Immobilization of Lead and Cadmium in ... - ScienceDirect.com

Pedosphere 25(4): 555–568, 2015 ISSN 1002-0160/CN 32-1315/P c 2015 Soil Science Society of China Published by Elsevier B.V. and Science Press

Immobilization of Lead and Cadmium in Contaminated Soil Using Amendments: A Review Amanullah MAHAR1,2 , WANG Ping1 , LI Ronghua1 and ZHANG Zengqiang1,∗ 1 College 2 Centre

of Natural Resources and Environment, Northwest A&F University, Yangling, Shannxi 712100 (China) for Environmental Sciences, University of Sindh, Jamshoro 76080 (Pakistan)

(Received September 9, 2014; revised May 4, 2015)

ABSTRACT Since the inception of industrial revolution, metal refining plants using pyrometallurgical processes have generated the prodigious emissions of lead (Pb) and cadmium (Cd). As the core target of such pollutants, a large number of soils are nowadays contaminated over widespread areas, posing a great threat to public health worldwide. Unlike organic pollutants, Pb and Cd do not undergo chemical or microbial breakdown and stay likely in site for longer duration after their release. Immobilization is an in-situ remediation technique that uses cost-effective soil amendments to reduce Pb and Cd availability in the contaminated soils. The Pb and Cd contamination in the soil environment is reviewed with focus on source enrichment, speciation and associated health risks, and immobilization options using various soil amendments. Commonly applied and emerging cost-effective soil amendments for Pb and Cd immobilization include phosphate compounds, liming, animal manure, biosolids, metal oxides, and biochar. These immobilizing agents could reduce the transfer of metal pollutants or residues to food web (plant uptake and leaching to subsurface water) and their long-term sustainability in heavy metal fixation needs further assessment. Key Words:

bioavailability, biochar, biosolids, heavy metal, public health, remediation, soil extraction

Citation: Mahar A, Wang P, Li R H, Zhang Z Q. 2015. Immobilization of lead and cadmium in contaminated soil using amendments: A review. Pedosphere. 25(4): 555–568.

INTRODUCTION The contamination of land resources is a major worldwide concern as urban sprawl and industrial units are continuously rising and leaving negative ecological footprints on the natural environment (Wong and Li, 2004). Lead (Pb)- and cadmium (Cd)-contaminated soils are a global environmental concern that results not only in human health and ecological hazards, but also in huge economic implications with respect to reclamation and restoration costs (Semenzin et al., 2007). Since the last few decades, the distribution and mobility of heavy metals in various soils have been documented. While few heavy metals are essential for living organisms in trace quantities, most are hazardous in high concentrations. Among the later, Cd and Pb, which are naturally present in soils, can accumulate in humans and cause serious health problems (Ok et al., 2004). Various unsustainable waste disposal practices have resulted in significant accumulation of a diverse range of toxic heavy metals including Cd and Pb. The contamination of toxic metals in food chain de∗ Corresponding

author. E-mail: [email protected].

pends upon the source, dosage, rate, and magnitude of plant metal uptake, physico-chemical properties of soil, and the extent of absorption by animals (Adriano, 2001). Worldwide public now becomes vocal on the detrimental health implications to humans and the natural environment (Marshall, 2001). On a historical note in human health, metal toxicity has gained significant attention as a result of serial poisoning at a large scale; i.e., a huge number of human poisoning tragic cases of Minamata disease due to consumption of contaminated fish by toxic metals were recorded, in Minamata Bay in Japan in the late 1950s (Knopf and König, 2010). Many countries have recorded excessive Pb and Cd concentrations in their soils. Significant negative effects of Pb and Cd on human health have been noted in China, India, and Bangladesh with an alarming large number of populations at risk from toxic metals (Bhattacharya et al., 2012). Cadmium is a serious concern for Australia and New Zealand, where it accumulates in offal of grazing animals and makes it unsuitable for human usage (Loganathan et al., 2008). Likewise, Pb in soils resulting from unsustainable use as a compo-

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nent of herbicides and fungicides during urban development of horticultural sites is also an increasing concern there (Pietrzak and Uren, 2011). Toxic heavy metals do not undertake chemical and microbial degradation and their total concentrations usually do not change due to their persistent nature after being released into the environment (Adriano et al., 2004). As a result, the scientific community focuses immensely on the development of soil remediation technologies as global masses are increasingly vocal on the animal and human health implications resulting from contaminated soils. Soil remediation technologies are of special attention since conventional soil reclamation practices, i.e., landfilling and excavation, are often very expansive and environmentally unfeasible, as compared to alternative options. Many soil amendment based-technologies such as soil immobilization/solidification are cost effective and less environmentally disruptive (Mulligan et al., 2001; Kumpiene et al., 2008). Unlike organic pollutants which can be destroyed, heavy metals impair their toxicity and mobility by triggering the important immobilization process, i.e., (ad)sorption, precipitation, complexation, and redox reactions, in the process of soil remediation (Adriano et al., 2004). In addition, biological and chemical stabilization of toxic heavy metals using organic (i.e., biosolids) and inorganic amendments (i.e., phosphate compounds and lime) are suitable options to minimize metal bioavailability (Park et al., 2011a). In the urban environment, as more localized toxic heavy metals are found, the process of metal stabilization including chemical washing and phytoextraction has been ap-

