A review on heavy metal pollution, toxicity and remedial measures: Current trends and future perspectives

Journal of Molecular Liquids 290 (2019) 111197

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Journal of Molecular Liquids journal homepage: www.elsevier.com/locate/molliq

Review

A review on heavy metal pollution, toxicity and remedial measures: Current trends and future perspectives Kilaru Harsha Vardhan a, Ponnusamy Senthil Kumar a,⁎, Rames C. Panda b,⁎ a b

Department of Chemical Engineering, SSN College of Engineering, Chennai, Tamilnadu, India Department of Chemical Engineering, CSIR- CLRI, Adyar, Chennai 600 020, India

a r t i c l e

i n f o

Article history: Received 27 March 2019 Received in revised form 12 June 2019 Accepted 15 June 2019 Available online 18 June 2019 Keywords: Adsorption Adsorbent Heavy Metals Toxicity Treatment Wastewater

a b s t r a c t Water is an exceptionally essential source for the presence of life on the earth. The water quality has seriously affected because of the overgrowth of the population, human activities, fast industrialization, unskilled utilization of natural water resources and unplanned urbanization. Heavy metal is a group of metal and metalloids with an atomic density higher than the 4000 kg/m3. The heavy metals are toxic in nature which causes serious health illness to human beings and animals, even at very low concentration. These heavy metals enter into the aquatic system through the agricultural runoff and industrial discharges. Different treatment methods are available to remove the heavy metals from the aquatic environment with a different degree of success. In any case, the deficiencies of a large portion of these treatment methods are due to the production of secondary waste, high cost for operation and maintenance etc. Hence, it is important to develop robust, eco-friendly and economically viable treatment methods for the removal of heavy metals from the aquatic system and to safeguard the environment. In this review article, definite data on the source and their belongings of substantial metal contamination to the plants, individuals and other living life forms have been clarified. The distinctive sorts of treatment strategies for the removal of the toxic metals from wastewater had been explained. The numerous types of low-cost adsorbents utilized by different researchers/scientists for removal of heavy metals from wastewater have been discussed in detail. The recommendations for further scope of research which can be done have been discussed in detail. © 2019 Elsevier B.V. All rights reserved.

Contents 1.

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Introduction . . . . . . . . . . . . . . . . . . . . . 1.1. Water pollution . . . . . . . . . . . . . . . . 1.2. Heavy metals . . . . . . . . . . . . . . . . . 1.2.1. Copper . . . . . . . . . . . . . . . . 1.2.2. Cadmium . . . . . . . . . . . . . . . 1.2.3. Zinc . . . . . . . . . . . . . . . . . 1.3. Heavy metals toxicity in the environment . . . . 1.3.1. Impact of heavy metals on plants . . . . 1.3.2. Impact of heavy metals on human health 1.4. Treatment methods . . . . . . . . . . . . . . 1.4.1. Chemical precipitation . . . . . . . . . 1.4.2. Chemical coagulation/flocculation . . . . 1.4.3. Electrochemical methods . . . . . . . . 1.4.4. Membrane filtration . . . . . . . . . . 1.4.5. Ion-exchange . . . . . . . . . . . . . 1.4.6. Bioremediation . . . . . . . . . . . . 1.4.7. Adsorption . . . . . . . . . . . . . . Adsorbents. . . . . . . . . . . . . . . . . . . . . .

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⁎ Corresponding authors. E-mail addresses: [email protected] (P.S. Kumar), [email protected] (R.C. Panda).

https://doi.org/10.1016/j.molliq.2019.111197 0167-7322/© 2019 Elsevier B.V. All rights reserved.

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2.1.

Adsorbents from industrial byproducts/wastes . . . . 2.1.1. Adsorbents from lignin . . . . . . . . . . . 2.1.2. Adsorbents from fly ash . . . . . . . . . . . 2.1.3. Adsorbents from sludge and blast furnace slag 2.1.4. Adsorbents from red mud . . . . . . . . . . 2.2. Adsorbents from agricultural byproducts/wastes. . . . 2.2.1. Adsorbents from rice husk . . . . . . . . . 2.2.2. Adsorbents from waste peels . . . . . . . . 2.2.3. Adsorbents from wheat wastes . . . . . . . 2.2.4. Adsorbents from clay materials . . . . . . . 2.2.5. Adsorbents from other agricultural wastes . . 3. Comparison of Langmuir adsorption capacities . . . . . . . . 4. Conclusion and scope for further research . . . . . . . . . . Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . References. . . . . . . . . . . . . . . . . . . . . . . . . . .

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1. Introduction 1.1. Water pollution Water is an exceptionally essential hot spot for the presence of life on the earth. It is a wellspring of life and imperativeness, yet a considerable number of people worldwide are suffering with the absence of new and clean drinking water. In the most recent century, the water demand for anthropogenic activities has expanded sevenfold due to the quadrupled worldwide population [1]. A report on“Progress on Sanitation and Drinking Water-2015 Update and MDG Assessment” by the World Health Organization (WHO) indicates that, around 2.6 billion peoples have accessed to enhanced drinking water source in the year 1990 may increase to 663 million people accessing to enhanced drinking water source by the year 2015. The number of individuals living in water scare areas will increase to around 3.9 billion by 2030, as assessed by the World Water Council [2]. The present and the future demand for water assets is expanding, because of increase in urban, industries and natural needs, and it is important to search for the arrangements that permit the effective cleaning of water for conceivable reuse in different ways [3,4,7]. The fast pace of industrialization, population explosion, furthermore, the spontaneous urbanization has leaded to serious contamination of water and soils. The primary wellsprings of freshwater contamination can be ascribed to the release of untreated clean and poisonous industrial wastes, dumping of industrial wastewater, and runoff from agrarian fields. It is remarkable that 70– 80% of all problems in developing countries are identified with water pollution, especially helpless for ladies and youngsters. The toxic pollutants released in wastewaters can be harmful to aquatic organisms which also cause the regular waters to be unfit as consumable water sources [5,6,8–12]. A substantial number of poisons substances, for example, toxic metals, pharmaceuticals, pesticides, dyes, surfactants, and others have polluted the water resources and are ecologically dangerous to individuals and creatures [13–22]. 1.2. Heavy metals Heavy metals are a general term which applies to the group of metals and metalloids and it has an atomic density more prominent than 4000 kg/m3 [23,24]. Almost all the heavy metals are toxic to human beings even at low metal ions concentrations [25–34]; [21]; [37]. Some of the examples for the heavy metals include copper, cadmium, zinc, chromium, arsenic, boron, cobalt, titanium, strontium, tin, vanadium, nickel, molybdenum, mercury, lead, etc. The heavy metals like copper, zinc, nickel, boron, iron, molybdenum are the basic needs for the growth of the plants but these heavy metals are harmful to the creatures and plants when their concentrations go beyond the permissible limits. Other few heavy metals such as lead, mercury, cadmium,

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and arsenic are not essential for the growth of plants and animals. The soil gets polluted due to the heavy metals because of the entry of the industrial wastewater, sewage sludge, fertilizers, utilization of treated wastewater in land application and weathering of soil minerals [24,35,36]. Heavy metals creates not only soil pollution, but it influence on food generation, quality and well being also [38]. Some heavy metals are poisonous to plants even at a very low concentration, while other heavy metals may accumulate in plant tissues to moderately abnormal states with no obvious side effects or decrease in yield [39–43]. The growing of the plants in these heavy metal-contaminated areas, bring change in their metabolism, physiological and biochemical means which results in metal accumulation, lower biomass generation and reduction in the biomass growth. Certain points need to be considered while doing research on the toxicity of heavy metals in plants. To start with, the impacts caused by contaminated soils are perpetual, and their subatomic behaviors must be considered. Most examinations have been done in hydroponic or in vitro culture, and have included the utilization of greatly high metal concentrations in the development media. The harmfulness of a heavy metal relies upon its oxidation state, for example, Cr(VI) is viewed as the most poisonous type of Cr, and as a rule, hap2pens related with oxygen as chromate (CrO24 ) or dichromate (Cr2O7 ) oxyanions. Cr(III) is less portable, less harmful, and prevalently bound to organic matter in soil and aquatic systems [44–46]. The capacity of heavy metals to endure in the soil in the form which is bioavailable to the roots was affected by their adsorption, desorption, and complexation in the soil matrix. These processes were strongly affected by soil composition, pH, and structure. Heavy metals have a tendency to be more portable in acidic soils. The heavy metal poisonous quality is species dependent. For example, metal-tolerant plants and certain plants known as hyper accumulators [able to gather no less than 100 mg/g Cd, As, and some other metals, 1000 mg/g Co, Cu, Cr, Ni, and Pb and 10,000 mg/g Mn and Ni [47]] have barrier mechanisms that dodge harm caused by the heavy metal-initiated pressure, despite the fact that the span and magnitude of exposure and other natural conditions, adds to heavy metal affectability [48]. The people exposed to high amounts of heavy metals may experience the ill effects of different ailments, for example, gastrointestinal and renal toxicity, cardiovascular issues, tumors, hematic, melancholy, tubular and glomerular dysfunctions, and osteoporosis. Newborn children's, kids, and young people are especially immune to heavy metal, bringing about formative difficulties and low insight remainders. Most of the countries have framed the norms for the heavy metals allowable limit in the food to avoid its consumption. The large portion of the heavy metal contaminations are from industrial wastewater which includes mining, pharmaceuticals, electroplating, rubber and plastics, metal finishing, tanneries, organic chemicals, pesticides, timber and wood items, and so forth [49–53]. The heavy metals are transported

