Rehabilitation of Heavy Metal Contamination and Soil Erosion Through Integrated Management

Soil is an essential element for life in natural environment. Soil erosion is a natural geological removal of top productive soil by water or wind and transported from high elevated areas to low elevated areas. Agricultural and industrial human activities such as removal of natural vegetation for plantation purpose (tea and coffee) (Rajagopal et al., 2014), intensive cropping, tillage practices detach and displace the soil aggregates that accelerate the soil erosion. In light textured sandy soils every year 0.5-200 Mg ha^-1 of top productive soil is removed and it led to the deterioration of soil fertility (Chambers et al., 2000). Topography and flooding are the major factors to cause erosion. In India, due to flood, 6000 MT of fertile soil was lost resulting in the loss of 5.5 MT of NPK (Meena et al., 2017). Hilly areas with >5% slope are more prone to soil erosion that decreases soil fertility in arable land and force to external synthetic inputs. Indiscriminative use of synthetic inputs, sewage sludge, improper management, industrialization and urbanization activities increase the heavy metal pollution that degrade the natural environment. Heavy metals have specific gravity >5 or having atomic number >20. Heavy metals include As, Cd, Cr, Cu, Fe, Hg, Mn, Ni and Pb. After Independence the development of industrial sector was increased. Industries generate enormous amount of wastewater with large amount of heavy metals and toxic chemicals. Central pollution control board (CPCB, 2011) reported that Gujarat, Maharashtra and Andhra Pradesh contribute 80% of hazardous waste (including heavy metals) in India. United Nations World Water Development Report showed that at global level, 80% of wastewater was discharged into environment as untreated which increase the BOD and COD level in waterbodies that degrade the aquatic ecosystem. Among different kinds of pollution heavy metals made significant problems to natural ecosystem (Nedelkoska and Doran, 2000). In aquatic and terrestrial ecosystems, high levels of heavy metals can act as ecological toxins (Nazemi, 2012). Heavy metals make problems to local, regional and global level. Apart from industries, roadways and automobiles contribute substantially to environmental burden of heavy metals since particulate matters in traffic emission include heavy metals like lead, cadmium and arsenic (Onat et al., 2013). Excessive heavy metal pollution adversely affect the plant metabolic activities, that affect the food production quantitatively and qualitatively. Heavy metals enter into food chain process, that ultimately affect the health of animals and humans. Heavy metal reaches human tissues through various absorption pathways such as direct ingestion, dermal contact, diet through the soil-food chain, inhalation and oral intake that may seriously affect their health. Among heavy metals Cd is the mobile element in the soil and readily available to plant. The present review covers the scenario of several management practices to minimize the soil erosion and heavy metal toxicity.
 
Reclamation methods  

Cultural methods

Contour farming
 
Contour is an imaginary line connecting two equal heights on the slope. Formation of contour bunds across the slope reduce the velocity of water and soil erosion. Contour bunding reduce 10% annual runoff of water and it reduce 49.5% soil erosion (Farahani et al., 2016). Contour farming means all the management practices from ploughing to harvesting takes place along the contour. Contour farming is most effective on slopes between 2 to 10% (Liu, 2014). It reduce soil erosion and increase the infiltration rate. On steeper slopes more than 10%, contour bunding should combined with other measures such as bench terracing to reduce the velocity of water. Low soil erosion conserve the native soil nutrients and avoid the external use of inputs and reduce the risk of heavy metal contamination. Many researchers reported that, contour farming is wildly used in mild slopes. It has significant impact on moisture regime, restrain the velocity of water flow and protect the soil from external barriers and contaminants.
 