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plied for remediation. For safe disposal of toxic metals from urban soil, phytoextraction process and the subsequent recovery are considered for commercial and research implications (Robinson et al., 2009). However, when phytoextraction is ineffective, alternate options, i.e., in-situ immobilization, are considered as important part of environmental management (Fig. 1). Both organic and inorganic soil amendments are components of remediation techniques applied to manage contaminated soils. The immobilization of Pb and Cd in contaminated soils depends upon the local availability and financial implications of soil amendments. Therefore, the main question to be answered here in this review is that what types of amendments have been commonly employed and are emerging as cost-effective technologies to immobilize Pb and Cd in contaminated soils by significantly decreasing bioavailability of these metals. SOURCES OF ENRICHMENT IN SOIL Soil functions as a final sink for heavy metal contamination in terrestrial ecosystems. Both anthropogenic and pedogenic sources contribute to heavy metal loads in the soil environment. Most toxic trace elements are naturally present in the parent material of soil, mainly in forms that are not readily bioavailable for plant absorption or uptake as compared to those of pedogenic contribution by anthropogenic sources which have extreme bioavailability (Naidu et al., 1996; Lamb et al., 2009). Anthropogenic activities such as manufacturing industrial processes, usage of phosphorus (P) fertilizers, and disposal of domestic and industrial wastes are the main sources of toxic heavy metals

Fig. 1 Illustrative diagram showing the relationship between immobilization, bioavailability and remediation of toxic heavy metals (Bolan et al., 2014). M = metal.

SOIL LEAD AND CADMIUM IMMOBILIZATION

in the soil environment (Adriano, 2001; Bolan et al., 2003a, b, c, d; Gray et al., 2003). Lead is a natural constituent in the Earth’s crust and is normally available at trace concentrations in plants, soils, and water bodies. Metallic Pb is rarely found in the natural environment (Cheng and Hu, 2010). Galena (PbS) and cerussite (PbCO3 ) are the important Pb minerals; anglesite (PbSO4 ) and pyromorphite (Pb5 (PO4 )3 Cl) also frequently occur but in less quantities (Crook, 1921). Lead is commonly found in ores which also contain copper (Cu), zinc (Zn), and silver (Ag) and extracted as a co-product of these metals. Lead is extremely malleable, ductile, and very easy to smelt, and its ores are extensive. Lead may exist in the Bronze and Copper ages and is considered as one of the seven antique metals. In 6 500 BC, the first Pb object was used and the processing of Pb minerals upgraded significantly around 6 000 years ago (Nriagu, 1983). Due to anthropogenic activities at a global scale, Pb has become a widespread toxic metal in the natural environment across the world. Today, Pb is used extensively in lead-acid batteries, weights, pewters, fusible alloys, and bullets, lead shots (pellets), solders, and building construction (Cheng and Hu, 2010). Atmospheric deposition due to emission of Pbbased petrol is a main problem in various countries where there is no restriction on consumption of leaded gasoline (Fenger, 2009). On the other hand, biosolids are the main source of toxic metals released in North America and Europe, and P fertilizers are considered to be the main source of heavy metals, especially Cd and Pb, in New Zealand, Australia, and China (McLaughlin et al., 1996; Bolan et al., 2003a; Loganathan et al., 2008; Haynes et al., 2009). The main sources of Pb contamination in soil are smelting, mining, industrial, coal burning, and waste incineration. Cadmium is odorless, whitish silver in appearance and has a firm bioaccumulative tendency. It is easily absorbed into animal, human, and plant tissues due to its chemical nature similar to Zn (van der Voet et al., 1994). This is a highly toxic metal and usually found in the industrial work places, extensively used in electroplating, industrial paints, sprays, and manufacturing of batteries, and results from the waste-water discharge of stearate Cd factories. Cadmium contamination due to P fertilizers used in agricultural soils is of specific concern because Cd originates from phosphate rocks which are used for P fertilizer manufacturing and becomes part of food chain through regular use of Cd-containing P fertilizers. The safe limits for P fertilizers have not been set, although various countries have stipulated threshold levels for Cd in soil due to