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by runoff water and defile the water sources due to the industrial activities. Every single living including plants, animals, and microorganisms rely upon water forever. Toxic metals can attach to the surface of microorganisms by bioaccumulation activities and it may even infiltrate inside the cell and can be chemically changed as the microorganism utilizes biochemical reactions to digest the food materials. Most heavy metals which include copper, cadmium, and zinc are generally connected with pollution and toxic issues, especially when they are available in dissolved form. The presence of any of these heavy metals in extreme level is dangerous to people and can interfere with numerous beneficials of the earth due to their harmfulness and portability. Moreover, these heavy metals are nonbiodegradable, which makes them harder to clean. Thus, it is Important to measure, comprehend, and control of these heavy metal pollution in nature. The details on these three heavy metals are as follows: 1.2.1. Copper Copper (Cu) is one of the regularly utilized heavy metals for different applications. It is generally utilized in electronic chips, batteries, cell phones, semiconductors, water pipes, fertilizer industry, pulp, and paper industry, fungicides, insecticides, catalysts, and metal processing products [54–58]. Copper can enter the environment from the mining of Cu and other metals and from industries which make or utilize the metallic Cu or Cu compounds. Cu is an essential element for human beings. Its quality is essential in the creation of hemoglobin in red platelets. Cu(I) is found in enzymes fit for conveying oxygen which is similar to the activity of haemoglobin. It can help fortify tendons and cartilage. Cu is also utilized in the correct working of a few enzymes. Like iron and zinc, it works as a micronutrient for human beings. Cu is basic for healthy development. In any case, high dosages of copper to living organisms can be to a great degree of unsafe. It can enter the human body through food, residue, and water. The lack of Cu in people can cause anemia, bone, and cardiovascular issues, weakening in mental and sensory systems, defective keratinization of hair, a decrease in levels of synapses, dopamine, and norephedrine, and imperfect myelination in the mind stem and spinal line. Free copper ions, Cu2+, is one of the most harmful types of copper in amphibian life. Utilization of copper-polluted water or foods can cause intense gastrointestinal side effects [59–62]. An intake of high measures of copper salt can cause queasiness and intense gastric aggravation. Numerous instances of detailed intense copper harming are regularly connected with suicide endeavors. Ingestion of a lot of Cu with a dosage surpassing 20 g may result in dazedness, laziness, and cerebral pain at a beginning period. The consequent side effects are epigastric torment, gastrointestinal bleeding, diarrhea, spewing, tachycardia, hematuria, respiratory challenges, hemolytic anemia, hepatocellular putrefaction in the liver, intense tubular rot in the kidney, and demise [63–76]. Patients with Wilson's sickness may hold expanded measures of copper in the liver amid childhood. The side effects typically show up between the ages of 6 and 40 years and if untreated results in liver failure. The permissible limit of copper in drinking water as given by the Bureau of Indian Standards (BIS) is 0.05 mg/L [77]. 1.2.2. Cadmium Cadmium (Cd) has been utilized broadly in batteries, ceramics, electronic and metal-finishing industries, electroplating industries, pigments, petroleum products, textiles, insecticides, solders, television sets, metallurgical industries, synthetic chemicals and photography [78–85,87,88]. The raised concentrations of cadmium in air, water, and soil happen near industrial sources, especially those of nonferrous mining and metal handling activities. Cd is a profoundly harmful metal that can accumulate in the human body and cause irreversible harms to various organic biological systems even at very low concentration. It has been accounted for that Cd is more successfully retained in the aviation routes than in the gastrointestinal tract. As the levels of cadmium in the open air are for the most part underneath 10 ng/m3, the presentation

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through the air isn't an issue of incredible worry, with the exception of in intensely industrialized territories [89]. The Cd will enter into a human being easily through eating routine and tobacco smoking. The high aggregation levels of cadmium are found in offal results of mammals, for example, the kidneys and liver [90–92]. The higher cadmium concentrations are available in a few types of fish, mussels, and clams, which are gotten in the contaminated seaside regions. For instance, the Cd levels in some consumable crabs purportedly came to around 30–50 mg/L [93]. Non-smoking people are highly affected by the consumption of cadmium-contaminated food materials. Tobacco smoking is a basic wellspring of introduction for smokers. It is the most critical human course of Cd allows in noncontaminated regions. One cigarette contains roughly 1 to 2 μg Cd, which is subject to the sort, the brand, and the area of production. A man smoking a pack for each day may retain around 1 μg cadmium. The Cd once entered into the human body, it is bound firmly to metallothioneins. Over half, Cd in the human body accumulates in the liver and kidney as a result of their capacity to orchestrate metallothionein [94]. Cd may, at last, be disposed of through urine [95–97]. In any case, the measure of Cd discharged day by day in urine is extremely constrained (0.005% to 0.01% of the entire Cd content). The low discharge rates of cadmium results in high retention in the body. Cd is outstanding as a tireless poison with a natural half-existence of over 20 years. As the principle stockpiling organ of toxicants, the kidney is dependably the basic target organ that showcases early indications of poisonous quality [98]. Chronic presentation to hoisted levels of cadmium can cause liver harm, bone degeneration, blood damage, and renal dysfunction. There are adequate confirmations in people for the cancercausing nature that outcomes from the presentation of both Cd and Cd compound [99]. The allowable limit of cadmium in drinking water as prescribed by the BIS is 0.003 mg/L [120]. 1.2.3. Zinc Zinc (Zn) is generally uncommon in nature, however, has a long history of utilization due to its available in restricted deposits and simplicity of extraction from ores. Zn available in various minerals which includes ZnO, ZnS, ZnCO3, Zn2SiO4, etc. Economically imperative ores are for the most part those of carbonate and sulfide. Despite the fact that zinc metallurgy is no less than 1000 years of age, zinc (depicted as false silver) has been known for nearly 2000 years. Smelting operation was conveyed to Europe from India and China around the eighteenth century and today zinc is mined and delivered in more than 30 nations. It positions fourth among the metals alongside steel, aluminum, and copper in yearly worldwide utilization. The principal ore, ZnS, available worldwide alongside lead deposits. ZnS oxidizes promptly to produce various optional minerals, for example, ZnCO3. The trace metals which include Ga, Ge, and Cd related with the ZnS minerals are separated during the extraction operation. A large number of the major zinc-lead deposits of the world are named strata-bound stores in carbonate rocks and generally available along and inside dolomite and calcite minerals. Most Zn generation on the planet starts with ZnS minerals. In the wake of mining and processing the ore, zinc sulfide concentrates are changed over into metallic Zn by either pyrometallurgical or a mix of pyrometallurgical and electrolytic operations. The most industrial utilization of Zn originates from its chemical and metallurgical characteristics. The biggest utilization of zinc is in galvanizing iron and steel items. This gives a corrosion resistant coating which can be done with an electroplated metal coating or natural coatings. Such items are utilized in construction, siding, apparatus housings, office hardware, heating and ventilation conduits, vehicle and building enterprises for roofing, vehicle door boards, and underbody parts. Zn is substituted by copper and plastic items in residential plumbing systems. New combinations, for example, zinc-aluminum alloy have been created as protective coatings. The zinc-based alloy, are utilized for indoor and window handles, grills, trim pieces, pumps, carburettors, door locks and in other automobile components. The greater part of

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zinc oxide delivered was utilized as a catalyst for the vulcanization of rubbers. Zinc oxide is likewise required for cosmetics, photocopy paper, agricultural products, paints, and medicinal products. Zn dust, a finely isolated type of the metal, is utilized in separating the silver and gold from the cyanide solution, colouring the textile materials, printing and fats purification. Climate safe paints in view of zinc oxide and zinc dust give a standout amongst the best and strong coatings on the exterior surfaces. The different applications of Zn which includes wood preservative (ZnCl2), pigment (ZnS) and rayon fibre manufacture (ZnSO4). Generally, Zn is a basic micronutrient and a considerably less dangerous metal. But an extreme level of Zn consumption which includes breathing Zn vapors and ingesting Zn-defiled foods and water can produce system dysfunctions, which causes a disability of growth and proliferation [101–103]. Zinc is discharged into the environment mainly due to the industrial operations which include steel manufacturing industries, mining, burning of coal and waste burning operations [104–111]. The zinc poisoning showed the acute symptoms which include diarrhea, liver failure, bloody urine, icterus, kidney failure, stomach cramps, abdominal cramps, epigastric pain, nausea, and vomiting. The zinc poisoning also showed the chronic symptoms which include pancreatic harm, anemia, and lower levels of high-density lipoprotein cholesterol [112–119]. The permissible level of zinc in the drinking water was suggested by the BIS is 5 mg/L [100]. 1.3. Heavy metals toxicity in the environment The release of heavy metals into the aquatic systems may result in various physical, chemical, and biological processes [121]. These can be isolated into two general classifications which include the effects of the heavy metals on the environment and the effects of environment on the heavy metals [122,123]. The first classification depends upon the natural conditions, there might be an adjustment in diversity, density, species composition of population and structure of the community. The nature and the degree of change depend to a great extent on the concentration of heavy metal species in the water and dregs. Hence, the physicochemical processes within the effluents and aquatic systems have a noteworthy, albeit indirect, impact on the biological responses [124,125]. The second classification stresses that the conditions in receiving waters may prompt an adjustment in the speciation and harmfulness of heavy metals. Such conditions incorporate differential contribution of anthropogenic and geochemical material, nature of industrial effluents, and suspended solids and concentration of chelators. The aquatic environment is described by [1] longitudinal varieties in colloidal particles, suspended solids, and natural/synthetic ligands and [2] vertical varieties in redox conditions, level of blending, and densities of living life forms [126]. The destiny of metals in characteristic waters is intensely reliant upon these factors. Changes, for example, methylation and decrease to the metallic shape establish impacts of the environment on metals. Likewise, descending development of metals to the base of regular water bodies results from rummaging by suspended solids and associative sedimentation. Organic ligands and chlorides complex metals, diminishing the sorption procedure and expanding habitation time in the water. Fundamentally, the speciation of metals is dictated by nature, and changes in speciation are responses to modifications [127]. The effects of heavy metal on the aquatic plants are profoundly variable [128]. Albeit the characteristic responses, for example, diminished diversity and density of populations, for the most part, happen in exceedingly defiled zones there are substantially more conflicting impact intolerably or softly contaminated regions. Likewise, the population responses to the toxic heavy metals are altogether impacted by the varieties in common natural parameters, for example, light and temperature [129,130]. Subsequently, biological observing programs in view of community criteria are liable to extensive characteristic problem. This suggests that the evaluation of heavy metals impact and management of these toxic heavy metal releases must not be exclusively based on the

