB cells adsorb HM ion on the surface of the cell wall. PAB can be observed as an aggregation of polymers (carbohydrates, cellulose, pectin, glycoprotein, etc.) that is capable of binding with HMs as adsorbents with the efficient and cost-effective wastewater treatment.

2.2 Cellular Sites Involved in HM Binding

HM ions bind to the AAB and PAB cell surface and are also transported within the cell, whereas the adsorptions process does not depend on metabolic process, requiring several metal transporters (Barakat 2011). Several AAB have metal efflux metabolism-driven systems for maintaining the HM concentration in intracellular space avoiding HM toxicity. Inside the cell HM ion can be distributed in cell vacuoles and organelle given in Fig. 3.3. Several cell-derived biomolecules (polyphosphates, phytochelatins, metallothioneins, metalloproteins, etc.) help in the sequestration of HMs from the wastewater. Besides this, the enzymatic reaction can alter the oxidation number of HMs and change into less toxic forms. Micro-precipitation of HM removal in the form of phosphates and sulfates by AAB is a potential approach to remove HMs from wastewater (Ungureanu et al. 2015). In case of PAB, drying and crushing should increase the metal-adsorbing capacity. The algal cell wall made up of microfibrillar exo-polysaccharides has typical chemical composition and contains functional groups as shown in Table 3.2 such as –COOH−, -OH−, -PO4−3, -RSH, SO4−2, etc.

Fig. 3.3
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Cellular mechanism of HMs: (a) inside the living cell; (b) on the cell surface

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Table 3.2 Functional groups involved in metal ion biosorption and adsorption
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These functional groups produce anionic nature to the cell wall and microfibrils. Since HM ion in wastewater is cationic in nature, they are adsorbed by the cell and microfibril surface. Cyanobacterial cell wall consists of mainly peptidoglycan, polymer of N-acetylglucosamine and β-1,4-N-acetylmuramic acid, which provides mostly –COOH functional group for HM adsorption. Few cyanobacterial cell walls bear capsule wall which is anionic in nature due to acid in nature and thus help in metal adsorption. Eukaryotic algae cell wall contains heteropolysaccharides, which offer -COOH and SO4−2 groups for HM adsorption (David et al. 2012). Plasma membrane and membranes of organelles, consisting of lipopolysaccharides and lipoprotein, contribute significantly to metal sorption by algae and cyanobacteria.

2.3 Ion-Exchange Concept

Microalgal cell wall plays significant role in an ion exchange as it is composed of polysaccharides, proteins, and lipids. These constituents contribute to various functional groups (carboxyl, hydroxyl, phosphate, amino, sulfhydryl, amide, alkyl, and aromatic compound) and hence possess an overall negative charge to the cell surface. The cell surface of alga acts as a strong binding site for metal cations and is involved in metal exchange through the ion-exchange mechanism (Monteiro et al. 2011). During the interaction between metal ion and protein on biological surface, metal ions coordinated in formation of complex groups. However, in marine system, a major part of active sites are bonded with protons at low pH or with alkaline earth metals (Ca, Na, and Mg) at higher pH. In the presence of cations such as Cu+2, Mn+2, Zn+2, Ni+2, Cd+2, Fe+3, and Pb+2, the previously bind protons and metals are released, and these cations are sorbed on cell surface. But in the case of anions, adsorption characteristics of algal significantly change toward the competitive binding of metal ions to the cell surface as shown in Fig. 3.4.

Fig. 3.4
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Basic principle of ion exchange through algae

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2.4 Physical Adsorption

Physical adsorption refers to a phenomenon in which aqueous metal ion binds to polyelectrolytes of algal cell wall through weak force of attraction such as van der Waals force, covalent bonding, redox interaction, biomineralization, etc. (Perpetuo et al. 2011). The pH of the adsorbing media has strong influence on the adsorption of the metal ion. It has been found that alkaline pH increases the attraction of metal cations and thus improves their adsorption on cell surface by replacing the functional groups containing negative charge such as polysaccharides, phosphate, amino group of nucleic acid, and amino and carboxyl group of protein (Majumder et al. 2015). Furthermore, electrostatic attraction has been found as the main mechanism for adsorption of metals such as uranium, cadmium, zinc, copper, and cobalt through passive algal biomass (Fig. 3.5).

Fig. 3.5
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Mechanism of physical adsorption of heavy metals. (Ayansina and Olubukola 2017)

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3 Factor Affecting Uptake of HM Ions

Biosorption has been regarded as the main mechanism for metal ion removal via algal biomass, but various factors are found to have influence on biosorption potential of alga such as pH, contact time, and temperature. In this context, the following section provides a detailed perspective on influence of these variables on biosorption of heavy metals (Ahalya et al. 2003).