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consumption of municipal biosolids (Wuana and Okieimen, 2011). SOIL EXTRACTION TECHNIQUES Bioavailability of Pb and Cd in soil can be investigated using chemical extraction and bioassay procedures that assess a bioaccessible fraction of these metals. Sequential and single extractions using reagents are the methods of chemical extraction (Ruby et al., 1996; Basta and Gradwohl, 2000). Animals, plants, and microorganisms are the tools for bioassay (Yang et al., 1991; Domene et al., 2010). Salt solutions (0.01 mol L−1 CaCl2 ), mineral acids (1 mol L−1 HCl), chelating agents (diethylene triamine pentaacetic acid (DTPA)), and buffer solutions (1 mol L−1 NH4 OAc ) are chemical extractants used in single extraction procedure for assessing bioavailability in Pb- and Cd-contaminated soils (McBride et al., 2009; Bakircioglu et al., 2011). Chelating agents (0.05 mol L−1 DTPA and 0.05 mol L−1 ethylenediamine tetraacetic acid (EDTA)) have been documented as more trustworthy in predicting the Pb and Cd availability in plants and are more impressive in eliminating soluble metal-organic complexes (Sims and Johnson, 1991; Zhu et al., 2012). Sequential extraction schemes are commonly used to determine the redistribution or partitioning of Pb and Cd in different chemical forms, i.e., adsorbed (exchangeable), soluble, occluded, and precipitated (Table I). With each successive step of the scheme, the bioavailability and solubility of Pb and Cd in soil reduces, though the extraction methods differ (Basta and Gradwohl, 2000; Zakir and Shikazono, 2011). Specific chemical species measured through chemical extraction procedures have been successfully linked with plant metal uptake in assuming the plant availability of metals in soils (Naidu et al., 1997; Abedin et al., 2012). The relative bioaccessibility leaching procedure (RBALP), physiologically based extraction test (PBET), gastrointestinal (GI) test, and potentially bioavailable sequential extraction (PBASE) are the physiologically based in vitro chemical extraction procedures for assessing bioavailability of Pb and Cd in soil (Drexler and Brattin, 2007; Ng et al., 2013). These advanced schemes predict the bioavailability of Pb and Cd in sediment and soil when ingested by human and animals. With reference to the conventional sequential procedures, the capacity of these schemes to solubilize Pb and Cd enhances with each following extraction step. Regardless of the identified non-specific nature of these chemical extraction procedures, their analytical simplicity and speed prove them to be most potential

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TABLE I Different methods used for extraction of Pb and Cd in soils or sediments from various parts of the world Soil(s) or sediments

Extraction techniquea)

Analytical References methodb)

Sediments from Barcelona Harbour, Spain

Modified BCR three-step sequential extraction method Modified BCR three-step sequential extraction method

ICP-MS

Guevara-Riba et al., 2004

GFAAS

Cuong and Obbard, 2006

Modified Tessier extraction method

AAS

Esslemont, 2000

Tessier extraction method

FAAS

BCR three-step sequential extraction method Four-step sequential extraction method

ICP-MS AAS

Wepener and Vermeulen, 2005 Yuan et al., 2004 Pempkowiak et al., 1999

Three sequential extraction methods BCR-sequential extraction method Optimized BCR three-step sequential extraction method Optimized BCR three-step sequential extraction method Optimized BCR three-step sequential extraction method Na2 -EDTA (0.1 mol L−1 ) extraction Kerstner-Forstner sequential extraction scheme Sequential extraction with H2 O, 0.1 mol L−1 Ca(NO3 )2 , and 0.05 mol L−1 EDTA Ahnstrom and Parker extraction scheme DTPA/TEA method

AAS GFAAS ICP-MS

Usero et al., 1998 Morillo et al., 2004 Zhang and Wang, 2003

FAAS

Elass et al., 2004

ICP-MS

Pueyo et al., 2003

AAS ICP-MS ICP-MS

Mahvi et al., 2005 Katasonova et al., 2005 Garau et al., 2007

AAS ICP-OES

Silveira et al., 2006 Contin et al., 2008

ICP-MS

Lee et al., 2009

Sediments from Kranji & Pulau Tekong Harbour, Singapore Sediments from Townsville Harbour, Queensland, Australia Sediments from Richards Bay Harbour, South Africa Sediments from East China Sea Sediments from Norwegian Sea and Baltic Sea Sediments from Huelva Estuarine, Spain Sediments from Southwest Coast, Spain Urban Soils from Hangzhou City, China Polluted soils and sediments from Morocco Soils affected by accidental spills from Spain Contaminated soils from Iran Soil from Moscow Region, Russia Contaminated soil from Southwest of Sardinia, Italy Tropical soils from Sao Paulo State, Brazil Contaminated soils from Carpiano, Milan, Italy Contaminated soil from Chungnam, Korea Contaminated soil from Arnoldstein, Austria Smelter site soils from Anghang, Seocheon, Chungnam, Korea Contaminated soils from Hadong Coast, Gyeongnam, Korea

Deionized water and 0.1 mol L−1 Ca(NO3 )2 extraction DTPA, CaCl2 , EDTA, and ammonium acetate extraction scheme USEPA TCLP method 1311

AAS

Tica et al., 2011

ICP-OES

Bade et al., 2012

Aqua regia extraction

ICP-OES

Ok et al., 2011

a) BCR

= Community Bureau of Reference; EDTA = ethylenediamine tetraacetic acid; DTPA = diethylene triamine pentaacetic acid; TEA = triethanolamine. b) ICP-MS = inductively coupled plasma-mass spectrometry; GFAAS = graphite furnace atomic absorption spectrometry; AAS = atomic absorption spectrophotometry; FAAS = flame absorption atomic spectrometry; ICP-OES = inductively coupled plasma-optical emission spectrometry; TCLP = toxicity characteristic leaching procedure.

