1. Identification, sources, toxicity and treatment of heavy metals
Heavy metals are considered to be the following elements: copper, silver, zinc, cadmium, gold, mercury, lead, chromium, iron, nickel, tin, arsenic, selenium, molybdenum, cobalt, manganese, and aluminum (Karnib et al. 2014). Heavy metals in water assets are one of the foremost vital natural problems for all countries. Due to modernization, the mechanical utilize of metals particularly, heavy metals, has risen alarmingly, in this way getting to be of earlier concern since of their poisonous quality to environment (Jothinayagi and Anbazhagan 2009).
However, they are danger for both the individuals and the total environment due to their toxicity and mutagenic, immunogenic, carcinogenic, teratogenic activity (Witek-Krowiak 2013). Plus, the chronic exposure of toxic dose of these metals in people comes about in various complications in nervous system, respiratory system, renal system, hepatic system and reproductive system. Also they reduce growth and development, whenever, the accumulation of metal ions in human bodies will occur through either coordinate admissions or nourishment chains.
Metals are too detailed to cause sensitivities and rehashed long-term contact with some metals or their compounds may indeed demonstrate carcinogenic and in extreme cases,death (Hegazi 2013; Karnib et al. 2014). Metals amass exceptionally effectively in living organisms that is why their destructive impact may show up indeed at exceptionally even at very low concentrations. Most of the heavy metals are well known poisonous and carcinogenic agents and represents a genuine risk to the human populace and the fauna and flora of the accepting water bodies as they are permanent and non-biodegradable. Heavy metals present in the wastewaters discharged from laboratories and industries. Fundamental anthropogenic sources of metals are: power plants, mining and metal ores processing, metallurgical, chemical, electronic, nuclear, agriculture and waste disposal industries (Witek-Krowiak 2013). Water contamination with heavy metals presents a critical natural issue requiring immediate mediation intervention (Witek-Krowiak 2013). When these metal ions present at intemperate levels in an fluid release, the stream remains unusable due to the unfavorable impacts related with utilizaion (Hegazi 2013). Fresh potable water resources are the most vital reservoirs in the world. It is important for all living living beings on earth to have access to adequate clean water. Available water resources are dwindling due to global population growth, emerging economies, and long-time droughts. Besides over-exploitation, defilement of natural water assets with refractory pollutants discharging from industries make the water deficiency more regrettable indeed in wealth water regions (Malato et al. 2009; Kang and Cao 2012; Mukherjee et al. 2016). It is conceivable to store rainfall streams or storm water to control water deficiency for a brief time, but the most perfect way is to treat and reuse wastewaters (polluted water coming from industries, businesses, and homes) (Chong et al. 2010).
Recuperation of heavy metals and their compounds from industrial waste streams is getting to be increasingly vital as society realizes the need for reusing and conservation of basic metals (Jothinayagi and Anbazhagan 2009). Therefore, heavy metals ought to be prevented from reaching the natural environment (Abdel Salam et al. 2011; Hassan et al. 2014). A few conventional methods have been utilized for example chemical precipitation, coagulation, ion exchange, electrochemical treatments, filtration, electro dialysis, membrane methods and biological methods in order to evacuate poisonous heavy metals from water systems as shown in Figure 1 (Azimi et al. 2017).
Figure 1. Some conventional methods for heavy metals treatment (Azimi et al. 2017).
2. Water treatment methods for heavy metals
2.1. Electrolysis treatment
2.1.1. Electrochemical treatment
Electrolytic recuperation is one innovation utilize to evacuate metals from wastewater streams. This process uses electricity to pass a current through an aqueous metal-bearing solution that includes a cathode plate and an insoluble anode. By transferring electrons through one component to another, energy can be produced. The electrochemical procedure of treating heavy metal wastewater is to precipitate the heavy metals in a weak acidic or neutralized catholyte as hydroxides. Electrochemical medications of wastewater include electro-deposition, electrocoagulation, electro-flotation and electro-oxidation (Shim et al. 2014).
2.1.2. Electro-flotation treatment
The electro-flotation (EFM) is a simple process that floats ions or strong particles, suspended or broken up in a liquid phase, by adhering on tiny bubbles of hydrogen and oxygen produced on cathode and anode and moving upward in the buoyancy cell. Hydrophobicity is the key to floatability of chemical compounds, the collector’s ratio to metal ion being an imperative factor; therefore, the amount of ion buoyancy collector utilized in ought to be at slightest stoichiometric with foamy properties (Nanseu-Njiki et al. 2009). The electrode choice permits configuring the system for particular process: soluble anodes, as made by iron or aluminum, produce coagulants agents in situ promoting the concurrent electrocoagulation (EC) phenomenon. It is generally accepted that there are three stages in the EC process involves. The primary stage consists of oxidation of electrode to produce coagulants agents. the water electrolysis happens at the same time. At stain-less steel electrodes, the following reactions occur according to Eqs. 1.1-1.3 (Emamjomeh and Sivakumar 2009).
