Bioavailability, Leachability, Chemical Speciation, and Bioremediation of Heavy Metals in the Process of Composting

Jiwan Singh, Ajay Kalamdhad

• Publisher: CRC Press
• Year: 2018
• Print ISBN:
  9781138598331
• eBook ISBN:
  9780429942839
• Topic: Biological Sciences
• Subtopic: Environmental Science
• Index: Biological Sciences

Currently, it is a serious concern to manage waste in the environment. Therefore, detailed knowledge of heavy metals, their eco-toxicological and health effects and ecofriendly approaches for their immobilization and detoxification is urgently required to control and minimize the environmental pollution. Composting is one of the popular methods in waste management and there are a lot of issues pertaining to composting. One of it is the leaching of heavy metals. This book discusses thoroughly, the availability, leachability and the speciation of heavy metals in the entire process of composting.

1 Introduction

1.1 OVERVIEW

The rapid rate of urbanization and industrialization has led to an increase in municipal solid waste (MSW) worldwide. Sewage sludge (SS), which is generally produced from wastewater treatment plants, plays a vital role in generating MSW. SS mainly comprises organic materials, macro/micronutrients, nonessential trace elements, microorganisms, and eggs of parasitic microorganisms (Wang et al., 2008). Nonessential trace elements decrease crop production and threaten soil safety and human health. A disposal and environmental management of nonessential trace elements are a worldwide concern (Wang et al., 2013). Water hyacinth (Eichhornia crassipes) is considered a green waste (GW) for its removal of organic and inorganic pollutants from water and soil. Water hyacinth is one of the most commonly used aquatic plants in constructed wetlands due to its fast growth rate and ability to absorb nutrients and contaminants (Singh and Kalamdhad, 2012). Composted residuals derived from MSW and GW have a high affinity to form organic complex of metals. There is a consensus in scientific literature that composting processes increase the complexation of heavy metals in organic waste residuals. Heavy metals are strongly bound to the organic fraction of organic matter (OM) and metal complexes, thus limiting their solubility and potential bioavailability in the soil (Smith, 2009).

Composting is the biological decomposition and stabilization of organic substrates (SS, GW, etc.), under conditions that allow the development of thermophilic temperatures as a result of biologically produced heat, to produce a final product that is stable, free of pathogens and plant seeds, and can be successfully applied to agricultural land (Singh and Kalamdhad, 2013a).

Aerobic microorganisms play a significant role in the decomposition of organic wastes in the composting process. Composting proceeds under three major phases: (1) mesophilic stage, (2) thermophilic stage, and (3) cooling (Neklyudov et al., 2008). Composting is a well-established technique for the treatment and disposal of MSW and GW due to material recycling (Villasenor et al., 2011). The process of composting involves the decomposition and degradation of organic waste by converting it into a more stabilized, humus-like material which is called compost. When applied on agricultural lands, it can significantly improve the physical properties and agricultural productivity of soils (Deka et al., 2011; Gabhane et al., 2012). Substances that are not biodegradable and have high concentrations of heavy metals do not decompose in the solid and hence cannot be used to improve the soil fertility of agricultural lands.

Heavy metals can accumulate in plants, which are then transferred into the food chain, and thus may cause a possible risk to animals and human beings (Iwegbue et al., 2007; Singh and Kalamdhad, 2012). The presence of heavy metals in compost can affect the growth, morphology, and metabolism of the soil microorganisms, thus decreasing the activity of microorganisms, and soil fertility (Bragato et al., 1998). The absorption of heavy metals by plants and the corresponding accumulation in human tissue and biomagnifications through the food chain not only causes environmental concerns but also human health concerns (Wong and Selvam, 2006). Figure 1.1 represents the fate and transference of heavy metals from compost to soil, plants, and the water system.

FIGURE 1.1 Fate and transport of heavy metals in the environment.

Thus, heavy metals are considered one of the major sources of soil pollution. Though, presence in various fractions of metals and their association with ecotoxicity is the major problem. Heavy metals are natural components of the environment and have high atomic weight with a density of >4 g/cm³ (Tchounwou et al., 2012). These metals are referred to as trace elements at a concentration level of <10 ppm in the environment (Tchounwou et al., 2012). Some heavy metals (such as Cu, Co, Fe, Mn, and Zn), are required for physiochemical and biochemical functions of both plants and animals. On the other hand, excessive additions of these heavy metals to the environment may have an adverse effect on both the animals and humans. Other nonessential metals (such as, Cd, Pb, Hg, and Ni), which are involved in biological functions and are serious to metal-sensitive enzymes, can initiate the cell death (Tchounwou et al., 2012; Swati and Hait, 2017). Determination of total concentration of heavy metal in compost/soil is indicator of pollution in soil but providing information about bioavailable fractions of metals. (Singh and Kalamdhad, 2012, 2013a). The bioavailability and speciation of heavy metals during the composting process suggested that toxicity of heavy metals depends on their different bioavailable fractions rather than total metal concentration detected in the digested solution (Kang et al., 2011).