density and diversity measures. The release of heavy metal may likewise create changes in physical conditions in receiving aquatic environment. These incorporate changes in water pH, organic content of the substrate and size of the particles in water. The plants in aquatic systems react to such bothers by a decline in density, diversity, and composition of species. Thus, the problem can be experienced in depicting impacts of heavy metal pollution from those physical impacts which are actuated indirectly [131–134]. 1.3.1. Impact of heavy metals on plants The plants are sessile life forms which must be adapted to different composition of the soils with respect to their living and reproduction. Soils are frequently contained with unnecessary levels of essential and non-essential elements, which may be harmful at high concentrations based upon the plant species and the soil qualities [109]. Numerous metals are commonly shared their basic lethality mechanisms, and the plants manage these metals by utilizing general removal routes. The effects due to the metal toxicity are made more intricate by rivalry, since the elevated amounts of one metal may imbalance the removal and movement of others and which provides the toxicity behavior [135–137]. The plants procure minerals from soil fundamentally in the form of inorganic ions. The expanded root and its capacity to assimilate ionic compounds even at low concentrations which makes mineral uptake to be more effective. The mineral compounds can be classified into two categories: (i) essential nutrients (macro and micronutrients) and (ii) non-essential nutrients. The essential macronutrients (nitrogen, phosphorus, potassium, magnesium, calcium, silicon and sulfur) and the micronutrients (sodium, manganese, iron, chlorine, boron, molybdenum, copper, zinc and nickel) are important elements to the plant structure and its metabolism [138]. The absence or deficiency of these nutrients may decrease the fitness and hinders the growth and development. Micronutrients are required in just little amounts and their excess amount in the soil (particularly Zn, Ni, and Cu) due to either natural availability or due to industrial activities. The different minerals, for example, chromium, cadmium, lead, mercury, antimony, silver, and arsenic, are dangerous to plants even at low concentrations [139–141]. The increase number of focuses for the toxicity of heavy metal implies that the negative impacts have a tendency to be seen in those cells that are exposed at first (cells in charge of the metal take-up) [142]. Heavy metals interfere with ionic homeostasis and the activity of enzymes, and these impacts are obvious in physiological procedures including single organs, (for example, supplement take-up by the roots) trailed by more broad procedures, for example, germination, development, photosynthesis, plant water balance, essential digestion, and multiplication. In reality, unmistakable side effects of heavy metal harmfulness incorporate chlorosis, senescence, leaf rolling and putrefaction, low biomass generation, shriveling and hindered development, restricted quantities of seeds, and in the long run demise [143–145]. Heavy metal pollution in an agricultural soil is a serious environmental problem due to the wide distribution of the heavy metals in the environment and its effects (acute and chronic) on the growth of the plant [24]. The higher concentration of Cu exposure in the plants leads to produce the reactive oxygen species and oxidative stress [146,147]. The growth of the plant and its metabolic activities and acceptance of oxidative harm in different species was observed due to the phytotoxicity of Cd and Zn [148,149]. The higher concentration of Pb in the soil instigate irregular morphology in many plant species [150]. The higher concentration of Ni content in plant tissues demonstrates disability of nutrient balance and results in scatters of cell membrane capacities [151]. The exposure of excess Cr to the plants leads to affect the photosynthesis process in respect to carbon dioxide fixation, activities of enzymes, photophosphorylation and electron transport. Indications of As phytotoxicity incorporate leaf putrefaction and shrinking, trailed by root staining and hindrance of shoot development [152–156]. Many literature showed that the likelihood of As activity through enlistment of flagging pathways, particularly those associated

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with membrane harm, electrolyte spillage, and the production of reactive oxygen species [157–159]. The physiological issues and the visible damage were observed in the plants due to the high-level exposure of Hg [160,161]. The toxicity of Mn in the few species begins with chlorosis of the more seasoned leaves which moves to the more youthful leaves with time [162]. The excess amount of Fe generates the free radicals which irreversibly weaken the cell structure and harms the membrane, proteins, and DNA.

1.3.2. Impact of heavy metals on human health The heavy metals in a concentration higher than the critical values can produce serious health issues. The toxicity due to heavy metals may harm or decrease the mental and central nervous activities, damage the lungs, liver, kidneys, blood compositions and other fundamental organs. The long period of exposure of toxic heavy metals may cause muscular dystrophy, Alzheimer's disease, different types of cancers and multiple sclerosis. The exposure of heavy metals into human beings mainly observed through the three noteworthy routes which include the oral ingestion, inhalation and the dermal exposure [163,164]. The essential route of heavy metals exposure to human being relies upon the characteristics of heavy metals. The oral ingestion is the essential pathway of copper enters into human body because of its water solubility in nature. Likewise, ingestion is the essential pathway of mercury by means on the intake of marine living beings in which organic appearance of Hg (methyl mercury) have bioaccumulated at high concentrations in the living tissues [165–167]. The mercury intake into the human being is mainly due to the consumption of dietary fish. A few variables influence the ingestion of heavy metals by means of the gastrointestinal tract which includes metal solubility, chemical structures and availability of different compounds. The food intake is considered as one of the essential routes of these heavy metal exposures while it was compared with inhalation and dermal exposures. Inhalation is considered an essential route of occupational exposure. A few metals importantly affect its potential for inhalation exposure and exist in nature as vapors. Mercury is one of these classifications, which exists in the climate in vaporous state establishing 80% of total atmospheric mercury [168]. The skin is the successful barrier against retention because of the structure of the external most keratinized layer of epidermal cells [169,170]. Along these lines, the heavy metals exposure through the skin path is just a worry for a few metals. A few metals have the capability to penetrate into the human skin when it was combined with other compounds. These days, heavy metals have become most well-known contact sensitizers in people, especially nickel, increases the rate of nickel hypersensitivity in kids, particularly in developed countries [171,172]. One of the essential wellsprings of contamination with heavy metals is road dust, which is portrayed by fine particles with gigantic surface regions that are effectively exchanged and stores the miniaturized scale contaminations (heavy metals). The food chain is perceived as one of the major pathways for human exposure to soil pollution [173,174]. The generation of heavy metals into the food chain is of unique worry because of the variously related wellbeing dangers in humans and animals. Heavy metals are extremely poisonous and can possibly cause serious harm, even at low concentrations [175]. The different concentration of heavy metals has been identified in different food items which include juices, beverages, wines, etc. The exposure of heavy metals on human beings provide the health impacts which include cancer, developmental retardation, kidney damage, immunological impact, endocrine disruption and neurological impacts [176–179]. The removal of toxic heavy metals from water/wastewater is important in light of the fact that they provide serious health dangers to people and in addition to other living creatures. In perspective of this, it is important to develop treatment strategies to remove harmful metal ions from various networks to decrease the pollution load to nature.

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1.4. Treatment methods The heavy metals enter into human food chains through bioaccumulation mechanism and provide toxicity to the biological systems due to increased metal ion concentration over the time period. These heavy metals can easily enter into the aquatic system due to industrial wastewater, agricultural runoff, household, and commercial applications. The different treatment methods available for removal of toxic heavy metals from the water/wastewater includes chemical precipitation, chemical coagulation and flocculation, electrochemical methods, membrane filtration, ion exchange, bioremediation, and adsorption (shown in Fig. 1). 1.4.1. Chemical precipitation Chemical precipitation is widely used in the removal of heavy metals from the wastewater because it is inexpensive and it is relatively simple to operate [180–182]. The pH of the wastewater will adjusted to the basic conditions at the start, then the precipitating agent will be added. It reacts with the heavy metal ions in the wastewater to form the insoluble precipitates. The formed precipitates can be separated either by sedimentation or filtration operations. The traditional chemical precipitation processes includes sulfide precipitation and hydroxide precipitation. The hydroxide precipitation process is the most widely used chemical precipitation techniques in the treatment on the removal of heavy metals from the industrial wastewater because of its low cost, simplicity and easy to control the pH [183]. The solubility of the different metal hydroxides is limited in the pH range of 8.0 - 11.0. The formed metal hydroxides can be separated by flocculation followed by the sedimentation operations. The different types of hydroxides had been used to precipitate the heavy metals from the wastewater based on the easy to handle and low cost [184]. The lime was used as an important hydroxide precipitating agent in most of the industrial wastewater treatment [185]. The heavy metals such as copper and chromium were removed from the wastewater using hydroxide precipitating agents such as Ca(OH)2 and NaOH [186]. In hydroxide precipitation process, the addition of chemical coagulants such as alum, salts of iron and organic polymers can able to increase the removal of heavy metals from the water/wastewater. Even though the hydroxide precipitation was widely used but this has limitation in industrial application due production of large volume of the low-density sludges which creates the dewatering and disposal issues [187]. Furthermore, few metal hydroxides are amphoteric, and the mixed metals make an issue in utilizing the hydroxide precipitation since the ideal pH for one metal may return another metal into the solution. Also, while complexing agents are in the wastewater, they will restrain the metal hydroxide precipitation. The sulfide precipitation is likewise a successful procedure for the treatment of harmful heavy metals [188]. One of the essential important on utilizing the sulfides is the solubilities of the metal sulfide precipitates are significantly lower than hydroxide precipitates and sulfide precipitates are not amphoteric. The sulfide precipitation process can accomplish a high level of metal removal over a wide pH range compared with the hydroxide precipitation methods [189]. Metal sulfide sludges likewise display preferable thickening and dewatering qualities over the metal hydroxide sludges. In any case, there are potential perils in the utilization of the sulfide precipitation process [190]. Generally, the heavy metal ions are regularly in acid conditions and sulfide precipitants in acidic conditions can result in the increase of dangerous hydrogen sulfide vapour. It is basic that this precipitation process is performed in the neutral or basic conditions. Besides, metal sulfide precipitation tends to frame colloidal precipitates that provides some separation issues in either filtration or sedimentation processes. The chemical precipitation requires a lot of chemicals to decrease the metals to an adequate level for release into the environment [191]. Other disadvantages include a large amount of sludge generation, increasing sludge disposal cost, slower metal precipitation, poor settling and the long haul ecological effects of sludge disposal [192].