3.1 pH

The pH of the aqueous solution (wastewater) is one the most imperative factors that directly influences the biosorption process to remove heavy metal from wastewater. pH affects the dissociation of functional groups of the active sites of biosorbent as well as chemistry of ionic solution. pH optima for biosorption via algae vary from metal to other metals. Ajayan et al. (2011) reported a significant decrease in pH ranging from 5.6 to 8.3 while removing heavy metal from tannery wastewater. Ritixa and Monika (2013) reported that pH optima for iron and copper were 8 for both with removal efficiency of 92–93%, respectively, in biosorption process. Dominic et al. (2009) reported that pH level of industrially polluted wastewater shows a drift from acid to alkaline, i.e., 6.0–8.1, after treatment with Chlorella vulgaris. Wastewater treatment with Synechocystis salina shows a slight drift in pH from 6.0 to 8.0, while the same wastewater shows variation in pH decrease (from 6.0 to 7.9) treated with different algal species Gloeocapsa gelatinosa. Therefore, it is clear from the above explanation that changes in pH have substantial potential to alter the biosorption potential through various processes such as affecting ionic chemistry and metal availability in medium and affecting algal growth in case of active algal biomass.

3.2 Temperature

The biosorption of heavy metals through algae is unaffected within the temperature ranging from 20 to 35 °C, while at 40–50 °C, biosorption efficiency increased, but such high temperatures may be responsible for permanent structural damage to the algal cells. As a consequence, it decreases in metal uptake. Biosorption is mainly based on adsorption reaction which is an exothermic process. The extent of adsorption of heavy metals through algae increases with decrease in temperature. It has been reported that the temperature optimum for S. cerevisiae was 25 °C for maximum heavy metal (Ni and Pb) biosorption. Ali et al. (2013) reported that metal uptake by S. platensis (PAB) increased gradually with increasing temperature, and it was found that metal (Cu) uptake was maximum (90.61%) at temperature 37 °C. Hence, temperature plays a vital effect on metal uptake through algal biomass as shown in Table 3.3.

Table 3.3 Metal uptake through various passive algal biomasses at different temperatures
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3.3 Contact Time

PAB adsorb passively HMs on the surface of cell wall rapidly within few minutes, while in living algal biomass metal sorption is a gradual process and follows the life cycle of alga (Vogel et al. 2010). Tuzen and Sari (2010) observed that PAB Chlamydomonas reinhardtii biomass adsorbs Hg+2, Cd+2, and Pb+2 and equilibrium is achieved within 60 min. According to Mata et al. (2009), PAB biomass of Fucus vesiculosus (macroalgae) removes Au+3 28.9 mg/g and 74.1 mg/g after 1 and 8 h; this process suggest that biosorption of HM ion is a passive process that occurs relatively on a rapid scale. But, in AAB the biosorption rate of Cd+2 by Cladophora fracta decreased by increasing time (Wang et al. 2010), but greater absorption capacity is found in old culture (Ozer et al. 2000). The issue with older culture gradual depletion of cell surface by nutrient; this will affect the biosorption capacity of HMs on the algal cell surface.

Thus, pH, contact time, and temperature determine the sorption capacity of heavy metal ions. Researchers have optimized these variables and achieved maximum biosorption potential of various algal strains, which are shown in Table 3.4. Temperature ranges (20–96 °C) with contact time (30–240 min) with optimal pH ranges were experimentally investigated by various researchers in the last 15 years, using different algae.

Table 3.4 Factor influencing the HM removal efficiency by algal biomass
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4 Algal Biomass-Based Remediation Approaches for Heavy Metals: Traditional vs. Advanced

Conventional practices for metal ion removal were dominated by chemical methods such as chemical-mediated precipitation, redox reactions, ion-exchange resins, organic polymers (starch, poly ions, and xanthate), coagulation, osmosis, chemical-induced extraction, adsorption via activated carbon, electroprecipitation, and electrodialysis (Lezcano et al. 2010). But these methods are found to be costly and less effective for HM removal (Plaza et al. 2013). Contrary to these methods, application of inorganic adsorbents such as clay, mud, ash, alum, and other organic adsorbents (waste biomass, agricultural residues, plant leaf, etc.) was found to be less expensive, but most of them resulted in incomplete remediation (Zhang et al. 2016; Lee et al. 2016). These conventional methods demand large amount of energy and chemicals (Majumder et al. 2015). So, there have been developed, formulated, modern, economical, and sustainable adsorbents for the removal of HMs and toxic substances from wastewaters. Most of the researchers have been escalating their efforts in developing suitable adsorbents for the complete removal of HMs.

