Heavy Metals in Sewage Treated Effluents: Pollution and Microbial Bioremediation from Arid Regions

GLOBAL METAL RESOURCES

Minerals and metals have played an important role in economy throughout the world. In general, minerals and metals are used in roads, general construction, and industries involving electronic devices, such as computers, telephones, semiconductors and other electric devices. Mineral resources are classified into two groups, metallic and non-metallic. The non-metallic minerals include dimension stone, gravel, uranium, gypsum and halite. The metallic minerals include aluminum, chromium, copper, gold, iron, lead, nickel, silver, tin, and zinc. The most abundant metal is aluminum followed by iron, titanium, chromium, zinc, copper, silver, platinum, gold and uranium [25]. Based on biological importance heavy metals are classified into essential metals with many biological functions (Ca, Co, Cu, Fe, K, Mg, Mn, Mo, Na, Ni, V, W, Zn), toxic metals (Ag, Al, Au, Cd, Hg, Pb, Sn, Ti), none essential nontoxic metals (Cs, Rb, Sr, T) and metalloids (As, Ge, Sb, Se) [2]. Economically, metals are classified into four categories: industrial, precious, critical and additional metals [26] (Table 1).

Metals and mineral resources are usually not renewable. Those minerals are not distributed evenly throughout the world. Some countries are mineral-rich while others are mineral-poor [25]. The share of the global metal industry varies among the producing countries with China taking the lead with a share of 27% followed by Australia (4%), Russia, USA, and Chile (3%) each, Canada (2%), while Brazil, DRC, India, Japan, Kazakhstan, Mexico, Peru, South Africa, Rwanda and Turkey (1%) [26]. The rest of the countries fall below those values. However, China has been seriously environmentally impacted with the rapid modernization in all aspects of industries, mainly depending on steel and other metals. For example, red alert of severe air pollution was issued in ten major cities in China in December 2015. Such magnitude of air pollution affecting a wide area due to industrialization rarely occurs. The smog pollution affected 30 other smaller cities, including Beijing. Pollution was caused by many industry sectors throughout China [7]. The Organization for Economic Co-operation and Development (OECD) [27] reported that the highest mortality rate from outdoor pollution between 2005-2010 was China followed by India with an estimated cost of USD 3.5 trillion a year. Number of mortalities in outdoor air pollution exceeded the rest of the world (GBD 2014) [28]. The Pearl River delta in south China and the Yangtze delta in east China were the most polluted regions due to industries and mining. The two regions are heavily polluted with arsenic, cadmium, lead and mercury. It was reported that about 12 million tons per year of food produced in China is contaminated with toxic heavy metals. China has introduced soil-heavy-metals-remediation project with the estimated cost of more than 4 billion USD [8]. These conditions have led China to environmental disasters coupled with serious public health related issues which may also heavily impact their economy.

There is a major global concern on the management of metals in industry and recycling. The increased demand for metals will lead to significant global deficiency, specifically the platinum group [29]. Recycling of metals is therefore a wise practice. With the current technologies and challenges, it was reported that the current recycling process of metals does not meet the demand of most metals used in various industries [26]. Only 1% of most metals mentioned in Table 1 can be recycled as indicated in Table 2. Most of the low-recycled metals are involved in many high-tech industries such solar PV, microchips, and consumer electronics. For example, sixty elements may be involved in manufacturing circuit boards and microprocessors, some of which are impossible to recycle due to technical limits or as a result of oxidation in the smelting slag [29]. If the current situation of metal loss continues, most of the current technologies will be facing a tremendous shortage. On the other hand, it was reported that it is not likely that there will be a complete depletion of useful mineral resources. However, he stressed that if the current economical supply deposits are depleted, and the cost will increase dramatically and the concentration factor will also increase [25].

The most recycled metals are aluminum and steel [30]. The plentiful metals such as Al, Cu, Ni, Pb, and Zn with recycling rates of more than 50% due to their existence in pure forms (Table 2). The rest of the metals are lost, causing accumulation in the environment. The other precious metals are at low-recovery rate. Some of unrecovered metals are highly toxic to organisms [29].

Heavy metal pollution should be dealt jointly in a global effort by setting up international regulatory standards to minimize its impact on the environment which directly affects public health. Several international organizations have established regulatory heavy metals standards. However, these standards are not unified [31-37] and have deficiencies in many elements mentioned in Tables 1 and 2. Also, most of the metals are not recycled, leading to their accumulation in the environment causing pollution [29].

Environmental Pollution & Health

Due to excessive industrialization accompanied by rapid global population growth and increase in agricultural practices, heavy metals As, Cd, Cr, Cu, Hg, Pb, and Zn are the most common environmental pollutants [1, 38]. Generally, the sources of heavy metal pollution are mining, smelting, metallurgical industries, corrosion, waste disposal, fossil fuel combustion agriculture and forestry [5, 38-48]. As a result, the contaminants spread through air and accumulate in soil infiltrating underground water. Consequently, this causes severe ecosystem damages, entering the food chain through contaminated soil and water [1, 3, 4, 11-15, 49, 50]. More than ten million polluted sites were reported across the globe. Most of the polluted sites are contaminated with heavy metals and metalloids causing more than ten billion USD annually [1]. It was reported in 2009, Belgium has the maximum heavy metal pollution in Europe, followed by Greece. In 2012, Poland exceeded Belgium followed by Romania, while Greece decreased to lower levels [51].