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Fig. 1. Treatment methods for the removal of heavy metals from wastewater

1.4.2. Chemical coagulation/flocculation Coagulation/flocculation can be utilized to treat the heavy metal contaminated wastewater [193–195]. Essentially, the coagulation treatment destabilizes the colloidal particles by using a chemical coagulant and results in sedimentation. Generally, the coagulation is followed by flocculation of the unstable particles into massive floccules to increase the size of the particle [196]. Finally, the massive floccules will be settled in the sedimentation tank. Numerous coagulants broadly utilized in the traditional wastewater treatment technologies includes alum, ferric chloride, ferrous sulfate, etc for the removal of pollutants from the wastewater [197]. Coagulation is a standout amongst the most vital techniques for wastewater treatment, however, the principal objects of coagulation treatment are for the suspended solids and hydrophobic colloids. Flocculation is the activity of polymers to make the connection between the flocs and tie the particles into huge agglomerates. Suspended solids are flocculated into bigger particles and are easily removed by sedimentation, filtration, and floatation or straining. The numerous types of flocculants includes poly-aluminum chloride, polyacrylamide, and polyferric sulfate are utilized in the wastewater treatment but these flocculants cannot be directly utilized for the removal of toxic metal ions from wastewater. Macromolecule flocculants have been utilized as an effective flocculant for the removal of heavy metals from the wastewater [198–201]. Generally, the heavy metals cannot be completely removed by the coagulation/flocculation treatment units alone, but it can be completely removed by considering other treatment along with this treatment [202–206]. Though it has advantages, coagulation/flocculation has restrictions, for example, high operational costs due to a large amount of chemical utilization.

1.4.3. Electrochemical methods The electrochemical methods have been known as an extremely effective wastewater treatment methods particularly for the removal of heavy metal ions from industrial wastewater. This method involves the recovery of the heavy metals in the elemental metallic state by using the anodic and cathodic reactions in the electrochemical cell. This method required a large amount of capital investment and more

power supply which limits its wide industrial applications. Due to the stringent environmental rules and regulations for the wastewater discharge into the environment, the electrochemical methods have been attracted many scientists and researchers to work in this research field. Some of the electrochemical methods include electrocoagulation, electrodeposition, and electroflotation techniques have been employed in the removal of heavy metal ions from wastewater. Electrocoagulation process comprises of electrodes which act as the anode and cathode where the oxidation and reduction reactions taking place, respectively [207]. Numerous physicochemical processes which include oxidation, reduction, coagulation, and adsorption govern the electrocoagulation process. Similar to other treatment methods, electrocoagulation process for the removal of heavy metals provides the easy handling and cost-effective methods on an industrial scale operation. This process involves in the production of coagulants in situ by dissolving electrically either iron or aluminium ions from iron or aluminium electrodes [208]. The anode produces the metal ion and the cathode releases the hydrogen gas during the process. This produced hydrogen gas used to float the flocculated particles from the water [209,210]. This treatment method has already applied for the removal of heavy metals, dyes, fluorides, nitrates, sulfates, pharmaceuticals and phenolic compounds from wastewater [211–214]. Electrocoagulation process is thought to be intended for any treatment capacity which gives better removal efficiency for the toxic heavy metals. In the electrocoagulation process, no need to add the chemicals because the electron is the essential reagent. This method is indicated as an eco-friendly technology when it was compared with the coagulation process because the electrocoagulation process produces a lower quantity of sludge, which is more stable. But few limitations in the electrocoagulation process which include higher operating and capital investment cost [215,216]. Electrodeposition method is widely used in the recovery of toxic metal ions from the wastewater by using suitable electrode [217]. It is a clean treatment method for the recovery of heavy metals from the wastewater without the presence of permanent residues. The major advantages of this method include the recovery of metal in the purest form, operating cost is low and no problem in the disposal of sludges

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[218,219]. Electroflotation is also widely applied in the removal of heavy metals from industrial wastewater [220–222]. It is a solid-liquid separation process which floats the toxic heavy metals to the water surface by small bubbles of oxygen and hydrogen gases produced from the water electrolysis [223]. Many heavy metals which include lead, copper, zinc, nickel, iron and etc. have been successfully removed from the wastewater by this treatment technique [224,225]. Electrochemical methods are quick and well-controlled treatment methods for the removal of heavy metals from industrial wastewater, which needs lesser chemicals, generate less sludge and provide the maximum metal ions removal. In any case, the electrochemical methods required higher initial capital speculation and the costly power supply, which confines its improvement. 1.4.4. Membrane filtration Membrane filtration treatment methods showed excellent results for the removal of heavy metals from the wastewater [226]. Membranes are in the form of complex structure which contains dynamic elements on the nanometer scale. The membranes of the recent reverse osmosis system are commonly a homogeneous polymer thin films bolstered by a permeable support structure. The permeability of the water through the membrane and the rejection of heavy metal ions in the membrane mainly depend upon the chemical and physical properties of the membrane [227,228]. The main advantage of this method is higher removal efficiency, lesser space requirement and operation is easier. The different types of membranes which include reverse osmosis, ultrafiltration, nanofiltration, and electrodialysis have been successfully employed for the removal of toxic metal ions from the wastewater. The reverse osmosis consists of a semi-permeable membrane which allows the water to pass through in it and rejects the toxic heavy metals from the wastewater. This method is one of the finest treatment methods for the removal of dissolved metals from the water/wastewater. The required membrane was used in reverse osmosis to remove the different toxic metals from the wastewater [229–232]. The important disadvantage of this method is handling of the rejection, membrane fouling, and high power cost. Ultrafiltration method is operating at low transmembrane pressures for the recovery of dissolved and colloidal solids [233]. Generally, the dissolved solids are easy passes through the ultrafiltration membrane due to the fact that the pore sizes of the ultrafiltration membrane were larger than the dissolved solids. The effective removal of heavy metal ions was achieved by the polymer enhanced ultrafiltration and micellar enhanced ultrafiltration methods [234–237]. Nanofiltration is the intermediate operation between reverse osmosis and ultrafiltration. This treatment method is employed for the removal of toxic heavy metals from the wastewater [238–241]. This treatment method has advantages which include high efficiency, relatively lower energy requirements, reliability and easy operation [242]. Electrodialysis is the type of membrane operation for the separation of ions from one solution to another solution using charged ion exchange membranes under an electric field. The anion-exchange and cationexchange membranes are the two types of ion exchange membranes used in the applications. Generally, this treatment method was successfully employed for the treatment of industrial wastewater, generation of drinking and process water from seawater, recovery of useful materials from industrial effluents and manufacture of salts [243]. This treatment method was also adopted for the removal of toxic heavy metals from the industrial wastewater [244–247]. Membrane treatment methods can able to successfully remove the toxic heavy metals from the wastewater but these methods have an issue which includes complexity in the process, higher cost, membrane fouling, and lower permeate flux. 1.4.5. Ion-exchange Ion-exchange process has been extensively employed in the removal of heavy metals from the wastewater due to its excellent removal rate, higher treatment capacity and rapid kinetics [248–252]. In this process,