Generally, algal-based HM remediation is considered as a part of bioremediation and involves biosorption either by passive sorption of pollutant independent of metabolic process or active sorption of pollutant depending on metabolic pathway. In case of active sorption process, energy generated by respiration is consumed in metal sorption; hence this process depends on the efficacy of physiological process of the living algal biomass. In addition to this, environmental variables such as pH, temperature, contact time, etc., nature of ionic species, biomass concentration, contact time, and nature of the adsorbent also affect the biosorption capacity.

4.1 Microalgae Potential in HM Remediation

Microalgae belong to the group of photosynthetic organisms and are found in fresh- as well as marine water environment. These organisms have tremendous photosynthetic efficiency, and about 32% of the global photosynthesis is carried out by microalgae (Priyadarshani et al. 2011). Microalgae perform the specific mechanism to uptake the essential heavy metals required to their cell growth. The benefits of microalgae include rapid capacity of metal uptake, reduced time and energy-efficient, eco-friendly, polynomial, recyclable, economical, highly efficient, large surface/volume (S/V) ratio, high selectivity (which enhances their performance), no synthesis required, and useful in all types of system (Cristina et al. 2012). Apart from possessing greater HM ion removal efficiency, microalgae perform easy recovery of HMs involving a few simple desorption physical and chemical methods. AAB requires minimum nutrients (nitrogen and phosphorus) and climatic condition, while PAB does not require nutrients. Moreover, they could also remove HM ion from wastewater and aqueous solutions too. Microalgae can effectively remove HMs, and use of transgenic approaches enhances the HM binding efficiency. Microalgae have the ability to bind polyvalent metal ions; thus they can be effectively applied to treat the wastewater contaminated with polyvalent metallic ion. With affinities for polyvalent metals helping to establish their potential application in cleansing of wastewater containing dissolved metallic ions (De-Bashan and Bashan 2010), particularly, Chlorella and Scenedesmus are microalgae of choice for metal removal. Passive algal biomass has been found to uptake a variety of heavy metals such as Fe, Co, Cu, Mn, Ni, V, Zn, As, Cd, Mo, Pb, and Se. Brinza et al. (2007) explored the potential use of PAB of Chlamydomonas reinhardtii, C. sorokiniana, C. vulgaris, C. miniata, Chlorella salina, Chlorococcum spp., Phaeodactylum tricornutum, Scenedesmus abundans, S. quadricauda, S. subspicatus, Spirulina platensis, (Gokhale et al. 2008) and Spirogyra sp. for biosorption of heavy metal ions.

Microalgal biomass specially produces peptide bond during the photosynthesis which is capable of binding HM ion and forming organometallic complexes, which are further reached inside the vacuoles to maintain the cytoplasmic concentration of HM ion, which neutralizes the toxic effect of the HMs. Most studies of HMs focus on Cu, followed by Cd, Ni, Pb, Zn, Hg, and Cr by microalgae. The efficiency to absorb metal was found to be different in macro- and microalgae strains. Metal uptake capacity in macroalgae is found to be directly related to the extent of alginate, its availability to provide sorption site, and specific macromolecular conformations. Despite having similar functional groups in Spirogyra and Cladophora sp., Lee and Chang (2011) found that Spirogyra has higher adsorption capacity for Pb(II) and Cu(II) than Cladophora sp. Hence, capacity for bio-removal of metal in macro- and microalgae, clear differences have been observed in accumulation.

4.2 Active Algal Biomass vs. Passive Algal Biomass

The difference of metal biosorption via AAB and PAB has been clearly explained in the above sections. However, ion exchange is the common process found in both biosorbents which has a large contribution in biosorption potential. Most of the researchers preferred PAB for biosorption process due to its possibility to recycle and reuse. In addition to this, PAB don’t require additional nutrient source, and its application to remove heavy metal ions under extreme environmental conditions was found feasible in comparison to the living algal biomass. PAB biomass can be pretreated by physical and chemical methods to improve biosorption efficiency, while in living algal cell, sorption potential is limited and depends on growth capacity of algal strain. Acidic and alkaline condition of growth medium can affect the growth rate of algae and causes metal ion precipitation, respectively. Table 3.5 summarizes the efforts of various researchers toward the algal-based metal biosorption.