Fe ? Fe2+ + 2e Eq. 1.1
H2O ? 2H+ + ?O2+ 2e Eq. 1.2
2H2O + 2e ? 2OH? + H2 Eq. 1.3
The second anode reaction will proceed in operation, and gas bubbles at both electrodes are detected. EC’s second phase involves destabilization. An electro-chemical reduction of metal cations may occur on cathode surface and the hydroxide ions produced at the cathode increase the pH of the wastewater thereby inducing precipitation of metal ions.
Electro-deposition is a practical and efficient method among the electrochemical processes for the recuperation of heavy metals (Oztekin and Yazicigil 2006). Electro-deposition is beneficial since no advance reagents are fundamental, during the process, no sludge is produced, it is also highly selective and low-cost (Chen and Lim2005). By deposition on ionic conductors (cathode and anode) it transforms dissolved metal ions into solid particles to secure them from corrosion (Saji and Cook 2012). Electro-deposition is a one-step clean strategy based on heavy metal ions reduction and oxidation in a cell consisting of one anode, a cathode, an electrolyte cell, and a current source (Koparal et al. 2004; .Saji and Cook 2012). Heavy metals are reduced and electroplated onto the cathode (Saji and Cook. 2012). The reaction is as follows according to Eq. 1.4 (Koparal et al. 2004):
Mn+ +ne- M Eq. 1.4
The ultimate size and dissemination of electrodeposits depends on the nucleation of deposits and their growth (Saji and Cook 2012). The anodes are favored to be insoluble or inactive(Chen and Lim 2005; Saji and Cook 2012), otherwise they will interfere with the recuperation process of heavy metals (Chen and Lim 2005). A common side reaction at the anodes is observed as given by Eq. 1.5 (Chen and Lim 2005):
4OH- O2+2H2O+4e- Eq. 1.5
Some competing reactions occur during the process. One of them is a reduction of H+ into hydrogen gas as given by Eq. 1.6 (Chen and Lim2005)
H++e- ? H2 Eq. 1.6
The effectiveness of the whole process is influenced by the initial concentration of the waste solution, physical operational parameters such as temperature and pH and the presence of complexing and chelating agents (Chen and Lim 2005). The primary use of Electro-deposition is that it can be applied to non-aqueous solutions or solutions containing chelating agents as well. Either of these solutions yields better removal of a pollutant than aqueous solutions. Customary aqueous solutions confront some problems like the freedom of hydrogen gas molecules, low thermal stability, and limit electrochemical window. Therefore, sometimes non-aqueous solutions are favored. In any case, there are some specialized issues in the way of commercialization of non-aqueous solutions like low current efficiency, cell heat balance and erosion of cell components (Simka et al. 2009). Another interesting case is the electro-deposition of aqueous solutions that contain chelating agents such as EDTA, NTA and citrate (Oztekin and Yazicigil 2006). These chelating agents are valuable since they bind with heavy metals cations to minimize the formation of insoluble salts and increment the efficiency of recuperation by creating stronger metal ions complexes (Chen and Lim 2005; Oztekin and Yazicigil 2006). Traditional methods of separation will not be required (Oztekin and Yazicigil 2006).
Electrocoagulation is an electrochemical strategy of treating water and wastewater, where sacrificial anodes erode due to oxidation and dissolute the metal ions from the anode (Moneer et al. 2018). Meanwhile, concurrent formation of hydroxyl ions and hydrogen gas at the cathode results in the formation of precipitates which then later form the floc with the contaminants from the water and can be removed through filtration leaving behind potable water. Electrodes used in the electrocoagulation can be of many metals such as iron, aluminum and platinum (Figure 2). However, iron and aluminum have gained the popularity over other metals because of the high removal efficiency, high ion dissolution rates, low cost and the ability to reuse the same metals for several experiments. Some of the major advantages of electrocoagulation are its ease of operation, pH buffering ability, low sludge production, low toxic content and easy handling with better stability. Flocs formed as a result of electrocoagulation are larger, acid resistant, and more stable and hence easy and fast to eliminate through filtration. Since no chemicals are used throughout the electrocoagulation experiments, the treated water is much healthier as opposed to chemical coagulation treatment. Very few disadvantages are known in this process such as corrosion of electrodes with time and need to be regularly replaced, formation of an oxide layer on the cathode with time resulting in a decrease in the effectiveness of the removal rate and expensiveness of energy consumption for electrocoagulation in some places (Chaturvedi 2013).
An electrocoagulation reactor for heavy metal treatment consists of two sacrificial Fe electrodes; anode and cathode submerged in an electrolytic cell containing metal sullied water (Chaturvedi 2013). Which when associated to an external DC power source, the anode metal undergoes electrolytic oxidation releasing ferrous ions (Fe2+), since ferrous ions are soluble they are vital to be oxidized to ferric ions (Fe3+), which can be done by increasing the amount of dissolved oxygen in the solution by sparging air onto the solution containing beaker. However, in many cases air sparging is not necessary as the atmospheric oxidation conditions might suffice the oxidation of Fe2+ ions to Fe3+ ions which are further hydrolyzed to polymeric hydroxides. Meanwhile, on the cathode side water gets electrolyzed producing hydrogen gas and hydroxide ions. Oxidized Fe3+ ions from the anode combines with hydroxides formed on the cathode to form an iron oxide precipitate also known as hydrous ferrous oxide (HFO) which is an excellent coagulant agent. The formed iron oxide then destabilizes the contaminants in the solution which then aggregates to form floc. The bubble in the solution carries the floc then to the surface through natural buoyancy. The floc can then be removed through filtration after the settling (Chaturvedi 2013).