Almost all mercury emission is from fossil fuel (46%) combustion, followed by gold production (24%), metal and cement production (10% each) and waste incineration (6%). The major source of mercury air emissions from the US industrial sources and power plants is estimated at about more than half of anthropogenic emission. About 33 tons of mercury are released from power plants annually [52].

In addition to natural process, as anthropogenic activities are the main sources of pollution, which include fossil fuel combustion, smelting, waste disposal, mining, farming and corrosion [1]. Some of the toxic metals (As, Cd, Hg, Pb, and Se) residence time in the atmosphere, land, ocean and sediments differs significantly. In general, metal residence time in sediments ranges between year 90.8 x 10⁶ for Hg to 99.8 x 10⁶ for As and Cd, while the least residence time in atmosphere ranges between years. 1.0 x 10^-1 for Hg to 3.0 x 10^-2 As and Se [53].

Toxicity of heavy metals in human health and various organisms has been reported by many investigators (Table 3). The effects range from low to severe cases with degenerative effects by interfering with specific cellular metabolic processes reducing natural detoxification mechanisms. For example, lead, mercury, and cadmium prevent glutathione S-transferase from liver peroxidase activity [43]. Other heavy metals mimic essential elements reducing co-factor activity at molecular level of cellular metabolism. Several factors are involved in heavy metal toxicity. These factors include route of exposure, type of metal and age.

Many heavy metals are used in traditional and international cosmetic brands, such as eyeliners, eyeshadows, lipsticks and powders. Most women use cosmetics daily without being aware of their contents and the health effects. The traditional known brands such as henna and kohol eyeliner are very common in many nations in the Middle East, Africa and India. Al-Dayel et al. [54] analyzed twenty-seven heavy metals and related elements from nine traditional and international brands from Bahrain, Bulgaria, India, Mauritania, Morocco, Nigeria, Oman, Pakistan, Saudi Arabia, the United Arab Emirates, the United Kingdom and the United States. They reported that the cosmetics contained different concentrations of Al, Ag, As, B, Ba, Ca, Cd, Co, Cr, Cu, Fe, Hg, K, Li, Mg, Mn, Na, Ni, Pb, Rb, Sb, Se, Sn, Sr, U, V and Zn. Some with high concentrations of those element, suggesting that thorough investigation and evaluation of such products are needed for their safety.

Traditional remedies and medicines from China, India, Middle East, and Pakistan were also reported to contain heavy metals. In Oman and the United Arab Emirates, the traditional medicine “bint al dhahab” contained Cd, Pb and Sb [55]. Some Asian herbal remedies were found to contain toxic heavy metals [56]. Saper et al. [57], reported that Ayurvedic herbal medicinal products contained As, Hg, Pb and that Pb was the main constituent of Indian traditional remedies. Most of herbal medicines are not regulated and are easily accessible to the public. On the other hand, in Pakistan, occurrence of Pb, Cd, Cu, Cr, Co, Fe, Ni and Zn contaminants due to environmental pollution was reported in herbal plants. These plants include Cuminum cyminum, Coriandrum sativum, Foeniculum vulgare, Glycyrrhiza glabra, Onosma bracteatum, Viola odorata, and Zingiber officinalis which is evident that a health risk is associated with the use of such herbal drugs [58].

Aluminum is commonly found in food and water. This element was reported in the brains of patients suffering with Alzheimer’s [42, 44].

Arsenic is found naturally in inorganic and organic forms. Arsenic intoxication is most commonly reported in cases with much toxic inorganic form. Seafood, for example, contains the highest concentration of As in an organic form [40].

Chronic Cd toxicity is rare but fatal and has no metabolic benefit in humans and other animals. It is mainly found in a marine environment. When ingested it affects internal organs, including spleen, kidney, liver and pancreas, while mimicking the zinc function in metabolic processes [46]. It is also reported to be carcinogenic [5, 6].

Toxicity due to iron is most common, affecting melanin and causing grey or bronze skin color pigmentation, damaging the liver, causing heart failure and joint diseases [41, 45].

The most frequent toxic metal reported is lead exposure from painted items, water pipes, children toys and some cosmetic items, such as mascara eyeliners [40]. It is one of the non-essential elements with no role in human metabolism, with children being more affected than adults [59]. Lead replaces calcium in bones and weakens bone structure and replaces Zn, blocking oxygen exchange in red blood cells. It is also interfering with some enzymes where magnesium is a co-factor affecting nucleotide synthesis [60].