The bioavailability of metals is used to indicate that part of the concentration of the metals are easily soluble in water and considered effortlessly accessible to plants and soil microorganisms. The bioavailability of metals in soil is a self-motivated process that depends on the explicit physicochemical parameters of soil. These parameters, such as pH, OM content, redox potential, cation exchange capacity of soil, and hydroxides, are present in soil, soil texture, and clay content (Guala et al., 2010). The pH and OM contents are the most important parameters involved in the absorption of heavy-metal by living organisms (Li et al., 2010).

The addition of soluble organo-metal complexes occurring in the final compost in the soil increases the potential risk of metals, since soluble metals will be more available to receptors (plants, microorganisms, etc.) (Zheng et al., 2007).

The water soluble fraction of metal is most readily bioavailable in compost when applied to soils. Hsu and Lo (2001) reported that water-soluble metals play a critical role in maximizing the potential for contamination of the food chain and water system (surface water as well as ground water).

It has been considered that the metals extracted with diethylene triamine pentaacetic acid (DTPA) solution may play a significant role in checking the bioavailability of heavy metals in soil and compost-added soil available for plant uptake (Fuentes et al., 2006; Singh and Kalamdhad, 2013b).

The toxicity characteristic leaching procedure (TCLP) test has been applied to assess the mobility of hazardous metals that are present in the compost/waste materials. If heavy metals contaminated compost is applied to soil, metals can be leached out in soil and may pollute ground water (Singh and Kalamdhad, 2013b). Leaching of metals is defined as the ratio of the amount of a heavy-metal discharge from the TCLP test to its total concentration; it is usually applied to evaluate the potential leaching ability of heavy metals occurring in the compost and amended soil (Chiang et al., 2007).

An estimation of the fractionation of heavy metals in the final compost helps to assess their bioavailability and the aptness of compost to be applied for land application (Wong and Selvam, 2006). The sequential extraction method affords a useful technique for determining the different chemical forms of heavy metals in sludge/compost. This technique has been applied to organic soils, which are very similar to composts, for studying metal mobility and bioavailability (Yuan et al., 2011). The mobility of the heavy metals is decreased roughly in the order of extraction categorization (Nair et al., 2008). Tessier et al. (1979) reported a sequential extraction method of heavy metals that are present in the compost. These metals involve the following five fractions: (i) exchangeable fraction (F1): this fraction can be transformed by changing ionic composition of water in addition to sorption–desorption process, (ii) carbonate fraction (F2): this fraction is depend on pH and have ability to convert into soluble and mobilized fractions under acidic condition, (iii) reducible fraction (F3): this fraction is thermally disturbed under anoxic conditions, (iv) oxidizable fraction (F4): this fraction will be released and solubilize after getting oxidizing conditions, and (v) residual fraction (F5): this fraction is perpetually bound with the mineral components of the compost and soil. This fraction will never be accessible for plants or soil microorganisms in standard natural conditions. In the sequential extraction procedure various chemical reagents are applied to extract various form of metals from the compost sample, chemical reagents are elaborate in dissolving the different constituents of the sample medium in order sequentially. On the contrary, a reagent should have the ability to liberate all fractions of the metals from a specific constituent of matrix (i.e., F1 and F2 fractions) without touching other constituents (Li et al., 2001). The sequential extraction technique provide important information about different fractions of metals and allows the forecast of metal leaching rates (He et al., 2009).

Some of the major sources to decrease metal noxiousness for the environment including human health are reduction of heavy-metal bioavailability, leachability, and their various fractions by the addition of a few chemicals while composting different waste materials. Lime is an alkaline material and can lead to a decrease in the mobility of heavy metals in the final compost (Chiang et al., 2007). Adding lime in composting biomass increases its pH level, thus resulting in a decrease in the accumulation of metals in soil (Wong and Fang, 2000). Lime can be used as a stabilizer for heavy metals that will increase the rate of degradation of organic materials in the composting process by offering a buffering to composting biomass during acid formation after degradation of organic materials resulting decrease in pH. Lime addition in appropriate supply sufficient amount of Ca to the composting microbes that enhanced the metabolic activity of microbes resulting in an increase in temperature and CO₂ progress without any negative impact on microbial community present in the composting mass (Fang and Wong, 1999; Gabhane et al., 2012). The addition of lime was very efficient in reducing bioavailability of heavy metals in the mature compost of SS. It might form less-soluble carbonate salts with metals ions (Fang and Wong, 1999). Zeolite is a naturally hydrated aluminosilicate mineral and can be classified as “tectosilicate” (Singh and Klamdhad, 2014). Zeolites can be used extensively in the composting process to improve physical and chemical characteristics of the compost and then immobilize metals in the SS composting (Sprynskyy et al., 2007; Villasenor et al., 2011). It improves the composting process by increasing the porosity of the substrate of the composting mixture (Zorpas et al., 2000). It has the ability to readily absorb almost all heavy metals that are bound to exchangeable and carbonate fractions (Zorpas et al., 2000). It can increase sodium and potassium in the compost through exchange with toxic metals.