for regular identification of metal forms and assessment of Pb and Cd bioavailability in situ although the dispersal of given metals among different fractions can only be measured as a valuation at best because of the subjectivity of the steps involved. As microorganisms are more sensitive to metal (Pb and Cd) stress than plant flora or soil microfauna, determining bioavailability and toxicity of metals in contaminated soils employing microorganisms are gaining more and more focus nowadays (Zhang et al., 2010; Anyanwu et al., 2011). The procedures using protozoa and microflora have showed the capacity to deliver an extent of short-term bioavailability of heavy metals and even enable the assessment of temporal

changes. On the other hand, macrofauna (enchytridae, invertebrates, and earthworms) and mesofauna (microarthropods) have the ability to demonstrate collective impacts. There are drawbacks in these methods: they are time consuming and can only give an overall impact of metal bioavailability to the species tested. In addition, molecular techniques are swiftly developed and applied, but they are generally relatively expensive; thus, the information given by the techniques requires to be well explained (Amaro et al., 2011; Frosteg˚ ard et al., 2011). PUBLIC HEALTH RISKS Soils contaminated with toxic heavy metals not


only possess threats to environmental health but also have huge financial issues, i.e., removal costs and restoration problems (Semenzin et al., 2007; Chapman et al., 2013). Lead is susceptible for inhalation or ingestion from different sources, i.e., food items, polluted ambient air, soil, and polluted potable water. A soil contaminated with Pb mostly enters human body through inhalation or ingestion and may leave harmful effects on the central nervous system (Needleman et al., 1990). Human adults are vulnerable to lead poisoning, causing problems to central and peripheral nervous system, blood pressure, and kidney (Needleman, 2004). Mild exposures to Pb may cause increased blood pressure, decreased neurological functions, weakness, and tingling in the extremities (Jennings, 2013). High Pb exposures may cause kidney damage, miscarriages, severe brain impairment, and ultimately, death. Compared to adults, infants and children are more vulnerable to Pb poisoning, triggering irreparable impacts on mental development in their early childhood (Godwin, 2001). Even mild exposures can affect infant’s neurological and physical development (ATSDR, 2007; NLM, NIH, 2012). Nowadays, Pb contamination is a global issue and the focus has shifted from susceptibility of adults to increases of emissions in industrial areas to a significant number of children populations being more prone to respiratory allergies and asthma with lesser contacts (Needleman, 2004). A consensus analysis showed that Pb is probably a human carcinogen and leaves multiple impacts on overall quality of environmental health (ATSDR, 2007). Without any inhibition of plant growth, Cd can accumulate in plant at concentrations harmful to human or animal consumption (Asami, 1984). Cadmium is poisonous to all living organisms and can enter into the human body through various different routes. When a human body absorbs Cd, it may show symptoms such as difficulty in breathing, salivation, vomiting, nausea, anemia, abdominal pain, diarrhea, and kidney failure. Furthermore, inhaling smoke or dust containing Cd may cause headache, dryness of throat, coughing, chest pain, bronchial complications, and increased uneasiness. Cadmium may also have harmful effects on the metabolic systems of living organisms (Doyle, 1977). Moreover, Cd might cause kidney disorder or ultimately death at the concentration of 200 mg kg−1 in the kidney (Alloway et al., 1990). A total of 64 persons died in the Jintsu Valley of Toyama, Japan in 1969 due to Itaiitai disease (Friberg, 1984). IMMOBILIZATION USING SOIL AMENDMENTS Many efforts have been made in developing reme-

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diation technologies to minimize or control Pb and Cd contamination in soil. In general, various remediation methods are needed to eliminate Pb and Cd from wastes containing exceeding levels of contaminants than stipulated safe limits. Various amendments are being employed to immobilize Pb and Cd in contaminated soils. Immobilization of toxic heavy metals can be achieved mainly through precipitation, complexation and adsorption, reactions which result in the redistribution of metals from solution phase to solid phase, thus reducing their transport and bioavailability in the soil (Bolan et al., 2003a; Porter et al., 2004). Phosphate compounds Various studies have reported convincing evidence for the mitigative value of both water-soluble (diammonium phosphate, DAP) and water-insoluble (apatite from phosphate rock, PR) phosphate compounds (Table II), which can immobilize metals in soils, thus decreasing their bioavailability for plant and human uptake, mobility, and transport (Bolan et al., 2003b, d). Phosphate compounds increase the immobilization of metals in soils through different processes, i.e., direct metal absorption/substitution by phosphate compounds and precipitation of metals with phosphate solution as metal phosphates. Through increased phosphate anion-induced metal adsorption and increased surface charge, soil application of phosphate compounds can cause direct adsorption of metals on these compounds depending upon the sources. Precipitation as metal phosphates has been proved one of the main mechanisms for the immobilization of Pb and Cd in soils (Bolan et al., 2014). These fairly stable metal-phosphate compounds have extremely low solubility over a wide pH range, which makes phosphate application an attractive technology for managing Pb- and Cd-contaminated soils, but phosphate is also a fossil source and high phosphate application rates cause eutrophication of water resources. In typical arable soils, precipitation of Pb and Cd is unlikely, but in modest Pb- and Cd-contaminated soils, this process can play a major role in the immobilization of such metals. The potential of apatite to immobilize Pb in contaminated soils or solution by Pb-phosphate precipitation has been well reported (Chrysochoou et al., 2007). The precipitants are usually termed as chloropyromo rphite or hydroxypyromorphite. Two methods have been recommended for the reaction of dissolved Pb with apatite. Firstly, Pb(II) in apatite can be a substitute for Ca(II). Thereby, through adsorption of Pb or dissolution of hydroxyapatite (Ca10 (PO4 )6 (OH)2 ), (Ca,