Application of manures
 
Adoption of conservation agricultural practices produce large quantity of waste material. Every year, 998 million tonnes of agricultural wastes are produced (Agamuthu, 2009). Application of organic manures increase the soil aggregate stability and improve soil resistance (Roose and Barthes, 2001). The abundant availability of agricultural by-products make it good sources of raw materials for natural sorbents. Large quantity of agricultural residues are partially combusted and made into carboneous rich products e.g. biochar. The incorporation of biochar into the soil can alter soil physical properties such as structure, pore size distribution and water holding capacity (Downie et al., 2009). Improved soil structure facilitate the infiltration of water and reduce the runoff and soil erosion (Sharma and Bhardwaj, 2017). Vithanage et al., (2014) reported biochar amended soils reduce the soil loss by 19.9% compared to untreated area. Recently, combined application of biochar and polyacrylamide significantly reduced the soil erosion (Lee et al., 2015). In addition application of biochar increase the nutrient availability and reduce the nutrient loses (Major et al., 2009). These conserved nutrients in field reduce the element leaching to waterbodies and reduce the metal contamination. It also absorb polar compound contaminants from the environment (Yu et al., 2006). It reduced the leachability of Ni (II) (from 0.35% to 0.12- 0.15%) and Zn (II) (from 0.12% to 0.01%) compared to control plots (Shen et al., 2016). Addition of P enriched biochar immobilize the Cd, Zn and Pb metals by forming the phosphate precipitates (Zheng et al., 2015). Highest application of orchard prune residue derived biochar reduced the leachability of Cd, Pb and Cr (Fellet et al., 2011). But the effectiveness of the biochar decreased with time (Cui et al., 2016). The organic substance present in the soil has a significant impact on absorption and translocation of heavy metals (Cu, Zn, Pb and Cd) in soil which turns into more stable forms (Kabata-Pendias, 2001). Addition of potato peel and tubers with 10 % compost or 10 % vermicompost amendment reduce the concentration of Pb and Cu in soil (Angelova et al., 2010). Saw dust and rice husk act as a good source of sorbents. Application of saw dust and rice husk act as a binding agent that reduce the availability of heavy metals to plant (Wan Nagh and Hanafiah, 2008). Bio-availability of metal ion to plants and their mobility in soils are largely determined by chemical equilibrium of metal ions in solid or solution phases and adsorption reactions of the soil. Saw dust contain appreciable amount of cellulose, hemicellulose and lignin. The lignin interacts with cations by exchanging with protons and subsequently by chelating with the metallic ions (Rafatullah et al., 2009). Pine (Pinus sylvestris) sawdust remove the hexavalent chromium from the wastewater (Sidiras et al., 2013). Application of manures improve the physical properties of soil, acting as a chelating agent and reduces the availability of heavy metals to plant. Application of farm yard manure reduce the availability of Cadmium by 35% (Singh and Prasad, 2014). Addition of high sorption capacity organic material such as peat, coal and compost reduce the mobility of heavy metals. Addition of clay to increasing the sorptive capacity of light soil (James and Strand, 2009). Addition of compost, biosolids and recycled paper waste stabilize the metals in the soil (Jones and Healey, 2010). Application of sewage sludge amended with coal fly ash could reduce availability of Cu, Zn, Ni and Cd in sludge. With increasing rate of fly ash amendment, the DTPA extractable metals (Cu, Zn, Ni and Cd) were reduced (Su and Wong, 2003). Manures are rich in nutrient status and are traditionally used for cultivation purposes which protects and improve the soil wealth. The adoption of intensive cultivation strategies produce plenty of organic materials. Use of organic material is simple and easy method for effective reclamation.
 
Biological methods
 
It is the productive utilization of living organisms, such as bacteria, fungi, algae and some plants to conserve the soil and to degrade, detoxify, immobilize, or stabilize toxic environmental contaminants into an innocuous state for safe environment.
 