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the ions (cations or anions) in the solution are replaced by the same kind of ions (cations or anions) on the insoluble material (called as ion-exchange resin). The heavy metal wastewater is entered into the ion-exchange column at the one end and is travelled through the bed, which actually removes the heavy metals in the wastewater. If the column is saturated with the heavy metals then the column is backwashed to remove the deposited heavy metals and the column was regenerated [253–256]. The ion-exchange resin may be of either natural or synthetic resin, has the specific role to remove the heavy metals from the wastewater. Among the different natural and synthetic resin materials used in heavy metal removal from the wastewater, the synthetic resins are mostly preferred because they are effective for the removal of heavy metals from the wastewater [257]. The cation exchange resin is most widely used to remove the heavy metals from the wastewater. The most general cation exchangers are a strong acidic resin with sulfonic acid groups and weak acidic resin with carboxylic acid groups. The presence of hydrogen ions in the acidic groups of the resins can act as an exchangeable ion with the cationic forms of metal ions from the wastewater. Other than synthetic resins, natural zeolites and natural silicate minerals have been widely applied to remove the heavy metals from the wastewater due to their large availability in the environment and its low cost [258–260]. Though the natural materials have applied in the removal of heavy metals, its application is limited as compared with the synthetic resins because it is employed in lab scale system only but not in industrial scale level. Even though ion-exchange method is widely used, the chemicals used to regenerate the ion-exchange resins produce serious secondary pollutants. These secondary pollutants must be handled properly. Ion-exchange process is expensive because it requires a large amount of resin to treat the large volume of the wastewater which consists of lower metal ions concentration. 1.4.6. Bioremediation Bioremediation process is a treatment method whereby the biological systems, plants, and animals which includes the microorganisms to remove the toxic pollutants from the aquatic environment [261–264]. Recently, microbe assisted bioremediation process has been widely used in the removal of heavy metals from the wastewater. The traditional treatment methods for the removal of heavy metals are not financially effective and may create unfavourable effects on the aquatic systems. The microbe assisted bioremediation and phytoremediation methods for the removal of toxic heavy metals are financially effective treatment methods [265–269]. Aquatic plants such as wetland ecosystems have exceptional properties to remove the heavy metals from the wastewater. Wetland ecosystems are much unrivaled in the examination with other traditional strategies due to the low cost, easy to handle, regular microorganisms growth and low cost for maintenance [270]. The rhizospheres in wetland ecosystems give an upgraded supplements supply to the microbial environments of plants, which effectively convert and remove the heavy metals in their biological functions [271]. Constructed wetlands have been effectively utilized for the treatment of toxic metals from rural overflow, municipal squanders and mine seepage [272]. Numerous aquatic plants which include Typha, Eichhornia, Phragmites, Azolla and Lemna have been utilized for the removal of toxic metals from the wastewater ([273,275–281],562). Phytoremediation is the low-cost method for the treatment of wastewater, groundwater and polluted soil [561]. Plants are extremely touchy to heavy metals however in phytoremediation wild and hereditarily altered plants, including herbs, grasses, woody species, and forbs, are for the most part utilized. The plants consume the metals through the method of phytoextraction, phytostabilization, rhizoremediation or phytofiltration [283–287]. Generally, the organic compounds are metabolized but the heavy metals are not metabolized but instead, it is accumulated in the plant biomass [288]. The biomass produced by phytoremediation stays exceptionally constrained in sum and holds on, while all the biomass can be used as forage, manure, mulch or for the generation of biogas [289,290]. Despite the fact that it is notable

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that metals are harmful to numerous plants, they have built up some inside components that permit the take-up, resistance, and collection of high concentrations of metals that would be lethal to different living beings. Being a low cost and easy process, phytoremediation can be executed for their improvement to metal aggregations and translocations. By and large, two systems of phytoextraction have been produced, which are: (i) ordinary phytoremediation of heavy metals from aquatic system through the plants in their whole development cycle and (ii) chemically instigated phytoextraction systems to clean up polluted water by utilizing metal-tolerant plants to evacuate the heavy metals [291–296]. The effectiveness of phytoextraction can be expanded by utilizing more biomass creating plant species and with the use of reasonable chelates. Hyperaccumulators or hyperaccumulating plants are equipped for aggregating a lot of heavy metals which includes lead, cadmium, zinc, nickel, and arsenic, in their over-the-ground tissues with no poisonous side effects [297–300]. Metals take-up in connection to the outside concentration of the harmful heavy metals may contrast because of the diverse genotypes of plants. The plants in which have low take-up of metals at very high metal concentrations are called excluders. These plants have some sort of boundary to stay away from take-up of heavy metals, in any case, when metal concentrations are at a high level this obstruction misfortunes its capacity, presumably because of the lethal activity of the metals. A few plants have certain detoxification system inside their tissue, which enables the plant to the aggregate higher quantity of metals [301–304]. In any case, phytoremediation on an industrial scale is constrained due to its low biomass generation, constrained growth rate, and time utilization. Algae, unicellular or multicellular organisms which are normally available in terrestrial, fresh and marine environments. These algae can also be available in the snow and the rocks in beneficial interaction with fungus, for example, lichen. Some important applications of algae such as vitality sources, fertilizers, pollution abatement, nourishment, etc. The algae have been effectively utilized in the removal of organic and inorganic pollutants from wastewater. Many research works have been already reported on the removal of heavy metals from wastewater using the different types of algae [305–316]. The algae cell wall consists of different functional groups such as amino, hydroxyl, carboxyl, and sulfate are act as the main binding sites for the removal of heavy metals from wastewater [317]. The properties of the algae, characteristics of the heavy metals and wastewater characteristics mainly affect the removal of heavy metals by the algae. Some of the major types of algae which includes green microalgae (in freshwater) and macroalgae (marine green, red and brown) have been effectively applied for the removal of different kinds of heavy metals from wastewater. The heavy metals generally enter into the cell wall of the algae (rapid process) followed by entering into the inside of the cell (slow process). Few green microalgae includes Chlorella Vulgaris, Chlorella miniata, Chlamydomonas reinhardtii, Cladonia rangiformis, Fucus spiralis, Laminaria Hyperborea, Sargassum filipendula, Sargassum filipendula, Sargassum wightii, Sphaeroplea, Turbinaria conoides, Spirulina platensis, etc., the green macroalgae (Caulerpa lentillifera, Cladophora fascicularis, etc.), the marine red macroalgae (Galaxaura oblongata, Gelidium sesquipedale, Corallina Mediterranea, Pterocladia capillacea, Jania rubens, etc.) and the marine brown macroalgae (Laminaria japonica, Ascophyllum nodosum, Pomacea canaliculata, etc.) have been successfully adopted for the removal of toxic heavy metals from the wastewater [318–322]. Bacteria (Escherichia coli, Bacillus cereus, Pseudomonas fluorescens, Bacillus cereus, Bacillus firmus, Kocuria rhizophila, Bacillus licheniformis, Micrococcus luteus, Cupriavidus metallidurans, Staphylococcus xylosus, Bacillus megaterium, Enterobacter cloacae, Streptomyces rimosus, Pantoea agglomerans, Salmonella typhi, Pseudomonas aeruginosa, Ochrobactrum intermedium, Desulfovibrio desulfuricans G20, Desulfovibrio vulgaris, Desulfovibrio desulfuricans, Desulfovibrio sp., etc.) and Fungi (Aspergillus niger, Aspergillus sydoni, Ganoderma

lucidum, Cephalosporium aphidicola, Pleurotus sapidus, Tolypocladium inflatum, Neurospora crassa, Trichoderma longibrachiatum, Saccharomyces cerevisiae, Mucar rouxii, Penicillium janthinellum, Rhizopus arrhizus, Rhizopus oryzae, Saccharomyces cerevisiae, Lepiota hystrix, Aspergillus brasiliensis, Aspergillus flavus, Penicillium cirtinum, Penicillium oxalicum, Aspergillus terreus, Pleurotus platypus, Phanerochaete chrysosporium, Penicillium citrinum, etc) species have also been effectively utilized for the treatment of heavy metal contaminated wastewater [323–327]. Bacteria (prokaryotes) is a single cell organism. This single cell organism may have a membrane-bounded nucleus or maybe a membrane-bounded organelle similar to chloroplasts and mitochondria. Fungi is a eukaryotic organism. Almost all the fungi species develop as tubular fibers called as hyphae. The interlaced mass of hyphae is called as mycelium. The surface of the walls and envelopes of these two organisms have the adequate functional groups to remove the heavy metals from the wastewater [328–332]. Most of the biomasses have the capacity to remove the heavy metals from wastewater but all of them cannot act as alternative materials in real industrial wastewater treatment. 1.4.7. Adsorption The adsorption process is currently perceived as most economic, efficient and selective treatment method for the removal of heavy metals from wastewater [333–338]. This process provides flexibility in operation and design in the complete recovery of heavy metals from wastewater. Adsorption is a solid-liquid mass transfer operation, where the heavy metal (adsorbate) is migrated from the wastewater to the solid surface (called adsorbent) and then bonded due to chemical or physical adsorption over the adsorbent surface. Physical adsorption is due to the weak Van der Waals forces' of attraction and the chemical adsorption is due to the strong covalent bond between the adsorbent and adsorbate. Sometimes, the heavy metal adsorption is a reversible process which makes the adsorbent to be regenerated by using the proper desorption methods. Activated carbon (AC) is a most widely used and popular adsorbent for the removal of heavy metals from the wastewater due to its higher surface area and its higher affinity towards the heavy metals [339–343]. The different forms of the AC which include granular activated carbon (GAC), powdered activated carbon (PAC) and woven carbon (also called activated carbon cloth (ACC)) have been employed in the water treatment [344,345]. AC is generally produced for the treatment of organic pollutants but it can also be used for the removal of heavy metals from wastewater. The commercially produced activated carbons are divided into two types which include H and L type adsorbents [346,347]. If the carbon is activated at high temperature and removes H+ ions then these type of material is called H type AC. The examples for the H type activated carbons are dust coal AC and coconut shell based AC (Filtrasorb 200 and Filtrasorb 400) [348,349]. If the carbon is oxidized at low temperature and remove OH- ions then these type of material is called L type AC (Ex: AC from wood-based materials). GAC is mainly used in both batch and column mode of operation for the removal of heavy metals from wastewater [350,351]. Sometimes, PAC is applied but it is not possible to completely recover the adsorbent for its regeneration [352]. The important surface properties of the AC such as surface area, functional groups, surface charge, size and porosity (micropore and mesopore) are the main reason for the removal of heavy metals from wastewater. The application of AC is restricted due to its high cost and limited regeneration behaviour. From the economic aspect, it is not feasible to utilize the AC for commercial or industrial wastewater treatment. To overcome this issue, the waste materials from agricultural operations that are available in large quantities can act as an alternative material to the AC for the treatment of the heavy metal contaminated wastewater. This treatment technique has emerged as low cost and effective alternative materials for the removal of heavy metals from wastewater. The advantageous of these materials over the conventional materials such as low cost, higher efficiency, higher adsorption capacity, easy operation, no sludge production, no