Cd, Pb Cd Cd, Pb

Cd Cd Cd Pb, Cd Cd Cd Pb, Cd Pb, Cd

Target metal(s) Transforming to less mobile fractions, reduced phytoavailability No reduction in uptake, transferring to the kernels of sunflower Reduced uptake by lettuce Decreased Cd uptake, little influence on Pb uptake by radish Phytotoxicity prevented by Ca(OH)2 , but not by CaCO3 Reduced uptake of Cd Reduced solubility of Pb and Cd, changes in microbial communities Increased residual fraction of metals in soils, decreased metal uptake by white lupin Reduced metal concentrations in grass Reduced Cd accumulation in plant and concentration in soil pore water Different combinations with cement instead of cement alone as fixing agents

Immobilization result(s)

Cd Enhanced immobilization, decreased plant availability Cd, Pb Reduced mobility and plant uptake Pb, Cd, Zn Reduced Pb level, but not Zn and Cd levels in earthworms Pb Decreased Pb uptake with increasing phosphate supply, decreased phosphate concentration in the root with increasing Pb supply Phosphate Pb, As Increased plant uptake of soil As, but no effect on soil Pb phytoavailability Pb Increased residual fraction, reduced translocation from root to shoot of plant H3 PO4 , Ca(H2 PO4 )2 , phosphate rock Phosphate rock, diammonium phosphate Cd, Pb Reduced metal elution Bonemeal (finely ground, poorly crystalline apatite) Pb Reduced concentration in the leachate and availability in soils Hydroxyapatite, phosphate rock, triple superphoCd, Pb Reduced plant uptake sphate, diammonium phosphate Metal oxide Hydrous Mn oxide Cd, Pb Reduced mobility and ryegrass uptake Mn oxide Pb Reduced bioavailable fractions Fe-rich waste (Fe (hydro)oxides) with redox cycles Cd, Pb, Reduced exchangeable fractions Mn oxide Cd, Pb Reduced exchangeable fraction and Chinese cabbage uptake Fe oxide waste byproduct Cd, Pb Reduced uptake by maize and barley Organic matter Biosolid Cd Reduced bioavailability Papermill sludge, sewage sludge Cd Increased uptake Biosolid Cd Reduced plant availability Biosolid Cd Increased phytoavailability in biosolid-amended soil by chloro-complexation of Cd Cow manure Cd No reduction in rice Chicken manure compost Cd Reduced soluble/exchangeable fractions and phytotoxicity Compost Cd, Pb Decreased Cd leaching, but increased Pb leaching Coal fly ash, peat Pb Reduced leaching in the field lysimeters Humus, compost Cd Reduced uptake by rice Pb, Cd Alleviated phytotoxicity of Pb and Cd and reduced bioavailability of Pb Biosolid, wood ash, K2 SO4 Hard wood biochar Cd Immobilization due to enhanced pH by biochar Chicken manure- and green waste-derived biochars Cd, Pb Immobilization due to partitioning from the exchangeable to organicbound fraction Oak wood biochar Pb Sorption due to increases in soil pH

Lime, red mud Cyclonic ash, lime Portland cement Phosphate compound(s) KH2 PO4 Apatite, zeolite, Fe oxide KH2 PO4 Phosphate

Liming material(s) Ca(OH)2 Lime Lime Lime Ca(OH)2 , CaCO3 Lime Red mud, natural zeolite, lime Zeolite, compost, calcium hydroxide

Soil amendment(s)

Promising soil amendments in the immobilization of Pb and Cd in contaminated soils

TABLE II

Ahmad et al., 2012

Kashem and Singh, 2001 Liu et al., 2009 Ruttens et al., 2006 Kumpiene et al., 2007 Ok et al., 2011 DeVolder et al., 2003 Beesley et al., 2010 Park et al., 2011c

Bolan et al., 2003a Merrington and Madden, 2000 Brown et al., 1998 Weggler-Beaton et al., 2000

Mench et al., 1994 Hettiarachchi et al., 2000 Contin et al., 2007 Cheng and Hseu, 2002 Chlopecka and Adriano, 1996

Creger and Peryea, 1994 Cao et al., 2003 Basta and McGowen, 2004 Hodson et al., 2000 Chen et al., 2007

Bolan et al., 2003b Chlopecka and Adriano, 1997 Pearson et al., 2000 Wang et al., 2002

Gray et al., 2006 Ruttens et al., 2010 Jang and Kim, 2000

Bolan et al., 2003c Li et al., 1996 Lehoczky et al., 2000 Han and Lee, 1996 Chaney et al., 1977 Brallier et al., 1996 Garau et al., 2007 Castaldi et al., 2005

