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Bioremediation for Sustainable Environmental Cleanup

Harmless removal of Pb from Pb-contaminated sites is the need of this modern century. Such

safe disposal and proper remediation of Pb from polluted sites have been done using various modern

tools and techniques such as bioremediation, especially phytoremediation. Some plants are very

capable of remediating contaminants from contaminated sites, and such processes are collectively

also known as phytoremediation (Jagetiya and Kumar 2020). Soils polluted with Pb are remediated

by other living forms such as fungi, algae, microbes, etc. A considerable number of studies have

been successfully carried out on Pb-bioremediation because these techniques are acceptable, safer,

cheaper and more ecofriendly than any other techniques for the removal of Pb.

11.2 Lead Toxicity in Soil Plant System

Various heavy metals act as pollutants and create serious threats to plants’ health by altering their

metabolism, growth rate and other physiology. Among them, lead (Pb) is the second most dangerous

heavy metal due to its potential toxicity to plants (Shahid et al. 2011). In the soil-plant system, Pb

comes from fumes of automobiles, factories, storage batteries, mining, metal plating and finishing

operations. Once it gets established in the soil, it is more easily accumulated in it (Hadi and Aziz

2015).

Plant nutrient dynamics is an important factor for determining the growth and yield of the crop.

In healthy soil, a good amount of plant biomass and yield is produced. Large numbers of organic acid

(having low molecular weight) are secreted by the plants into the soil. Most of the microorganisms

get their raw food material from them and enhance the nutrient mineralization processes. Studies

showed that approximately 40–65% of the photosynthesis products were released by the living

plants as root exudates. These exudates are constituted of amino acids, plant growth hormones, etc.

(Dotaniya et al. 2020). The toxic levels of Pb inside soil negatively impact the normal root exudation

processes and decline their secretion. These conditions are very critical in the presence of higher

concentrations of Pb as the fertility of soil decreases up to a level from where land will not produce

even a single blade of grass. Pb creates oxidative stress on plants and generates reactive oxygen

species, which are phytotoxic. The most common symptoms of Pb toxicity are loss of chlorophyll

content, disruption in respiration and photosynthesis, etc. It has been reported that younger plants

and leafy crops are more susceptible to Pb toxicity as compared to older and less leafy crops.

Similarly, peri-urban vegetables also tend to be more susceptible to lead toxicity. Once established

in the food chain, Pb toxicity greatly affects human health, especifically infants. Such infants show

symptoms like stunted growth, mental retardation, hair loss and organ and metabolic process failure.

Different plant species get affected to varying degrees of toxicity due to Pb. Some plants have their

own excluder strategies through roots to reduce the effect of Pb toxicity. Some plants have a higher

potential for tolerating Pb toxicity. These plants are referred to as hyperaccumulators, and are also

used in phytoremediation (Dotaniya et al. 2017, Dotaniya et al. 2018a).

11.2.1 Lead Toxicity in Soil

Lead is found naturally in the crust layer of the earth, having levels less than 50 mg kg^–1 (Arias

et al. 2010). But anthropogenic activities may lead to change this level very frequently. These

anthropogenic activities may result in the accumulation of Pb onto the surface layer of the soil.

Further, its content declines with an increase in depth (Cecchi et al. 2008). According to ATSDR

(2020), the Pb level in soil was increased by one thousand-fold in the past three centuries because

of the availability of more advanced techniques that can detect Pb in very small fractions (ATSDR

2020). In soil, Pb may be present in many forms, including free metal ion, complex forms (devise

complexes with inorganic constituents like carbonate, bicarbonate, sulfate and chloride ion) or maybe

found in its organic forms such as fulvic acid, amino acid and humic acid. Alternatively, adsorption

of Pb occurs on the surface of particles like iron oxides, biological and organic matter, etc. (Vega

et al. 2010, Sammut et al. 2010). However, due to strong binding affinities of Pb with colloidal/

organic material, it created the possibility that only a tiny fraction of Pb is soluble in soil, and