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extra nutrients needed, the possibility of regeneration and recovery of attached metals from the spent adsorbent. 2. Adsorbents The heavy metals from the different industries include electroplating, batteries, electrical, tanneries, pesticides, mining operation, fertilizers, ore refining, etc. were entered into the aquatic system and pose serious environmental issues to the living organism even at very low metal ions concentration. Even though, AC is widely employed in the removal of these heavy metals from the wastewater its usage is limited due to the high cost and limited regeneration ability of these materials. Many researchers/scientists worked/working on the preparation of low cost and efficient adsorbent to replace the available commercial AC for the removal of heavy metals from wastewater. Recently the attention has been focussed towards the utilization of biomaterials which are available in a huge amount such as the agricultural waste products and the byproducts or the industrial wastes, for the treatment of heavy metal contained wastewater. The natural resources in the environment are grown in a shorter period due to agricultural activities and these natural resources are considered as the inexhaustible assets. Furthermore, the greater part of the agricultural waste biomasses contains chemical compounds that have high carbon constituents, which make them reasonable antecedent for adsorbent preparation. Accessibility of low-cost by-products from biomass generation and preparing industry has ended up being a potential precursor for the preparation of adsorbent. A few endeavours have been made to recognize financially savvy strategies and antecedents to deliver adsorbents. 2.1. Adsorbents from industrial byproducts/wastes The industrial byproducts/wastes are one of the low cost and efficient adsorbents for the removal of heavy metals from wastewater. These materials are produced either as byproducts or leftover materials from the industrial operations. These industrial byproducts/wastes often required less processing to improve its adsorption capacities. Because of the large availability, higher adsorption capacity, higher efficiency, and low cost these materials can be utilized as an effective alternative adsorbent as compared with the commercial adsorbents for the removal of heavy metals from wastewater. Industrial byproducts/wastes include lignin, fly ash, sludge, blast furnace slag, and red mud have been applied for the removal of heavy metals from water/wastewater. 2.1.1. Adsorbents from lignin Lignin is a natural polymer, which is the second most plentiful natural polymer after the cellulose polymer. This lignin is essentially found in the cell mass of woody tree species [353,354]. The fundamental wellspring of lignin promptly accessible for use on a bigger scale originates from spent pulping alcohols and chemical freedom of wood strands from the paper and pulp enterprises. Lignin has not yet been changed over into high-esteemed items on an expansive scale and today lignin is fundamentally utilized for vitality recuperation at the pulp plants. The conventional pulping methods include Kraft and sulphite cooking processes. Kraft lignin is typically separated by the precipitation that remaining parts subsequent to cooking via acidification and protonation of phenolic groups. Contingent upon the value of pH to which the black liquor is acidified, a various composition and yield of the lignin is acquired. A hydrosoluble type of lignin that contains an increase number of charged groups, the lignosulfonates, is readied from the byproducts of the sulfite pulping process where the lignins are sulfonated, debased and solubilized. The organosolv procedure is a pulping procedure utilizing liquid organic solvents with the lignocellulosic biomass isolating the lignin piece by means of solubilization. The homogeneity of the organosolv handled lignin is larger than that of lignosulfonates and Kraft lignins. The extracted lignin from black liquor

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has been applied for the removal of heavy metals from wastewater [355–367]. The important functional groups are available in the lignin materials to include hydroxyl, phenolic, methoxy, benzyl alcohol, aldehyde, carboxyl, and sulfonate groups but it varies with the pulping operations [368]. The mechanism on the removal of heavy metals by the lignin was predicted many researchers and this may be due to the surface adsorption, ion exchange, and complexation. But the detailed adsorption mechanism for the specific removal of heavy metals from wastewater by lignin must be reported. The adsorption influencing parameters like pH, temperature, contact time, initial metal ions concentration and adsorbent dose have been experimented and many kinetic and isotherm models have been analyzed by the various researchers. 2.1.2. Adsorbents from fly ash The thermal plant is generally producing a large amount of fly ash during the combustion of coal [369]. The fly ash provides the two important solutions for the environmental problems which include the removal of pollutants from the wastewater and the solid waste management. The fly ash mainly consists of silica, alumina, calcium oxide, ferric oxide and carbon [370]. The composition of the fly ash is varied with the operational behaviour and quality of the coal during its combustion operation. The low adsorption capability of the fly ash has been improved by the surface modification procedure which includes chemical or physical modification procedures [371]. The importance of the surface modification procedure was to improve the surface properties of the adsorbent, particularly, surface area and surface affinity towards the heavy metals removal from wastewater. Fly ash is a strongly alkaline material and which provides the pH of 10-13 when it was added to the water. The surface of the fly ash was acquired negatively charged at higher pH values. Generally, the removal of heavy metal ions from wastewater by the fly ash was either may be due to the electrostatic adsorption or precipitation. The fly ash is in raw or surface modified form has been effectively utilized for the removal of different heavy metals from wastewater [26,372–380]. 2.1.3. Adsorbents from sludge and blast furnace slag The sludge is generated from the wastewater treatment plants and electroplating industries. The produced sludge is dried and becomes the solid waste, which is being utilized as an effective adsorbent for the removal of heavy metals from wastewater [365,381–385]. Blast furnace slag is generally produced in large quantities from the steel processing industries [386,387]. These wastes have attracted many scientists/researchers to utilize these materials as an efficient adsorbent for the treatment of heavy metal contained wastewater [388–392]. This blast furnace slag has a higher adsorption capacity because this consists of aluminium and iron oxides with them [393–395]. The different adsorption affecting parameters such as pH, contact time, temperature, adsorbent dosage, and initial metal ions concentration have been studied to observe the optimum conditions for the highest removal of heavy metals from wastewater. The adsorption kinetics, equilibrium, and thermodynamic analysis have been experimented to predict the nature of the adsorption process [390,396,397]. The column analysis on the removal of heavy metals from the wastewater by these waste materials has also been experimented by many researchers [398]. 2.1.4. Adsorbents from red mud Red mud is a solid waste generated from the bauxite ores during the process of alumina extraction [399]. It has a corroded red tint emerges from the presence of ferric oxide. Approximately, 120 million tons of this dangerous material delivered far and wide annually [400,401]. Red mud is a standout amongst the most vital disposal issues in the mining and metallurgical industries. The amount of generation varies enormously relying upon the kind of bauxite and method utilized in the refining procedure. The release of red mud is harming to the earth as a result of its high alkalinity, various substantial metals and a little measure of radioactive components [401,402]. In this manner, it is a

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gigantic task for the alumina ventures. The bauxite residue has limited applications in paving, construction materials and land reclamation [403]. Store and dumped adrift are the primary disposal strategies for handling this waste. The huge tract of land utilized by red mud stockpiling raises the financial expense as well as strengthens the danger of natural contamination, for example, surface/groundwater contamination and fog, for neighbouring networks [404]. In this manner, endeavours and government should give careful consideration to reusing and dealing with the sharp increase and huge volume of this strong waste. The advancements have been made in utilizing red mud as low-cost adsorbents for fumes gas and wastewater treatment and also it was utilized as catalysts for catalytic reactions [405,406]. The red mud is effectively used as low cost material either is in raw form or surface modified form for the removal of toxic heavy metals from wastewater [407–414]. 2.2. Adsorbents from agricultural byproducts/wastes Agricultural byproducts/wastes, residues are generated from the agricultural and forestry activities. The primary kinds are agricultural and forestry biomass which includes mainly straw, dross, grouts, husks, and morsels. The enormous measure of agricultural waste has numerous focal points in wastewater treatment which includes low cost, bounteous source, short recovery cycle, chemical stability, inexhaustible cycle, green energy and eco-friendly. In addition, these materials have a high porosity and larger surface area, which makes these materials, can act as efficient adsorbent to remove the toxic heavy metals from the wastewater [80,415–422]. The straw is an agricultural byproduct, generated from the agricultural harvest which includes sugar cane, rice, beans, wheat, oil, cotton, and corn. The produced byproducts have been effectively applied for the treatment of heavy metal contained wastewater. In this manner, the use of agricultural waste as an adsorbent to treat heavy metal contamination in water is a promising technique. Specifically, this not just serves to completely use a large amount of straw resources, yet in addition, gives a long haul improvement course of green cutting edge and natural protection. Agricultural waste generally consists of cellulose, lignin and hemicellulose compounds in it [423,424]. The composition of the agricultural wastes of these components is varied and which depends upon the types of waste and its local environmental conditions. Generally, it is the range of 35–50% of cellulose, 20–30% of lignin and 15–30% of hemicellulose in the agricultural wastes [424,425]. Lignocellulose is largely available in agricultural waste [563]. Cellulose is a linear syndiotactic polymer of glucose and which is formed by β-(l→4)- glycosidic bonds [415]. The primary objective of the cellulose extraction is to expel the lignin and hemicellulose from the agricultural wastes [426]. Generally, in the industry, the cellulose is collected from the chemical pulping methods by using alkali and sulfite treatment procedures [427]. Lignin is a crosslinked aromatic polymer which is comprised of coniferyl alcohol, p-coumaryl alcohol, and sinapyl alcohol. Hemicellulose is comprised of different uronic acid groups. It is a heteropolymer of Dxylose, D-glucose, D-galactose, D-mannose, L-arabinose and Dglucuronic acid. Agricultural by-products are mainly consists of lignin and cellulose as major constituents. This may also consist of other polar functional groups of lignin such as aldehydes, alcohols, ketones, ether, phenolic and carboxylic groups. These functional groups are having the ability to remove the heavy metals from the wastewater [428,429]. Some of the agricultural byproducts/wastes being widely employed in the removal of heavy metals from wastewater include rice husk, waste peels, wheat wastes, and clay. 2.2.1. Adsorbents from rice husk Rice has developed on each landmass aside from Antarctica and positions second just to wheat as far as overall region and generation [430]. when rice or paddy is husked, the rice husk is produced as a waste and for the most part, every 100 kg of paddy rice generates