Reference

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Pb) apatites could be formed, followed by co-precipitation of mixed apatites. Secondly, by precipitation of pure hydroxypyromorphite (Pb10 (PO4 )6 (OH)2 ), Pb2+ can react with apatite through hydroxyapatite dissolution. A substitute process, which happens to be significant in temperate soils, involves metal-ligand complexation in solution and followed reductions in cation charge, which most likely decreases adsorption (Harter and Naidu, 1995). The formation of soluble Cdphosphate complexes reduces Cd(II) sorption onto soils in the presence of phosphate (Krishnamurti et al., 1996). The activity of free Cd(II), rather than total dissolved Cd(II), is a regulatory factor in Cd(II) sorption (O’Connor et al., 1984). The efficacy of phosphateinduced metal immobilization can be improved by enhancing solubility of phosphate compounds in the soil. The combination of rock phosphate and phosphoric acid has the potential to immobilize Pb (Cao et al., 2003). To enhance Pb immobilization in soil, phosphate-solubilizing bacteria (PSB) were used to slowly release phosphate from insoluble phosphate rock (Park et al., 2011a, b) (Fig. 2). There are some concerns related to the usage of phosphate compounds for metal immobilization. The leaching of phosphate induced after the phosphate treatment must be considered. A mixture of soil with phosphate addition in the molar ratio of H3 PO4 :hydroxyapatite of 0.75:1 is suggested to be optimum to reduce phosphorus (P) leaching (Zupanˇciˇc et al., 2012). Furthermore, bacteria affect the stability of pyromorphite. It is assumed that the microbes enhance the dissolution of mineral P, promoting its transformation into pyromorphite. The controversy of the method stems from the need of introducing a living, extraneous strain of bacteria into an uncontrolled environment, although the long-term effect of such a treatment is unknown. Park et al. (2011a, c) showed that PSB can affect the stability of pyromorphite and the effectiveness of the process depends on the availability of dissolved phosphates in solution. However, the interaction between microbes and minerals are complex and some aspects of the potential involvement of PSB in remediation treatments remain unclear. There is some evidence that various organic compounds, microbial metabolites, and plant activity may increase the dissolution of pyromorphite and cause a secondary Pb release (Sayer et al., 1997; Fomina et al., 2004; Manecki and Maurice, 2008; Debela et al., 2010, 2013; Topolska et al., 2013). Lime treatment Lime is basically used to ameliorate soil acidity but

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Fig. 2 Immobilization of Pb by phosphate amendments with or without phosphate solubilizing bacteria (PSB) as measured by the Pb concentration in soil extracted with ammonium nitrate (NH4 NO3 ) (Park et al., 2011b; Bolan et al., 2014). The soils tested, a naturally Pb-contaminated soil collected from a shooting range and a Pb-spiked soil collected from an uncontaminated site, are a sandy loam with a pH of 5.8, water-holding capacity of 38.5%, electrical conductivity of 34.5 μS cm−1 , and organic matter of 7 g kg−1 and a silt loam with a pH of 5.23, waterholding capacity of 73%, electrical conductivity of 66.9 μS cm−1 , and organic matter of 109 g kg−1 , respectively. CK = control without phosphate amendment; TP = tricalcium phosphate; HA = hydroxyapatite; PR = phosphate rock.

at the same time it is getting wide acceptance as a potential option among scientific community to reduce metal poisoning in soil (Fig. 3). Liming materials are in a diverse range with differences in their potential to neutralize the acidity (Table II). Over the years, liming has been employed as a regular traditional practice to decrease levels of Cd and Pb in edible parts of agricultural crops. Liming increases sorption of metals through decreasing the H+ concentration and enhancing negatively charged ions. Addition of alkaline materials such as lime and red mud increases the concentrations of residual fractions of Cd and Pb in contaminated soil. The pH increases in red mud and lime result in the heavy metal precipitation (Garau et al., 2007). The competition between Ca(II) and metal ions and the reduced mobility in soils by precipitation and adsorption result in reduced metal uptake by plants with liming on the root surface (Gray et al., 2006). An in-situ field trial of heavy metal remediation in contaminated soil proved that combing red mud and lime enhances soil pH and reduces metal bioavailability, thus forming vegetative cover in soil contaminated by Pb and Cd. Lime can be employed in combination with other amendments to decrease metal availability. When lime is mixed with biosolids, it decreases electrical conductivity (EC) and enhances pH, precipitating soluble ions

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Fig. 3 Immobilization of Cd and Pb by eggshell, oyster shell and calcium carbonate treatments as measured by the Cd and Pb concentrations in soil extracted with 1 mol L−1 calcium chloride (CaCl2 ) after treatments (Ok et al., 2011; Ahmad et al., 2012). The soil tested is a sandy loam with a pH of 6.21, electrical conductivity of 0.22 dS m−1 , organic matter of 38.69 g kg−1 , total N of 1.49 g kg−1 , and cation exchange capacity of 15.11 cmol(+) kg−1 . CK = untreated control; CC10 = calcium carbonate at 10 g kg−1 ; CC50 = calcium carbonate at 50 g kg−1 ; ES10 = eggshell at 10 g kg−1 ; ES50 = eggshell at 50 g kg−1 ; OS10 = oyster shell at 10 g kg−1 ; OS50 = oyster shell at 50 g kg−1 .

(Fang and Wong, 1999). Composting with lime addition also effectively increases pH and decreases leaching and bioavailability of metals (Singh and Kalamdhad, 2013). Cement-based solidification/stabilization Soil contaminated with Cd and Pb usually needs solidification/stabilization, aiming to lower the leaching rate and bioavailability. Cement is the most adaptable binder currently available for the immobilization of Cd and Pb. The selection of cements and operating parameters depends upon an understanding of chemistry of the system (Chen et al., 2009). Cement-based stabilization/solidification technology is an attractive option for the management of soil polluted with Cd and Pb to facilitate land use and reduce the release of contaminants into the environment. The efficacy of cement-based solidification/stabilization can be improved by modifying cement phase compositions and controlling temperature, water/solid ratios, particle size, and other factors that affect setting and strength development and long-term durability of solidified waste forms. The potentially deleterious Cd and Pb may adversely affect the cementing matrix; pretreatment to render such substances harmless, e.g., by adsorbent addition, is necessary in some cases (Jang and Kim, 2000). Portland cement (15%, weight/weight) was used