Table 3.5 HMs removal efficiency using active algal biomass and passive algal biomass
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4.3 Immobilized Algae

Initially, immobilization of algal cells was proposed to deal with the challenges associated with harvesting and dewatering of algal cells. However, immobilization offers several advantages over the algal cells grown in free suspension such as (1) immobilized algal cells occupy less surface area; (2) immobilized algal cell has been found with increased photosynthetic activity, biosorption capacity, and bioactivity; and (3) immobilized algal cells are found to be resistant to harsh environmental condition and less exposure to toxicity. Immobilization increases the applicability of entrapped algal cells for repetitive biosorption process (Eroglu et al. 2015). Researchers have developed various techniques (adsorption on surface, flocculation, liquid-liquid emulsion, covalent coupling, etc.) for entrapment of algal cells, but application of synthetic polymer (poly acryl amide) or natural polymers (agar, cellulose, and alginate) is the most preferred technique.

Immobilization of algal cell using polysaccharide gels has often been used for the purpose of wastewater treatment (nutrient uptake and metal ion removal). Entrapment of algal cells in alginate has been found to have sufficient immobilization and improved removal efficiencies from aqueous environment (Table 3.6). Maznah et al. (2012) reported higher biosorption capacity (Cu, 33.4 mg/g; Zn, 28.5 mg/g) in algal cells (Chlorella sp.) immobilized by sodium alginate than that of the free biomass. Recently, it has been found that incorporation of polyethyleneimine in alginate algal beads substantially increases the sorption capacity for Pt (II) and Pd (IV) (Wang et al. 2017). In addition to high capacity for metal biosorption, Lopez et al. (2017) reported that algal cells encapsulated in alginate yielded more nucleic acid than the free-living cells; hence higher concentration of nucleic acid indicates the more active cells in alginate algal beads than the free-living cells. Limitations were also observed in cell entrapment such as poor diffusion of carbon dioxide and oxygen, larger volume-to-surface ratio of encapsulating materials, etc. However, to overcome these challenges, researchers (Wang et al. 2017; Mujtaba and Lee 2017) have made several attempts by introducing new entrapment matrices.

Table 3.6 HM removal efficiency by immobilized algal biomass
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4.4 Metal Ion Biosorption Enhancement Using Molecular Tools

Investigating biological mechanisms at the molecular level to produce bioengineered organism with higher biosorption capacity can be used for effective bio-removal of heavy metal. The high cost of conventional technologies to reduce toxic metal ion concentrations in industrial wastewater to acceptable regulatory standards has prompted exploitation of genetic and protein engineering approaches to produce cost-effective “green” biosorbents (Valls and de Lorenzo 2002). Another emerging area of research is the design and development of novel algal strains with increased affinity, capacity, and selectivity for biosorption of heavy metal ions (Zvinowanda et al. 2009; Karthikeyan et al. 2007). Many genes are involved in metal uptake, detoxification, or tolerance (Ayansina and Olubukola 2017). Cysteine-rich peptides such as glutathione (GSH), some lipopolysaccharides, phytochelatins (PCs), and metallothioneins (MTs) bind metal ions (Cd, Cu, Hg, etc.) and enhance metal ion bioaccumulation (Zhang et al. 2015).

Based on the above said technologies (AAB, PAB, immobilized algal cell, molecular tools), various efforts have been made to commercialize the algal-based heavy metal removal (Zhou and Haynes 2010). In this context, Alga SORB is a commercial product comprising of PAB to remove the HM ion from aqueous solution. Another biosorbent, BIO-FIX, consisting of Sphagnum peat moss, algae (Ulva sp.), bacteria, and fungus encapsulated in polysulfone has also been found to have the potential to remove a variety of heavy metal ions. Thus available commercial technologies are dominated with adsorption process. Although microalgal potential to absorb a variety of heavy metal ions is known, sustainability of process is still under concern.

5 Conclusion

This review revealed the contribution of algal biomass for heavy metal removal from wastewater. Low-cost cultivation and high HM ion uptake capacity, with suitable environmental conditions (pH, temperature, contact time, etc.), make algae biomass as a potential source for wastewater bioremediation. Microalgae, predominantly, possess numerous considerable sequestering mechanism HM ions and hence are demarcated as promising biosorbents. Several reports suggest their supremacy over various traditional and physiochemical methods and their usefulness in large-scale remediation of wastewater. Based on the biomass productivity of algae on wastewater is attractive dual-use algae cultivation coupled with other downstream or hybrid production systems. A suitable and sustainable approach needs to be developed in the field to select the most appropriate biosorbents and favorable physical conditions and to find out the major challenges involved. However, it is essential to deliberate on various avenues of microalgal remediation technologies as eco-friendly alternatives for a better environment.