Using micro organism
 
World is abundant with microorganisms. Using microorganism to protect the environment is a promising and ecofriendly technique. Application of slime forming bacteria to the soil enhances the soil strength (Yang et al., 1994). Maleki et al., (2016) and is found that application of microbial-induced carbonate precipitation on soil reduce the wind erosion in sandy soil. Application of Azospirillum and other bacterial species improve the growth of desert plants and reduce the soil erosion in arid zone (Bashan et al., 2008). Mycorrhizal fungi plays an important role in forest ecosystem. It might be important to stabilize soil aggregates. The mycorrhizae release mycorrhizal protein glomalin with a strong cementing capacity of soil particles which increases the soil aggregates and reduce the soil erosion (Haddad and Sarkar, 2003). Rillig et al., (2003) results showed the mean weight diameter of macro aggregates (1-2 mm dia) was significantly higher in mycorrhizal soils compared to non-mycorrhizal soils. Using microbes is a passive process which acts as a biosorbent that has highly efficient regeneration of biosorbents, metal recovery, minimal sludge formation and cost effective (Hashim et al., 2011). The biomass acting as a ion exchange matrix binds and exhibits its intrinsic properties in order to remove heavy metals from very dilute aqueous solutions. Cell-wall structure of certain algae, fungi and bacteria are responsible for biosorbent (Volesky and Holan, 1995). It is very effective method for the remediation of metal-containing effluents. This process has advantage of selective removal of metals over a broad range of pH and temperature, rapid kinetics of adsorption and desorption of metals. The development of such biosorbents is found to have high metal affinity toward the different types of hazardous heavy metals released from the industrial waste and combination of distinct non-living microorganism also used. The immobilized biomass is recommended for large-scale application compared to native biomass. Acidithiobacillus ferrooxidans BY-3 is a chemolithotrophic bacterium acts as a natural biosorbent which is isolated from the mines. It has been extensively used for the removal of organic and inorganic arsenic compounds from the aqueous solutions (Yana et al., 2010). Gallionella ferruginea and Leptothrix ochracea microorganisms remove the arsenic by biotic oxidation. Deposition of iron oxides in the filter medium along with the microorganism provides the supportive environment for metal adsorption and its removal from the aqueous solution. It avoids the use of chemical reagents for the oxidation of trivalent arsenic in the sorption processes, because the iron oxides are produced continuously in the medium. Escherichia, Citrobacteria, Klebsiella, Rhodococcus, Staphylococcus, Alcaligenes, Bacillus and Pseudomonas are the organisms that are commonly used in bioremediation (Gomathy and Sabarinathan, 2010). Bioremediation consists of various remediation strategies, such as bioaugmentation, that is, natural attenuation process by using indigenous microorganisms, stimulated process by adding nutrients (biostimulation) and biomineralization involving the thorough biodegradation of organic substances into inorganic components (Joutey et al., 2013). Fukushi et al., (2003) used to remove arsenic by sequestering the metal into insoluble sulfides by the metabolic action of the sulfate-reducing bacteria, using a wide range of organic substrates with SO₄ as the terminal electron acceptor under anaerobic conditions. Kostal et al., (2004) worked on the genetic manipulation of Escherichia coli strain and over expressed ArsR genes which results in the accumulation of As. It increase the accumulation and binding of arsenic in selective ligands and removal of arsenic. Chauhan et al., (2009) discovered a new As (V) resistance gene (arsN) which encodes a protein similar to acetyl transferase. Over expression of this protein leads to higher arsenic resistance in E.coli. Different metal (As, Mg, Al, Cu, Fe, Zn and Ni) contaminated sludge was treated in up flow anaerobic packed-bed reactor using sulfate reducing bacteria, more than 77.5% of the initial arsenic concentration were removed (Jong and Parry, 2003). The degraded lands are fruitfully reformed by the application of natural and transgenic microbes. It helps to sustain the natural equilibrium and protect the nature from further degradation.
 