1.2 EFFECTS OF HEAVY METALS ON THE ENVIRONMENT

1.2.1 EFFECTS OF METALS ON SOIL

In recent years, natural anthropogenic activities can cause elevated concentrations of heavy metals in the soil. Consequently, the environment is deteriorating because of the negative impact of heavy metals (Su et al., 2014; Zojaji et al., 2014). The quality of the soil may be represented by the microbial and enzymatic activity of the soil. Microorganisms present in soil are an important sign of the level of pollution in the soil. Microbial activity can be prevented in the soil that has been polluted by heavy metals (Su et al., 2014). Soil pollution by heavy metals is one of most significant concerns for the whole industrialized world (Hinojosa et al., 2004).

Metals present in soil not only causes an adverse effect on different properties of soil related to plant life but also can cause deviations in composition, size, and activity of the microbial community (Yao et al., 2003). Thus, heavy metals are considered to be a main factor responsible for soil contamination. There are several metals, such as Cu, Ni, Cd, Zn, Cr, and Pb, which are responsible for soil contamination (Hinojosa et al., 2004). The soil’s biological and biochemical properties are affected by heavy metals. According to Speira et al. (1999), the soil parameters, such as OM, clay contents, and pH value, have key effects on biochemical properties.

Soil enzymatic activities were affected by toxic metals indirectly through shifts in the microbial community (Shun-hong et al., 2009). A key microbial process can also be affected by heavy metals in soil by decreasing the number of soil microorganisms and their activities. If the long-term exposure of heavy metals affects the tolerance of the bacterial community and fungi (arbuscularmycorrhizal fungi), then these microorganisms can play a vital role in the restoration of metal-polluted ecosystems (Mora et al., 2005). Heavy metals can decrease the productivity of many bacterial species, can increase actinomycetes in soil, and can affect both the biomass and diversity of the bacterial communities in polluted soils (Chen et al., 2010).

Enzymatic activities of soil are affected in different ways by different metals due to the different chemical binding properties of the enzymes in the soil environment. It has been considered that Cd is highly toxic to enzymes due to its better mobility and low-binding properties when compared with soil colloids. Cd contamination had a negative effect on the activities of protease, urease, alkaline phosphatase, and arylsulfatase, whereas it did not show any significant effect on the invertase (Karaca et al., 2010). Cu toxicity inhibited the β-glucosidase activity, whereas it did not affect the cellulose activity. The Pb contamination had a negative effect on the activities of urease, catalase, invertase, and acid phosphatase (Karaca et al., 2010; Singh and Kalamdhad, 2011). Soil microbes have important roles in recycling nutrients for plant, conservation of soil structure, decontamination of harmful chemicals, plant pest control, and control of plant growth communities that represent directories of soil quality. Repeating contamination of soil by heavy metals is an important study about the functioning of soil microorganisms (Wang et al., 2007).

Chromium is commonly present in soils as Cr (III) and Cr (VI). Cr (III) is a micronutrient and a nonhazardous species, whereas Cr (VI) is a strong oxidizing agent and is 10–100 times more toxic than Cr (III) (Garnier et al., 2006). With higher concentrations, Cr (VI) could cause shifts in the composition of soil microbial populations, and is also known to cause harmful effects on microbial cell metabolism at higher level (Shun-hong et al., 2009). Ashraf and Ali (2007) reported that the toxic effect of heavy metals on soil microorganisms prompts a change in the diversity, population size, and overall activity of the soil microbial communities. Generally, an increase of metal content shows an adverse effect on soil microbial metabolic activities, such as respiration rate and enzyme activity. These activities are very valuable indicators of soil contamination. The soil microbial profile was changed with the toxicity of lead (Pb) in the soil system.

1.2.2 EFFECTS OF METALS ON PLANTS

The availability of both essential and nonessential heavy metals at certain concentration levels in the soil can inhibit the growth of most plants. However, if the high concentration of metals is supplied to the plant that is exceeding threshold limit, resulting death of the plant (Su et al., 2014). Some heavy metals, such as As, Cd, Hg, Pb, and Se, are not necessary eleme...

Table of contents

• Cover
• Half Title
• Title Page
• Copyright Page
• Contents
• List of Figures
• List of Tables
• Preface
• Acronyms and Abbreviations
• Authors
• Chapter 1 Introduction
• Chapter 2 Composting Process
• Chapter 3 Analysis of Compost Samples
• Chapter 4 Composting and Heavy Metals
• Chapter 5 Bioavailability of Heavy Metals during Composting
• Chapter 6 Chemical Speciation of Heavy Metals during the Composting Process
• Chapter 7 Effects of Chemical Amendments on Bioavailability and Fractionation of Heavy Metals in Composting
• Chapter 8 Reduction of Bioavailability of Heavy Metals by Microorganisms
• Chapter 9 Leachability of Heavy Metals during the Composting Process
• Index