30 Control of Soil Pollution

Meenakshi Nandal

epgp books

 

 

 

Objectives:

  • To study different methods to control Soil Pollution.
  • To study the 3R’S approach.
  • To study about the solid waste management.

 

31.1 Introduction

 

Soil pollution have adverse effects on plant growth, animal health and human beings and is defined as the invariable increase of persistent toxic compounds, chemicals, salts, radioactive materials, or disease causing agents in the soil. Soil is one of the essential and valuable natural resources of the nature. Soil health is necessary for sustaining life and living on the earth.95% of human food is derived from the earth. Making plan for having healthy and productive soil is essential to human survival. Entrance of materials, biological organisms or energy into the soil will cause changes in soil quality. This problem causes soil to remove from its natural state. This problem causes soil to remove from its natural state. For dealing with contaminated sites there are three main approaches: recognition of the problem, evaluation of the nature and extent of the hazard, and the best choice of remedial action. Reclamation approaches includes physical, chemical and biological environmental sciences.

 

 

To control soil pollution most favored options have been given in Fig 31.1. We can limit construction in sensitive area in order to prevent soil erosion. If we could all adopt the three R’s: Reduce, Reuse, and Recycle we would need less fertilizer and fewer pesticides also. This way less solid waste will be produced. The contaminated soil can be treated by in situ and ex situ technologies. Without removing the bulk soil in- situ technique degrade, remove, or immobilize contaminants by applying chemical, biological, or physical processes to the sub surface of the soil. In- situ techniques have the advantage that the soil does not have to be removed or transported in the process whereas the techniques lack contaminant removal efficiency. Soil contaminants remove more effectively in ex situ techniques and due to excavation process addressing to deep contamination is also there but it is cost effective technique due to the costs involved with excavating and transporting of the soil.

 

31.2 Soil pollution control and remediation methods

 

31.2.1 Soil pollution control measure

 

Reducing use of chemical fertilizer and pesticides

 

Applying bio-fertilizers and manures can reduce chemical fertilizer and pesticide use and thereby minimize soil pollution. For instance in gardens, make use of organic fertilizers and pesticides, because firstly they are usually made of natural substances, secondly are bio-degradable and thirdly do little harm to the natural balance in the soil. Aware and insist people on buying natural and organic food, because chemical pesticides and fertilizers are not used in their growing process. Not only are organic foods healthier for the environment, they are also healthier for you and your family. Examples of some common organic pesticides include Cayenne pepper spray, Soap spray, Tobacco powder, Pyrethrin, Neem, Sabadilla etc.

 

Reusing, recycling and recovery of materials

 

Materials such as glass containers, plastic bags, paper, cloth etc. can be reused at domestic levels rather than being disposed, reducing solid waste pollution. This decreases the volume of refuse and helps in the conservation of natural resources. For example, the recovery of one tone of paper cans save17 trees.

 

Reforestation

 

Deforestation or the cutting down of trees are the main cause of erosion of the soil due to which the loss of fertility of the soil occurs which leads to soil pollution. Reforestation is an effective method of  preventing soil pollution. Another can be cut down the usage of paper or use recycled paper. This will lead fewer trees to be cut down and therefore reduced deforestation.

 

Restoring forest and grass cover to check wastelands, soil erosion and floods can be attempted to control land loss and soil erosion .On the other hand crop rotation or mixed cropping can improve the fertility of the land.

 

Waste minimization

 

The 3 R’s principle: Reduce, Reuse, and Recycle help in     help in minimization of waste production.Reduce the quantity and toxicity of refuse and trash that you discard. Reuse containers and try to repair things that are broken.Recycle products wherever possible, which includes buying recycled products i.e., recycled paper books, paper bags etc.

 

Some of the major types of solid waste which are major cause of soil pollution are as follows:

 

Municipal Solid Waste (MSW) Hazardous WastesIndustrial Wastes Agricultural Wastes Bio-medical Wastes Management of solid waste disposal can be done by adopting proper methods such as industrial wastes can be rendered less hazardous by treating physically, chemically and biologically. The insoluble biodegradable material should be permitted to degrade under controlled conditions; acidic and alkaline wastes should be first neutralized before being disposed. New areas for storage of hazardous waste should be regularly investigated such as deep well injection and more secure landfills to protect our soil from damaging effect cause otherwise.

 

Further soil pollution can be restricted by

 

  • Strong regulatory programs to reduce soil contamination need to be implemented.
  • Reduce consumption and reduce your trash is the best way as the less rubbish we create the less chance of the waste that it will end up in our soil.
  • Use of plastic bags which are a major cause of pollution must be ban.
  • Most appropriate and secure disposal of waste including nuclear wastes.
  • Encouraging social and agro forestry programs.
  • Undertaking many pollution awareness programs.
  • There is a need to educate the public about the harms done when they litter.

 

31.3 Soil pollution control techniques:

 

Removing the contaminated soil and replacing it with clean soil is one of the simplest physical methods for remediation. This process including dig, dump and replace, only be possible if extent and depth of the contaminated region is small. The contaminated soil excavated from the site can be either disposed off in an engineered landfill or subjected to simple washing as shown in fig.31.2.