The effects of mercury were reported by many researchers [61-65]. Sällsten et al. [66] reported that exhalation in low-level-mercury-exposed humans is treated with small doses of ethanol.

ROLE OF SEWAGE EFFLUENT POLLUTION IN ARID ZONE REGIONS

Rainfall scarcity in arid zone region is a common problem. Many countries with seacoast depend on seawater desalination plants for household and industrial uses. The produced sewage effluents are treated and recycled. The reclaimed water is used in agriculture, industries and public gardens [13, 50]. Some of the reclaimed water is disposed to sea or injected to aquifers [49]. However, aquifer recharging is an expensive process [76].

Oman and the rest of the Gulf Corporation Countries (GCC) are in the arid regions with severe lack of rain, depending mainly on desalinated water and recycling of sewage effluents [50]. Many studies in Oman have reported heavy metals in well water, soil, crops and marine habitats of fish and marine turtles [15, 50]. Some industrial applications of heavy metals (Y, Yb, Te and Zr) were detected in marine green turtle eggs (Chelonia mydas) at Ras Al-Hadd Turtle Reserve, Oman, suggesting that marine habitats are contaminated with industrial polluted effluents [14]. Treated sewage effluents were found to be the main source of such contamination. It is clearly indicated that the current technologies used in treating sewage effluents are not sufficient for removal of heavy metals and pathogenic microbes which are resistant to many antibiotics. The microbes were found to resist chlorination and were also found in well water, fish and marine turtles [14, 15-50]. For example, some researchers reported that microbes resistant to antibiotics are also resistant to heavy metals. Probably these microbes use the same mechanisms of resistance [77]. Such microbes were found in heavily polluted sewage effluents where they can be potentially used for bioremediation of heavy metals and solve the downsides of conventional methods which produce harmful secondary products.

Treatment of sewage effluent mainly relies on biological functions of microbes. Such microbes play an important role in bioremediation by breaking down toxic compounds under aerobic and anaerobic conditions where each microbial population plays a specific function. However, researchers reported that elimination of heavy metals from sewage effluent is inefficient [15, 73]. Probably the existence and interaction of appropriate microbial population at appropriate environmental conditions could be a key. Hence, isolation and identification of the right microbes to remove heavy metal toxicity are highly essential [78].

MICROBIAL BIOREMEDIATION

Heavy metal pollution continues to be one of the most important global threats. Out of twenty classified toxic heavy metals, half of them are released at higher concentrations with high potential to human health and the environment [78]. One of the main sources of water pollution is the discharge of industrial contaminated sewage effluents with heavy metals, such as Ag, As, Au, Cd, Co, Cr, Cu, Hg, Ni, Pb, Pd, Pt, Rd, Sn, Th, U and Zn. Continuous discharge of heavy metals into the environment and their accumulation cause adverse effects on terrestrial and aquatic environments as well as the population.

Well established physicochemical technologies to remove heavy metals and other contaminants from sewage effluents already exist. These include cementation, ion exchange, precipitation, electrocoagulation and electrowinning [73, 78]. However, these technologies are not cost-effective and needs skilled personnel for operation, generation of hazardous by-products and are inefficient for detoxifying low heavy metal concentrations [73, 79, 80]. Compared to conventional technologies, bioremediation is easy to operate, low metal concentration can be detoxified with high efficiency, self-sustaining system where the right microbes grow depending on the availability of the contaminant, harmful products are not produced and it can be operated in combination with conventional technologies [78, 79, 81]. The isolation and identification of heavy metal resistant bacteria in sewage effluent may contribute to efficient technologies to remove heavy metal pollutants from sewage effluents before being discharged to environment.

Microorganisms convert many toxic heavy metals through the process of metabolism into other non-toxic useful forms [82]. Microorganisms bioremediate heavy metals by several methods including bioleaching, enzyme-catalyzed transformation, biomineralization, biosorption and intracellular accumulation [83]. Most of heavy metal bioremediation occurs through the biosorption mechanisms [84]. However, ideal environmental conditions have to be met for the efficient bioremediation process.

In metal sulfide ores, ideal environmental conditions for maximum rates of bleaching at mining sites was reported at an optimum elevated temperature where high yields of metal extraction were achieved [85]. It was reported that the addition of appropriate nutrients enhances oxygen demand for Pseudomonas species and hence the active uptake of metals [86]. On the other hand, at optimum anaerobic conditions, Klebsiella pneumonia was reported to bioaccumulate high concentrations (15 mM) of heavy metals [87].

Basha and Rajaganesh [73], reported that Bacillus licheniformis, Pseudomonas fluorescence, Salmonella typhi and Escherichia coli, isolated from textile industry effluents were used effectively to bioremediate zinc, lead, cadmium, other toxic chemicals. Both free or complex ionic forms of zinc, lead and cadmium are discharged by various industries, such as textile industries [88].

The main disadvantage of microbial bioremediation appropriate conditions to achieve maximum detoxification of pollutants [78, 86-88].