20 kg of rice husk [431]. Obviously, the production of rice husk may fluctuate with various rice species. Hence, in many rice generating nations, the use of this large amounts of waste is of awesome importance. Rice husk is considered as lignocellulosic materials of agricultural byproduct which contains roughly lignin (21.44%), cellulose (32.24%), hemicelluloses (21.34%) and mineral slag (15.05%) [432]. The level of silica in its mineral slag remains is around 96.34% [433]. This high level of silica combined with most of the lignin content, an auxiliary polymer, is extremely strange in nature. It has made rice husk not just impervious to water infiltration, contagious disintegration and it does not biodegrade effortlessly. Compared to other cereal byproducts, the rice husk has the most minimal level of total digestible supplements which is approximately less than 10%. It likewise contains low carbohydrates and protein and also it contains a higher amount of unrefined fiber and ash [432–434]. Rice husk is a solid waste generated from the rice manufacturing process. From the waste utilization point of view, the rice husk is an asset yet to be completely used and abused. The researchers/scientists are interested in working to utilize the rice husk completely. Endeavours have been made to use rice husk as a building material [435,436]. Rice husk is utilized to protect dividers, floors, and rooftop depressions on account of its magnificent properties, for example, great warmth protection, does not radiate smell or gases, and it isn't destructive [437–439]. Shockingly, the expense of building materials made utilizing rice husk as the aggregate is not good as compared with the other aggregates in the market. Hence, another intriguing probability for using this modest and promptly accessible resource may be as an ease adsorbent in the removal of toxic metals from wastewater [440]. The magnificent attributes of the rice husk which includes good chemical stability, insolubility in water, high mechanical quality and granular structure allow this probability to be higher [441]. Many researchers have been worked on the utilization of rice husk in its native or modified form for the treatment of toxic metals contained wastewater [442–446]. 2.2.2. Adsorbents from waste peels The waste peels from the agricultural activities mainly consist of a lignocellulosic constituent in it. Generally, these waste peels are considered as an environmental burden to the community. The waste peels which include vegetables and fruit peels comprise the most elevated level of waste in most kitchens refuse containers. Besides, these wastes are being generated in large quantity from the industries during its processing particularly during the selection, sorting operation and boiling operations [447,448]. The vegetables and fruits processing industries generate the various types of solid peel wastes. A large number of the vegetable and fruits peels are disposed of in the waste or nourished to domesticated animals [5]. The vegetable wastes, fruits wastes, and byproducts from the food processing industries produce serious issues in the living environment. These wastes must be properly handled or utilized. Since a decade ago, endeavours have been made to enhance techniques and methods for reusing vegetables and fruits wastes. Generally, these wastes are frequently used as a feed or manure. They are high esteem items and their recuperation will be financially appealing. Recently, the useful products are produced from these wastes which include edible oils, essential oils, pigments, food additives, dietary fibres, bio-degradable plastic, polyphenolic compounds, enzymes, anticarcinogenic compounds, bio-ethanol and other miscellaneous items [449–454]. The waste peels from agricultural activities are natural, environmentally friendly, renewable resources and low-cost materials and can be utilized as effective adsorbents either in native or surface modified form for the removal of heavy metals from water/wastewater. Some of waste peels include citrus waste peels (lemon peels, citrus peels, pomelo peels and grapefruit peels), cassava peels, banana peels, pomegranate peels, jackfruit peels, garlic peels, yellow passion fruit peels, potato peels, mandarin peels, ponkan peels, watermelon peels, mango peels, lychee peels, Cucumis sativus peels and mangosteen peels [419,448,455–464].

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2.2.3. Adsorbents from wheat wastes Wheat is a noteworthy nourishment harvest of the world, which generates a lot of straw and bran during its agricultural activities as a waste or byproducts [465]. Wheat straw has been utilized as feed and in the paper industry to generate low-quality sheets or packing materials. In some places of the world, the stems are fired directly for the purpose to produce the energy but a large number of air pollutants are being generated into the environment. The wheat straw mainly consists of 37-39% of cellulose, 30-35% of hemicellulose, approximately 14% of lignin and little amount of sugar [466–469]. Wheat straw typically comprises different functional groups which include hydroxyl, carboxyl, amide, sulfhydryl, and amine. The composition of the wheat straw varies all over the world due to many factors but the components will be remaining the same. The wheat straw and wheat bran have been widely tested for the removal of different heavy metals from wastewater [470–473]. 2.2.4. Adsorbents from clay materials Clay materials are largely available in the environment and it is also cheap material. It is mainly comprised of silica, alumina, weathered rock and water [474,475]. Clay has a property to indicate plasticity through a variable scope of water content, which can solidify when dried [476]. The clays have discovered applications in different industries, for example, foundry sand, drilling liquid, characteristic fillers in papermaking, separating and freshening up specialists in the oil refineries, decolorizing operators in the oil enterprises, extenders in fluid-based paints, adsorbent, building material and catalyst [477]. The source of clays might be from steady surface weathering and optional sedimentary operations

11

of an assortment of minerals, regularly silicate-alumina bearing rocks or low-temperature aqueous adjustment over extended stretches of time [478,479]. The properties of various clays such as swelling ability, elasticity, plasticity, specific gravity, hardness, crystallinity, etc. fluctuate extensively, contingent upon the place of the source and local environmental conditions [480]. The water is added to the clay material and it becomes a mud. This mud can be mould into the desired shape and size of the product after the material was dried at a proper temperature. In view of this property, the ceramic and the pottery industries used to deliver the products like children's toys, bowls, cups, pipes, plates and so forth [481,482]. The different forms of clays and clay minerals have been utilized as an effective adsorbent for the removal of toxic heavy metals from the wastewater [483,484]. The clay materials have important advantages as compared with other conventional adsorbents which include higher adsorption capacity, low cost, non-dangerous nature, high potential for ion exchange, large availability and high specific area [422]. Clays likewise contain interchangeable cations and anions held to the surface and therefore, the consideration of researchers worldwide has been centered on utilizing natural or surface modified clay materials as an adsorbent for the treatment of heavy metals contained in wastewater [485]. The clay minerals mostly consist of negatively charged on its surface and which is exceptionally powerful and widely used to remove the cations from the wastewater because of its cation exchange behaviour, pore volume and larger specific surface area [422,486]. The adsorption of heavy metals by clay minerals includes a progression of complex adsorption mechanisms, for example, ion exchange, coordinate holding between metal cations and the negatively charged surface of the clay minerals,

Table 1 Langmuir monolayer adsorption capacity of the adsorbents for the removal of Cu(II) ions. Adsorbents Ultrasonic assisted Spirulina platensis Sulphuric acid modified Spirulina platensis Surface modified graphene oxide Chemically modified orange peel Graphene oxide Ultrasonic assisted jujube seeds Sulphuric acid modified Strychnos potatorum seeds Chitosan coated magnetic nanoparticles Nanoporous metal-organic framework Crosslinked chitosan Cellulose-graft polyacrylamide/ hydroxyapatite composite hydrogel Chitosan/polyvinyl alcohol magnetic composite Graphene oxide-CdS composite Fly ash/iron ore tailing geopolymer Ca-DTCS/ALG beads DTPA-modified chitosan micro-gels (DTCS) Plain alginate (ALG) beads sulfuric acid-modified Eucalyptus seeds Porphyra tenera-derived biochar Hydroxyapatite nanoparticles Cellulose hydrogel Nano-scale zero valent iron supported on rubber seed shell Nano-scale zero-valent iron impregnated cashew nut shell Biochars derived through farmyard manure (DBC-FYM) Grape bagasse activated carbon Biochars derived through poultry manure (DBC-PM) Guazuma ulmifolia seeds activated carbon Active coal pecan shell Untreated pomegranate peel Raw Spirulina platensis Sulphuric acid modified Caryota urens seeds Granular activated carbon Cashew nut shell Luffa Actangula Carbon Unmodified Strychnos potatorum seeds Raw Caryota urens seeds Activated alumnia

Langmuir monolayer adsorption capacity, qm (mg/g) 817.7 431.4 357.14 289 277.77 259 248 236.7 236.12 200 175 143 137.174 113.41 103.3 102.7 102.0 76.94 75.1 70.92 52.3 48.18 48.05 44.50 43.47 43.68 36.496 31.70 30.12 26.24 24.92 20.833 20.00 12.47 8.649 5.056 4.32

pH 6.0 6.0 6.0 6.0 6.0 5.0 5.0 6.0 5.2 5.0-6.0 4.0 5.0 6.0 6.0 3.0 3.0 3.0 5.0 5.5 6.0 4.7 6.0 5.0 2.0 5.0 2.0 6.0 4.8 5.8 6.0 5.0 5.0 5.0 6.0 5.0 5.0 4.0

Temperature (oC)

Time (min)

References

30 30 25 30 25 30 30 30 25 35 45 45 25 40 28 28 28 30 20 30 30 30 30 25 45 25 30 30 40 30 30 30 30 30 30 30 20

60 60 60 180 60 30 40 30 30 60 1440 200 60 90 2160 2160 2160 10 2880 90 360 30 30 1440 180 1440 60 1440 120 120 60 30 30 120 60 120 120

[551] [551] [497] [498] [497] [548] [543] [547] [69] [499] [500] [501] [502] [503] [504] [504] [504] [545] [505] [506] [75] [550] [549] [55] [507] [55] [542] [348] [508] [551] [546] [541] [540] [509] [544] [546] [57]