Using plants
 
Plants are the gift of nature and it covers more than half of the land areas. Using plant in hilly areas is an effective and stable method to reduce the runoff water (Singh et al., 2007). Plants are playing defensive role against erosion. Native plants have important role in erosion control. Medicago arborea is a native shrub decreased soil loss by 41.7% compared with bare soil (Andreu et al., 1994). Grasses have close canopy soil cover and profuse root system which binds soil particles that provide excellent protection against runoff and erosion. Grass canopy intercepts rainfall and reduce splash erosion. Grasses and trees are often used to stabilize gully banks. The effectiveness of plants in reducing runoff and soil erosion depends on the distribution of fibrous roots (Vannoppen et al., 2015). Eleusine floccifolia, Tephrosia vogelii and Rosa abyssinica had the strongest roots, which strengthen the gully banks (Li et al., 2016). Raising perennial grasses is recommended for shallow gravelly degraded soils. In India vetiver (Vetiveria zizanioides) grass was used for soil and water conservation during mid 1980’s. It has some extradinary features such as deep root system, tolerance to extreme climate variation, soil acidity, salinity, sodicity and elevated levels of heavy metals. Presently more than 100 countries had adopted vetiver system for soil and water conservation (Paul Troug, 2000). The grass was planted in row, vetiver grow thick hedge acting as a living barrier which slow and spread the runoff water. Comparative study of conventional and vetiver system followed in India resulted in the reduction of soil loss  from 14.4 t ha^-1 to 3.9 t ha^-1 and runoff loses narrow down from 23.3 to 15.5% (Rodriguez, 2000). Controlling of flood erosion in floodplains of Australia, vetiver hedge was established at 90 m intervals and the system had successfully reduced the velocity of water flow and limited the soil movement (Dalton, 1997). Strip intercropping is recommended for intensive cultivated sloppy land in which erosion permitting and erosion resistant crops are planted in alternate strips of 2-3 m width across slope. Pennisetum purpureum and Cynodon dactylon have been used as strip crops which successfully reduce the runoff and erosion (Bravo, 2002; Poonia and Singh, 2006). Effective soil surface covering compact pile plant reduces the effects of polluted soil erosion is termed as Phytostabilization. For Less than 2% sloppy areas, cover crops such as Cenchrus ciliaris, Cenchrus glaucus, Dinanath grasses are used to cover the soil surface. It acts as a barrier to prevent direct contact with contaminated soil and it also prevent soil erosion and distribution of the toxic metal to other areas (Raskin and Ensley, 2000). Plants have a protective function, anti-erosion and also stimulate the processes of sorption of metals in soil, reducing the risk of leaching of metals. Phytoremediation is a promising technology using plants to clean up contaminated air, soil and water (Behera, 2014). The families like Asteraceae, Brassicaceae, Caryophyllaceae, Cyperaceae, Cunouniaceae, Fabaceae, Flacourtiaceae, Lamiaceae, Poaceae, Violaceae and Euphorbiaceae showed remediation property (Sarma, 2011). Phytoaccumulation is an ability of plants in accumulating and showing effective tolerance to heavy metals (Baker et al., 2000). The ratio of metals between soil and plants parts should be more than one for phytoremediating species (Barman et al., 2000). Hydrilla plants are grown in field condition in the presence of contaminated water to remove the concentration of contaminants from soil (Kumar and Gopinath, 2016). Lee et al., (2002) reported that plutonium is accumulated ten times higher in Indian mustard (Brassica juncea) than sunflower (Helianthus annuus). Phytodegradation is a process of breakdown of contaminants by plants through metabolic processes within the plant. The roots of Indian mustard are effective in removal of Cd, Cr, Cu, Ni, Pb and Zn (Prasad and Freitas, 2003). B. napus accumulates high concentration of Cd and Pb (Selvam and Wong, 2008). Ensley (2000) have found that only lower contamination from groundwater, surface water and wastewater can be removed by rhizofiltration. Sunflower, Indian mustard, tobacco, rye, spinach and corn are able to remove the lead from water through rhizofiltration (Jadia and Fulekar, 2009). Some plants take the contaminants to respiration site and transpire the modified forms of the contaminants from plant to atmosphere (Ali et al., 2013). Reed bed system has more diverse activities, which consist of different layer of various size colloidal particles with plants. Plants such as Typha and Water hyacinth are used in reed bed system to absorb the heavy metals from industrial effluents (Menka Kumari and Tripathi, 2015). This method reduce the BOD, COD level and heavy metal contamination in waste water and make it more suitable to irrigation. The efficiency of phytoextraction of heavy metals from soils is usually very low and no chance of removing significant amount of metals from the soil in a time period. But attempts are made to increase the efficiency of phytoextraction by introducing desirable traits to plant (Aken, 2008). Biotechnological tools are used in order to improve the performance of plants in effective removal of metals from environment. Bacterial merAB operon was transferred to the chloroplast genome of tobacco plant to increase resistance toward highly toxic organic mercury (Heaton et al., 2005). To increase metal uptake, the yeast metallothionein CUP1 was introduced into tobacco plants to increase the phytoextraction of Cu and Cd (Thomas et al., 2003). Introduction of TcHMA4 protein gene into plant, which contain His and Cys repeats residues helps in heavy metal binding. Expression of this gene can be used for enhancing metal tolerance and phytoremediation potential of higher plants (Papoyan and Kochian, 2004). Transgenic plants removed up to 6% Zn and 25% Cd of the soil metal. Genetically modified tobacco have a gene for mercuric reductase, which convert ionic mercury (Hg (II)) to less toxic metallic mercury (Hg (0)) and volatilize it (Meagher et al., 2000). Farmers are preferred to grow grasses and other plants for reclamation. Native and transgenic plants play significant role in long term erosion control and metal reclamation. It is an ecofriendly and cost effective method.
 