 

Fig. 31.2 Soil washing for granular soils contaminated with inorganic pollutant

 

  1. a) Electroremediation

 

The technique is used to remove heavy metals or polar compounds as pollutant migrate in an electric field. Migrating particles have a permanent electric charge or they are polarized. Under the influence of an electromagnetic field contaminants migrate through the soil towards the cathode or anode area, where contaminants are removed by any of the methods: chemical precipitation, adhesion to the electrodes surfaces and removing and processing the contamination away from the remediated site.

  1. b) Ex-situ soil vapor extraction (SVE)

Volatile and some semi-volatile contaminants are removed in this ex situ soil vapor extraction (SVE) technology. Volatilization of organics is encouraged by placing excavated soil over a network of aboveground piping to which a vacuum is applied (Hyman and Dupont, 2001).

 

Directional drilling, pneumatic and hydraulic fracturing, and thermal enhancement (e.g. hot air or steam injection) helps in improving the potency of SVE technique. Gas flow through the subsurface is generally boosted by directional drilling and fracturing enhancements peculiarly in lower permeability zones and the latter such as hot air or steam injection helps in boosting of subsurface soil temperature and thus improvement of volatility of contamination.

 

 

Fig. 31.3 Ex-situ soil vapor extraction (SVE) system

 

Source: https://www.researchgate.net/figure/Ex-situ-solidification-stabilization_fig11_287574661

 

 

  1. c) Extraction and separation techniques

 

In solvent extraction, an extracting agent in general (an aqueous solution but preferably an organic solvent) along with the contaminated & oil is mixed. Potential applications include the removal of metals using a mineral solution, zinc lead, organo-metallic compounds and some cyanides using sodium hydroxide solution. Hydrocarbons and halogenated hydrocarbons can also be removed. Usually Contaminants are present in the finer or coarser fraction of the soil or the organic components, which can be removed in some cases using a process which separates the soil into fractions by any of the process like specific gravity or particle size or settling velocity.

  1. d) Thermal methods

Destruction of the contaminants directly or indirectly at an appropriate temperature can be done by any of the two ways of heat treatment methods; removal of contaminants by evaporation either by direct heat transfer from heated air or an open flame or by indirect heat transfer. The gas so leaving the  heating device must be treated to destroy or remove any contaminants or unwanted products of combustion. Stream stripping is another process in which steam is injected into soil which leads to evaporation of volatile contaminants which may be water soluble or insoluble.

 

  1. e) Chemical methods: Chemical methods are developed on the basis of different chemical processes, which include oxidation and reduction, extraction, precipitation of sparingly soluble chemical compounds, pH stabilization. During the process detoxification is completed only when intimate contact between soil and chemical is there regularly.

 

  1. f) Soil washing: The method uses a wide spectrum of leaching solutions from water to strong inorganic acids. There are two types of washing firstly separation of fine particles with adsorbed contaminants in a stream of dissolvent (water or solutions of inorganic salts Ca or Mg) and finally extraction of contaminants. It is a fast and efficient method for cleaning up of contaminated soil but high cost, generation of additional chemical waste and invasive in nature results in low level of community acceptance of this technique.

 

 

Fig. 31.4 Schematic diagram of soil washing Source: Dadrasnia et al., 2013

 

  1. g) Chemical and photochemical reduction

 

In this method, the total mineralization (by chemical reactions) of organic contaminants and the effective transformation of organic and inorganic contaminants into non-toxic, less toxic or chemically inert forms takes place. Theoretically, this method could be applied both ex situ and in situ; however, in practice, because of its high invasivity, only ex-situ techniques are used. The method is suitable for the removal of organic compounds (oil compounds, organic solvents, pesticides) and inorganic ones (ions or heavy metals, oxyanions). The reagents used in this method are: ozone, hydrogen peroxide, chlorates, chlorine monoxides and other oxidation or reduction reagents. In the case of the photochemical process, an additional factor is ultraviolet radiation.

 

  1. h) In- situ immobilization of contaminants:

 

This method is used for the immobilization of inorganic contaminants such as heavy metals (it is rarely applied to organic contaminants). The method is based on the introduction into the soil of different substances (cement, poly epoxide resin, zeolites) which bond strongly with the contaminants or create scarcely soluble chemical associations (carbonates or phosphates). The effects of remediation technique are also the modification of the soil chemical properties and hence causing the immobilization of heavy metals or their chemical transformation into less mobile forms by pH changes. This method is often used in emergency cases to prevent the contaminated area from spreading, often in connection with fit stabilization as a supporting technique. This method is applied to medium or low contaminated areas.