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Table 2 Langmuir monolayer adsorption capacity of the adsorbents for the removal of Cd(II) ions. Langmuir monolayer adsorption capacity, qm (mg/g) pH Temperature (oC) Time (min) References

Adsorbents Milled eggshell Mesoporous activated carbon from oil palm shell ultrasonic-assisted Caryota urens seeds Ultrasonic assisted jujube seeds Ligand-based nano-composite material Surfactant-modified chitosan Lentil husk Garbage ash Hybrid hydrogel composite Surface modified Eucalyptus seeds by sulphuric acid Olive cake Surface modified Eucalyptus seeds by hydrochloric acid Graft copolymerization of acrylamide (AAm) onto GL (GL–cl–PAAm) Scolymus hispanicus Grapefruit peel Pen shells Banana peel Jatropha seed coat Jatropha fruit coat Sugarcane bagasse Commercial activated carbon Freshwater mussel shells (FMS) Thermal activated serpentine Treated fly ash Unmodified Strychnos potatorum seeds Cork Peat Pine bark Chemically modified wheat straw Eggshell Alhaji maurorum seed Spruce wood Bael tree leaf powder Bagasse fly ash

surface complexation and so forth. In numerous examinations, the surface of the clay is modified to improve its adsorption capacity for the removal of metals ions from the wastewater. This surface modification methods were used to improves the surface area, pore volume and active sites of the adsorbent [488]. Due to this surface modification methods, the clay minerals become organophilic and hydrophobic for the removal of non-ionic organic species. In this manner, use of muds and its materials would take care of disposal issue, and furthermore access to more affordable material in the wastewater treatment. Due to the low cost for the generation of clays, there is no compelling reason to recover them and also it gives more points of interest in utilizing

329 227.27 183.4 182.5 148.32 125 107.31 100.25 78.13 71.15 65.40 64.16 54.95 54.05 42.09 37.63 35.52 22.83 21.97 14.80 32 26 15.21 14.33 7.023 6.0 5.0 4.0 3.833 3.8 3.748 2.0 1.890 1.20

7.0 6.0 5.0 5.0 5.5 7.0 5.0 6.0 6.0 5.0 6.0 5.0 6.0 6.5 5.0 4.0 8.0 6.0 6.0 6.0 6.0 6.0 6.0 5.0 5.0 5.0 5.0 5.0 6.0 6.0 6.5 5.0 6.0 6.0

25 30 30 30 25 25 30 25 25 30 28 30 25 25 30 25 30 30 30 30 30 30 25 30 30 30 30 30 30 25 30 30 30 30

120 180 60 30 15 600 60 60 60 10 1440 14 60 60 150 60 30 180 180 60 60 60 1440 30 90 4200 4200 4200 60 4200 45 4200 60 60

[510] [79] [559] [548] [511] [82] [80] [512] [81] [557] [513] [557] [81] [514] [515] [84] [516] [274] [274] [517] [518] [518] [519] [520] [544] [521] [521] [521] [522] [523] [524] [521] [553] [525]

clays as an effective adsorbent for the removal of toxic heavy metals from wastewater [264,422,489–491]. 2.2.5. Adsorbents from other agricultural wastes Many agricultural waste materials other than above have been discussed for the removal of various heavy metals from the wastewater. The different operating parameters have been studied and it is optimized for the maximum removal of metal ions from wastewater. The adsorption mechanism, kinetics, isotherms, thermodynamics on the removal of heavy metals by these adsorbents have been analyzed and the results are presented by the various researchers. Few researchers have

Table 3 Langmuir monolayer adsorption capacity of the adsorbents for the removal of Zn(II) ions. Adsorbents Sulphuric acid treated cashew nut shell Aspergillus flavus modified eucalyptus bark Amino-functionalized Fe3O4@SiO2 magnetic nano-adsorbent Sulphurised activated carbon Mesoporous geopolymeric powder Mesoporous modified chitosan surface modified Strychnos potatorum seeds nano zero-valent iron impregnated cashew nut shell Ultrasonic assisted Caryota urens seeds Eucalyptus seeds activated carbon Natural bentonite Rosa centifolia Activated carbon prepared from palm oil mill effluent Sodium dodecyl sulphate-coated Fe3O4 Fly ash coated by chitosan Physic seed hull Chitosan–PVA Blend Cedar leaf ash Bael tree leaf powder

Langmuir monolayer adsorption capacity, qm (mg/g) 455.7 287.8 250.0 169.5 147 138.98 107.21 98.75 94.46 80.91 80.37 68.4931 73.8 59.8802 59.52 55.52 12.79 5.917 4.79 2.083

pH 5.0 5.0 5.1 5.0 6.5 8.0 6.0 5.0 6.0 5.0 5.0 6.76 5.0 5.5 6.0 6.0 6.0 5.0 5.0 5.0

Temperature (oC)

Time (min)

References

30 30 30 25 30 25 40 30 30 30 30 30 30 30 30 20 24 30 20 25

30 360 100 120 300 120 180 30 30 60 16 120 1440 360 5 180 240 30 720 60

[554] [526] [527] [528] [529] [111] [530] [555] [560] [531] [558] [532] [533] [534] [535] [536] [538] [556] [539] [552]

K.H. Vardhan et al. / Journal of Molecular Liquids 290 (2019) 111197

been reported the column analysis on the removal of different heavy metals from the wastewater by the agricultural waste materials either in its natural or surface modified form. Some of these agricultural wastes include bagasse, white ash, neem bark, pine sawdust, plum kernel, apricot shell, almond shell, coconut shells, cottonseed shell, gingelly seed shell, olive stone, garden grass, hazelnut shell, lentil shell, woodderived biochar, walnut shell, cane pith, coir pith, pongam seed shell, sunflower stalk, white rice husk ash, rice shell, cashew nut shell, soya meal hull, groundnut shell etc. [420,492–496]. The studied materials by the different researchers are not able to show the high adsorption capacity for the different heavy metals and also fail to show the regeneration ability after few cycles of adsorption operations. Still, many researchers/scientists are working in the process to select the suitable adsorbent materials for the removal of heavy metals from the wastewater. 3. Comparison of Langmuir adsorption capacities The results of the isotherm studies done by various researchers were compared for the removal of heavy metals from aqueous solution to check the feasibility and adsorption efficiency of the adsorbents. The comparison was made based on the important isotherm parameter such as Langmuir monolayer adsorption capacity of the adsorbents. The comparison was given in Tables 1–3. The results of the comparison table give an input that the studied adsorbent has an excellent adsorption capacity. The maximum adsorption capacity of the adsorbent leads to indicate the highest quality of the adsorbent.

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removal mechanism for the removal of heavy metals must be explained with proper technical inputs. In this review, the studied adsorbents showed excellent adsorption potential for the removal of metal ions from the aqueous solution. However, more toxic pollutants such as dyes, pharmaceuticals, phenolic compounds, oil and grease, and other toxic pollutants must be experimented with the same prepared adsorbent. The surface modified adsorbent must also focus on the selectivity of the pollutants in the multicomponent system. The present review explained mostly on the removal of single pollutant from the model wastewater using batch and column adsorption techniques. Further research must be important to validate the finding on the real industrial wastewaters for single and multi-pollutants. The distinctive wellsprings of waste materials must be considered to set up the powerful minimal effort adsorbent for the removal of lethal poisons from the wastewater. The surface modification techniques must be streamlined in the amalgamation of the practical adsorbent. It is essential to incorporate more active functional groups on the adsorbent that can evacuate an extensive variety of contaminants, keep up great movement for quite a while, and meet the necessities for the down to earth treatment of wastewater. Some dynamic substances with great proclivity have been utilized to alter the adsorbent surface and improve their toxin removal efficiencies toward an extensive variety of contaminants. The life cycle analysis of the adsorbent must be carried out to ensure the effective reuse, reduce and disposal of the adsorbents. It is also important to do much research works on the selection and optimization of the regenerating agents for the betterment of the regeneration operation.

4. Conclusion and scope for further research Acknowledgments The importance of low-cost adsorbents for the removal of toxic metals from wastewater have been reported. The different adsorbents which include industrial and agricultural wastes have been studied extensively for the removal of heavy metals from aquatic environment. However, authors identified that many issues and shortcoming must be addressed in resolving the removal of heavy metals from wastewater by different treatment methods. It is important that the surface modified agricultural waste has started enthusiasm is apparent as substantial volume of published paper in this research focus. In spite of the huge collection of literature, more research will be expected to completely comprehend the physical and chemical properties of the surface modified adsorbent. There are challenges in the generation and refining of surface modified agricultural waste, the antecedent of the adsorbent. The production of the adsorbent through the surface modified procedure is as yet difficult and not reasonable for substantial scale production. For instance, this technique requires a concentrated acid or base or heat treatment, which may not be down to earth for large scale production of adsorbent and its subordinates. In this manner, the production techniques reasonable for substantial scale production should be produced. Also, since there are distinctive methodologies in the production of surface modified agricultural waste, with every strategy influencing the attributes of the prepared adsorbent, there is an important need to refine these techniques. It is an important research step to predict the structure of surface modified adsorbent to upgrade its fantastic adsorption properties. The considerable amounts of literature are already available on the surface modification procedures of agricultural waste but more studies will be expected to comprehend the mechanism of surface modification procedures. More research will be required towards seeing how the available functional groups impact the stability and the active sites of the surface modified adsorbent. Regardless of the huge research exertion being accounted for, the connection between the useful functional groups and the adsorption capacity of the adsorbent is yet to be completely comprehended. Additionally, the important active functions groups present on the adsorbent surface should be clearly identified and its

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