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for solidification/stabilization of Cd- and Pb-contaminated soils from a former industrial site (Voglar and Leˇstan, 2010a). Soils formed solid monoliths with cement. Concentrations of Cd and Pb in the water extracts and the toxicity characteristic leaching procedure (TCLP) extracts were lower in the solidified/stabilized soils than the original soils. Formulations of 15% (weight/weight) ordinary Portland cement (OPC), calcium aluminate cement (CAC), and pozzolanic cement (PC) and additives including plasticizers cementol delta ekstra (PCDE) and cementol antikorodin (PCA), polypropylene fibers (PPF), polyoxyethylene-sorbitan monooleate (Tween 80) and aqueous acrylic polymer dispersion (Akrimal) were used for solidification/stabilization of soils from an industrial brownfield contaminated with up to 157 and 32 mg kg−1 Cd and Pb, respectively (Voglar and Leˇstan, 2010b). Based on the model calculation, the most efficient formulation was CAC + Akrimal, which greatly reduced the leachability of soil Cd and Pb into deionized water below the limit of quantification and into TCLP solution and the mass transfer of the elements from soil monoliths. There are several areas ripe for further investigations in the field of cement-based solidification/stabilization of Cd and Pb in contaminated soils, for example, the phase development of cement pastes in the presence of Cd and Pb, the dissolution and precipitation of stoichiometric phases and solid solutions, and surface phenomena controlling Cd and Pb immobilization (Akhter et al., 1990). Animal manure and biosolids Animal manures and biosolids are the significant sources of organic composts. Biosolids are considered traditionally as one of the key sources of metal contamination in the soil. Advances in the industrial wastewater and sewage treatment technologies are successfully decreasing metal contamination in biosolids. Moreover, alkaline materials are used for stabilizing metals in biosolids. Zeolites are also valuable as a metal scavenger in the metal-contaminated biosolids. Natural zeolites stabilize Cd and Pb through transforming them in the exchangeable and carbonate to residual fractions (Zorpas et al., 2000). Cattle, poultry, swine, and dairy are key sources of animal manure byproducts worldwide. The majority of the manure products are less contaminated with metals except As in poultry and Zn and Cu in swine manure. However, recent developments in the manure byproduct treatment have brought reduction in the bioavailability of metals. For example, treatment of


poultry manure with alum (Al2 (SO4 )3 ) reduced the concentration of water-soluble Cd. Manure byproducts lowly contaminated with metals can be employed in stabilizing metals in soil (Bolan et al., 2014). Various studies have assessed the role of biosolids in the heavy metal contamination in the soil; however, only few have documented the advantageous impact of organic amendments as a sink for stabilization of toxic metals in the soil (Li et al., 2000; Brown et al., 2003). Alkaline-stabilized biosolids, known as designer sludge with exceptional quality of being low in metal bioavailability, can be employed as a potential sink for decreasing metal bioavailability in sediment and soil (Table II). Complexation, redox reaction, and adsorption are the immobilization processes accomplished through such amendments. Metals form both soluble and insoluble complexes with organic constituents in soil; the mechanisms specially depend on the type of the organic matter. The organic constituents in soil have a great affiliation to metal cations due to the existence of functional groups or ligands which can form chelates with metals. The alcoholic, phenolic, carbonyl, and carboxyl functional groups dissociate due to increasing pH, thus enhancing ligand ion affinity towards metal cations. Through prevention of sulfide oxidation/hydrolysis, the decreases in the metal concentration can be credited to an increase in soil pH (Hartley et al., 2004). Likewise, compost amendment increased the growth of white lupin and decreased uptake of Pb through reducing metal bioavailability in the soils (Castaldi et al., 2005). High fulvic and humic acid concentrations of compost are credited for the high metal-binding capacities of compost (Perminova and Hatfield, 2005; O’Dell et al., 2007). Oxides of metals Oxides of metals such as iron (Fe), aluminum (Al), and manganese (Mn) play an important role in metal geochemistry in soils. Atmospheric nature and highly active surface area make them potential for immobilization and sorption of diverse soil pollutants (Table II). Co-precipitation, formation of inner-surface complexes, and specific sorption result in a strong metal binding by metal oxides. Synthesized, industrial byproducts and naturally occurring oxides have been documented in their potentiality to be employed for soil remediation objectives (Kumpiene, 2010). Recent developments in the application of metal oxides and their precursors for chemical immobilization of metals in contaminated soils have been studied well (Komárek et al., 2013).