Nanotechnology
 
Nanotechnology is an advanced approach. Nanomaterials (NMs) are defined as the materials having size ranging from 1 - 100 nm with a minimum of one dimension. It provides new types of materials which offer the unique and important solutions to the limitations of other conventional materials (Kim, 2012). Wind erosion is a widespread phenomenon in arid region because of structurally unstable and sandy texture making these soils highly erodible for most seasons. The application of nanoclay on arid region, which stabilize the soil structure by fixing the sand, increase aggregation and reduce the soil erosion (Padidar et al., 2018). Zhou et al., (2018) found that application of nanocarbon below the soil surface at 5cm depth increase the soil water holding capacity and delay the initial runoff time compared to the control plot. Nano particles have high chemical stability and large specific surface area, which adsorbs the heavy metals (synthesized nano chelates reduce 42-72% heavy metals) (Edwards, 1999). Many of these materials have been explored for application in wastewater treatment. They utilize the size-dependent properties of NMs, such as high surface area, high reactivity, strong sorption and faster dissolution. Yang et al., (2006) found that application of nanostructured materials can be used as adsorbents or catalysts to remove toxic and harmful substances from wastewater and air and finally from soil. Liu and Zhao (2007) prepared a new class of iron phosphate (vivianite) nanoparticles for in situ immobilization of Pb⁺² in soil and showed that the nanoparticles could effectively reduce the leachability and bio accessibility of Pb⁺² from soil. Liu (2011) reported synthesized apatite nanoparticles also used for remediation of a lead-laden soil. Carbon-based NMs are extensively used for the removal of heavy metals because of its nontoxicity and greater adsorption capacity (Mauter and Elimelech, 2008). Multi-walled carbon nanotubes (CNTs) were used successfully for the removal of Copper (II), Lead (II), Cadmium (II) and Zinc (II) from aqueous solution (Salam, 2013). The efficiency for metal ion removal by CNTs was observed around 10-80%, which could be improved to 100% by selectively functionalizing CNTs with organic ligands (Yu et al., 2014). Application of carbon nanoparticles reduce the 75-92 % Ni contamination from soil and 99% reduction from water system (Rathore et al., 2013). CNT sheets have been used as an adsorbent for divalent heavy metals such as Cu²⁺, Zn²⁺, Pb²⁺, Cd²⁺ and Co²⁺ (Tofighy and Mohammadi, 2011). Singh et al., (2013) used zero-valent iron nanoparticles for removing Cr from contaminated soil and reported 99% Cr was removed from contaminated soil. Chitosan methacrylic acid nanoparticles are used for adsorption of Pb(II), Cd(II) and Ni(II) ions from aqueous solution (Gupta et al., 2011). Many of nanomaterials have been explored for application in contaminated sites. But many nano-based technologies are successful only on the laboratory scale, not in large-scale wastewater treatment. Now special attention has given to the use of nanomaterials to replace the conventional methods. It is an advanced and rapid method for reclamation when compared to other methods. This innovative methods effectively reclaim the small scale degraded area.

Increasing man made activities accelerate the degradation of soil and natural ecosystem. Recently, people are aware about natural degradation. It helps to develop the technologies that help in reclaiming the natural system. In order to reduce the soil erosion and heavy metal contamination, several remediation technologies have been implemented. These techniques include cultural biological and nanotechnological methods. In Northern China, the adoption of contour farming has significantly increased. Farming experience in Northwest China shows that building terraces has enhanced the rainfall utilization and reduce the run off. China implements a policy for remediation during fallow at 2016 to reduce the contaminants from the land. The organic amendments can reduce the risk of exposure to humans by reducing the availability of metals in soil and water. The combined application of earthworms and pig manures was significantly increased the soil organic carbon and decreased the extractable heavy metals in water. A European team has reported different microbial communities and trees which maintain the soil stability and prevent the soil erosion. In India, vetiver system has been promoted for riverbank protection. Department of Land Development in Thailand has established a policy to promote the vetiver system among farmers. Vetiver system has been used to rehabilitate the mine waste in South Africa, Thailand and Australia. Nanomaterials are practically used for water purification in many water effluent plans. Each method has some limitations. For long term and eco-friendly remediation process all the techniques are get integrated, where soil erosion and heavy metal contamination is a serious problem.