For example, in the electrochemical process, reduction of Cr(VI) to Cr(III) can be accomplished using electrochemical units. The process consists of electrolysis using an aluminum or iron anode. Iron electrodes are reported to be more efficient than aluminum electrodes (Mouedhen et al., 2009). However, some studies stated that aluminum electrodes are also effective (Zongo et al. 2009; Heidmann and Calmano 2008). The application of this technology for hexavalent chromium removal requires a reduction step followed by a precipitation step. The main reactions that occur at the anode during electrolysis using Al or Fe electrodes are (Zongo et al. 2009):
Al ? Al3+ + 3 e- E0 = -1.662 V Eq. 1.7
Fe ? Fe2+ + 2 e- E0 = -0.447 V Eq. 1.8
H2O ? 2H+ + ? O2 + 2e- E0 = 1.229 V Eq. 1.9
Mouedhen et al. (2009) believed that the reduction of Cr(VI) occured by Fe(II) anodically generated. The main reaction occurring at the cathode electrode during Cr(VI) removal by electrocoagulation is given as follows (Mouedhen et al. 2009; Zongo et al. 2009):
2H2O + 2e-? H2 + OH- E0 = -0.828 V/NHE Eq. 1.10
At the cathode made of iron, the following reaction may occur at acidic pHconditions:
6H+ + Cr2O72- + 6 e-? Cr2O42- + 3 H2O (pH < 6.5) (Mouedhen et al. 2009) Eq. 1.11
However, at neutral to alkaline pH, there is no Cr2O72- available to be reduced; the following reactions are thought to occur. There are disagreements among scientists about the reduction mechanisms at neutral to alkaline pH values at Fe cathodes (Thella et al. 2008; Mouedhen et al. 2009; Zongo et al. 2009).
Cr6+ + 3Fe2+ ? Cr3+ + 3 Fe3+ Eq. 1.12
3Fe2+ + CrO42- + 4H2O ? 3 Fe3+ + Cr3+ + 8OH- (6.5 < pH < 7.5) Eq. 1.13
CrO42- + 3 Fe(OH)2 + 4H2O ? 3Fe(OH)3 +Cr(OH)3 + 2OH- (pH >7.5 ) Eq. 1.14
CrO42- + 8H+ + 3Fe2+? Cr3+ + 3 Fe3+ + 4H2O E0 = 0.760 V/NHE Eq. 1.15
Cr2O72-(aq) + 6Fe(OH)2(aq) + 7H2O ? 2Cr(OH)3 + 6Fe(OH)3 +2OH- Eq. 1.16
2HCrO4- + 6Fe2+ + 14H+ ? 2Cr3+ +6 Fe3+ +8H2O Eq. 1.17
HCrO4- + 7H+ +3 e-? Cr3+ + H2O Eq. 1.18
At a cathode made of aluminum, an electrochemical reduction of hexavalent chromium to trivalent chromium occurs according to the following reaction equations:
Cr2O72- + 6 e- + 14H+ ? 2Cr3+ + 7 H2O (acidic pH) Eq. 1.19
CrO4- + 3e- + 4H2O ? Cr3+ + 8OH- (alkaline pH) Eq. 1.20
Most researchers concur that Cr(III) precipitates as Cr(OH)3(s) (Thella et al. 2008; Mouedhen et al. 2009; Zongo et al. 2009) when utilizing either aluminum or iron cathodes, nonetheless, Cr2O42- has also been reported to precipitate in the vicinity of Fe cathode as FeCr2O4 (Mouedhen et al. 2009).
Removal of Cr(VI) from aqueous solutions is favored at acidic pH conditions (Mouedhen et al. 2009; Thella et al. 2008; Zango et al. 2009). It has been stated that pH 2 with Fe electrodes is optimum for Cr(VI) removal (Thella et al. 2008). The pH ranges between 2-8 (Thella et al. 2008) and 4-8 (Adhoum et al. 2004; Reddithota et al. 2007) electrocoagulation with iron and aluminum electrodes was reported to be favorable for Cr(VI) removal by, respectively. 100 % Cr(VI) removal efficiency was achieved over pH range of 2-7.8 with Fe electrodes (Mouedhen et al. 2009). The majority of groundwaters have pH values above 8, and after electro-coagulation of Cr(VI), the final pH values could increase to as high as 11. pH values less than 4 or greater than 8 have been reported to reduce the efficiency of Cr(VI) removal by electrocoagulation with Al electrodes (Adhoum et al. 2004; Reddithota et al. 2007). It was also reported that the amphoteric behavior of Al(OH)3 explains the decrease in Cr(VI) removal efficiency (Reddithota et al. 2007). At pH > 8, Al(OH)- gets to be the ruling species of Al, and at pH < 4, Al3+ is the predominant species. Al3+ and Al(OH)- do not precipitate, while