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Oxides of Mn (birnessite and phyllomanganates group of minerals), oxyhydroxides (goethite, ferrihydrite, lepidocrocite, feroxyhite, and akaganeite), and oxides of Fe (magnetite, hematite, and maghemite) mostly occur in soil. Arsenat (AsO3− 4 ) and Pb(II) form inner-sphere surface complexes with hydrous ferric oxides and Pb and Cd form mononuclear complexes on goethite and ferrihydrite surfaces (Knox et al., 2001). The surfaces of Fe hydrous oxides play a significant role in metal retention; Pb and Cd were immobilized with amalgamation of Fe(II) and (III) sulfates (Hartley et al., 2004). The re-use of Fe oxide-based drinking water treatment residuals may be a beneficial retention of heavy metals and suitable soil amendment for various cations and anions (Impellitteri and Scheckel, 2006). Manganese oxide minerals such as cryptomelane, todorokite, hausmannite, and birnessite exist in soil (Hettiarachchi et al., 2000). Birnessites exhibit the highest adsorption capacity of Cd and Pb among these Mn oxide minerals. The sequence of birnessite sorption capacity is Pb(II) > Cd(II) and the maximum adsorption of metals by birnessite is attributed to hydrolysis which shows metal adsorption through Mn oxides present primarily in the form of hydroxylation cations (Feng et al., 2007). Oxides of Mn(IV) are well known to precisely adsorb Pb. Cryptomelane is representative of Mn oxides and was employed to decrease Pb bioavailability. Fungi and bacteria enzymatically oxidize Mn(II), produce insoluble oxides of Mn(III, IV), and form biogenic Mn oxides, which have a great sorption potential for metals including oxidation of metals (Miyata et al., 2007). Sorption of metals by manganese oxides is through complexation above or below the structural vacancies and at structural sheet edges, and incorporation into the vacancies by isomorphical substitution for Pb oxidation by biogenic Mn oxide produced by bacteria. Nanosized metal oxides such as Mn oxides, ferric oxides, titanium oxides, aluminum oxides, cerium oxides, and magnesium oxides are referred to as potential adsorbents for metal remediation due to their high activities and surface areas (Hua et al., 2012). Ammonium acetate fuel has been used for the synthesis of nanosized alumina and is effective in the Pb adsorption in soil, but there are limited available studies which employed metal oxides in soil environment (Rahmani et al., 2010). Biochar Recent research development has proved that bio-

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char has the capacity to adsorb metals in contaminated soil and serves as a green environmental sorbent. Various parameters are involved in assessing the main role of biochar for environmental protection. Feedstock types and pyrolysis conditions are the important factors affecting the sorption capacity of biochars (Joseph et al., 2010; Kookana et al., 2011). However, research studies have documented the reduction of Cd and Pb in contaminated soils amended with biochar (Table II). Therefore, the assessment of the biochar efficiency in immobilization of contaminants in multi-metalcontaminated soils should be conducted including the metal immobilization mechanism of biochar to examine the long-term efficacy. Hardwood-derived biochar is effective in immobilizing Cd in contaminated soils. Cadmium solubility decreases at alkaline pH induced by biochar (Beesley et al., 2010). However, biochar produced at low pyrolysis temperature regimes immobilizes Pb (Uchimiya et al., 2011). Remarkable Pb immobilization by biochar produced at low temperature results in the augmented release of available Ca, K, and P in the soil environment. Biochar derived from dairy manure having a great quantity of available P stabilizes Pb through making insoluble hydroxypyromorphite (Pb5 (PO4 )3 (OH)) in soil (Cao et al., 2011). The cotton seed hull-derived biochar produced at 350 ◦ C contains a high oxygen concentration, resulting in high uptake of Pb and Cd due to the role of oxygen-containing functional groups on biochar surfaces in metal binding (Uchimiya et al., 2011).

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soil are summarized. More research studies are desirable on the influence of immobilizing agents on the physiological processes of the plant, i.e., photosynthetic activity, hydrolysis, plant nutrient uptake, tropism, development of plant hormones including their functions, nastic movement, photomorphogenesis, circadian rhythms, photoperiodism, plant response to environmental stress, seed dormancy, germination, osmotic pressure, turgor potential and stomata function in relation to plant-water nexus. Further studies are required on the chelate-assisted phytoremediation and the root-soil interface. Further phytotoxicity research is required with focus on the effects of Pb- and Cd-contaminated soils amended with solidifying agents on the crop plants in both field and laboratory conditions and the activity of enzymes required for photosynthesis, cell division and respiration, water absorption and transpiration, and chlorophyll, carotenoid, and adenosine triphosphate (ATP) synthesis. More in situ studies are needed to validate the significance of a diverse range of immobilizing soil amendments to restore contaminated soils. These kinds of field-based experiments are essential to assess the effects of soil amendments on co-contaminants. There is a dire need to develop procedures to determine immobilization efficacy that could be used to assess the in-situ short and long-term environmental stability of metal immobilization. Additional in-situ studies should be conducted to determine phyto-poisoning, ecological receptor endpoints, and human biomagnification to demonstrate the heavy metal risk management efficiency resulting from soil amendments.

CONCLUSIONS REFERENCES Unlike organic contaminants, Pb and Cd do not undergo microbial or chemical degradation and persist for a long time after their introduction. Bioavailability of Pb and Cd plays a vital role in the remediation of contaminated soils. This paper reviews the remediation of Pb- and Cd-contaminated soils through manipulating their bioavailability using a range of soil amendments. Metal-immobilizing amendments such as precipitating agents and sorbent materials decrease the bioavailability of Pb and Cd. Immobilizing agents can be used to reduce the transfer of Pb and Cd to food chain via plant uptake and leaching to groundwater. One of the major limitations of the immobilization technique is the long-term stability of the immobilized Pb and Cd that needs to be monitored. In the current review, the immobilization options with regards to engineering the bioavailability Pb and Cd and following the remediation of polluted

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