  1. i) Biological treatment methods

 

In biological treatment, there is involvement of the use of microorganisms or vegetation or phytoremediation. Microorganisms like bacteria, fungi can mutate harmful chemicals to such substances which are less harmful than original one. Microbial bioremediation occurs under both aerobic and anaerobic conditions and at contaminated sites as either intrinsic or enhanced biodegradation. Bioremediation technologies can be largely classified as ex- situ or in- situ. Ex-situ treatment involves physical removal of contaminated area to another area within the site. Various ex-situ treatments processes like bioreactors, land farming, anaerobic digestion, composting, biosorption can be used in practice. In contrast to ex- situ techniques, in- situ techniques comprised of treatment of the contaminated material in place like bioventing which involves treatment of the contaminated soil and biostimulation of indigenous aquifer microorganisms. In bioventing, gas is injected into the subsurface to increase the biodegradation process of the contaminant. To expedite degradation of microbes, low amounts of oxygen is outfitted to contaminated unsaturated soils. With the help of oxygen, the microbes gain energy and carbon for their growth by oxidizing the contaminants and this oxygen is supplied by air injection wells that push air into the subsurface. Hydrocarbons and some chlorinated compounds are depraved by aerobic venting. Whole range of organic contaminants including phenol, polychlorinated hydrocarbons, oil and oil products, dioxins, etc can be treated by  applying microbial treatment methods which appear to be more promising and the problem can be solve by any of the two methods.

 

  1. Microbes already presented on the site may be collected and cultured in the laboratory.
  1. In the  laboratory strains  of  microbes  can  be  developed  that  is  capable  of  metabolizing particular chemicals.

For creating optimum conditions excavation of the soil prior to treatment offers the greatest scope. Using standard earth moving techniques excavated soil can be placed on thin layers to various depths and then microbes and nutrients applied using standard agricultural techniques such as fertilizing, ploughing etc. For example oil zapper used for treatment of oil contaminated soils

 

 

Bioreactors: Bioreactors treated the contaminated soil either in solid or slurry phase. Because of more manageable, controllable and predictable conditions in a bioreactor system the rate and extent of biodegradation are greater than in situ or in solid-phase systems. Before being placed in a bioreactor soil requires pre treatment (e.g., excavation) or the pollutant can be removed from the soil via soil washing or physical extraction process (e.g., vacuum extraction) (Vidali, 2001).Mixing in a closed container confirms the pollutants, water, air, nutrients and microorganisms are in permanent contact. The pH can be maintained by adding acid or alkalinity (Van Deuren, et al, 2002). A slurry bioreactor is a containment vessel and apparatus used to devise a three-phase (solid, liquid, and gas) mixing condition. To make possible the bioremediation rate of water-soluble pollutants and soil bound contaminants mixed to form water slurry and biomass capable of degrading target contaminants.

 

  1. j) Phytoremediation

 

In this technique plants are used to extract, immobilize or degrade the contaminants in soil, groundwater and other contaminated media. Large areas of shallow contamination can be best treated by this process. However phytoremediation has its limitation of possible bioconcentration of contaminants up the food chain. The contaminated vegetation and root zone may impact plant-eating animals and soil organisms. The applicability of phytoremediation is limited by several factors. First, it is important that the polluted site of significance is able to sustain plant growth. This requires proper climate, soil characteristics such as pH and texture, and sufficient water and nutrients. Second, Phytoremediation is practical only in situations where contamination is shallow because plant roots go deep. Third, for situations requiring rapid treatment, it is not suitable since the time requirements for phytoremediation are sometimes long relative to some conventional technologies such as land filling and incineration. The process of phytoremediation takes place through several mechanisms (Fig 31.5):

 

  1. Direct uptake, and absorption into plant biomass of contaminants
  2. In the subsurface Immobilization or Phytostabilization of contaminants
  3. Discharge of plant enzymes into the rhizosphere that proceed directly on the contaminants
  4. In the rhizosphere stimulation of microbial mediated degradation

 

Summary

 

 

Different types of soil pollution, namely agricultural soil pollution, industrial waste causing soil pollution, urbanization causing soil pollution. Soil fertility and mineral content decreases due to these different types of pollution. Therefore, measures have to be taken for controlling and remediation of soil pollution. Due to the lack of awareness about SWM impact and proper waste management process research findings revealed that there are significant issues with unauthorized waste disposal practices. Recycling, composting and waste-to-energy are integral parts of the solution to solid waste management therefore should be developed from the primary level and hence are all required to solve  the India’s SWM crisis. Bioremediation has grown into a green, attractive and promising technique as it can selectively degrade the pollutants without damaging the site or its indigenous flora and fauna, it is more cost-effective and alternate to traditional physico-chemical techniques for the remediation of hydrocarbons at a contaminated site. Due to the constraints imposed by environmental variability and substrate however bioremediation technologies have had limited applications such as limited biodegradative potential and viability of naturally occurring microorganisms.

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References:

  • Arezoo Dadrasnia, N. Shahsavari and C. U. U. Emenike :2013 Remediation of Contaminated Sites
  • Hyman M., Dupont R.R., (2001), Groundwater and Soil Remediation. Process Design and Cost Estimating of Proven Technologies, ASCE Press Vidali, Bioremediation. An overview. Pure Appl. Chem. 73 (7), 1163–1172 (2001)
  • Van Deuren, J., Lloyd, T., Chhetry, S., Raycharn, L., Peck, J., (2002) Remediation Technologies Screening Matrix and Reference Guide, Federal Remediation Technologies Roundtable, 4.