Introduction
This chapter presents a general overview of arsenic in soils and the dangers associated with it. It covers the need for the remediation of arsenic contaminated soils; the technologies currently used for arsenic contaminated soils; the general use of amendments, what they are, the various types in use and the amendments for the remediation of arsenic contaminated soils and narrowed down on the use of iron oxyhydroxides for the immobilization of As in contaminated soils.
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Arsenic in soil
Arsenic (As) contamination of soil and water has attracted serious attention in environmental studies (Lee et al., 2011and Huang and Matzner, 2007). This is rightly so because arsenic is carcinogenous (Hartley and Lepp, 2008) and a small amount of this element in the body system is enough to cause serious harm (Warren et al., 2003). The main route of As into the body is ingestion and inhalation (Miretzky and Cirelli, 2010). The wide spread of As in the human environment is attributable to human (mining and agricultural activities) and natural processes (weathering and microbial activities) (Garcia-Sanchez and Alvarez-Ayuso, 2003). It has been estimated that two third of As in the environment come from anthropogenic activities (WHO, 2001).
Arsenic exists in organic and inorganic forms in the soil. The organic species which are dominant in forest soils are not harmful while the inorganic species such as arsenite and arsenate which are known to be very toxic in nature could be found in organic and mineral soils (Huang and Matzner, 2007). The inorganic species are well known threat to the environment and human health (Jain et al., 1999). Arsenite is said to be 25-60 times more toxic than arsenate (Miretzky and Cirelli, 2010). Arsenic toxicity, bioavailability and mobility are closely associated with its redox potential, chemical association and the textural class of the soil as well as the prevailing environmental conditions.
In the soil, arsenic is usually associated with iron, aluminum and manganese hydroxides, clays and mineral oxyanions and organic matter. The goethite and ferrihydrite significantly influence the mobility of arsenic in soil (Miretzky and Cirelli, 2010). This is one reason why oxides of these elements are useful in the immobilization of As in contaminated soils.
The formation of the organic species such as Monomethylarsonic acid (MMA), dimethylarsinic acid (DMA), or trimethylarsine oxide (TMAO) in soil which are associated with microbial methylation are attributed to soil type, soil condition and microbial community (Huang and Matzner, 2007). It can be seen from above that microbial activities contribute to the formation of arsenic species. This explains why microbial activities are being considered as important factor in long term stability of As in any amended soil.
Since As toxicity, bioavailability and mobility are influenced by its redox potential, chemical association, the textural class of the soil and microbial activities, it imperative that any remediation measure that will be effective must take these environmental factors into consideration.
The need for the remediation of Arsenic contaminated sites
Contaminated sites usually have elevated amount of contaminants which are normally above acceptable limits. For instance, the World Health Organization (WHO, 2001) puts the acceptable limit of As in soil at 10 mg/kg. This is value is by far lower than what is obtainable in many old mine sites and agricultural lands. When contaminants are present in high amount, there is the tendency for the mobile species to increase in quantity. This presents a dangerous trend.
The danger associated with mobile inorganic contaminants lies in their bioavailability to plants, micro-organisms and consequently to humans through the food chain. It is important to reduce the amount of mobile As species in the soil. This way, the danger associated with the element and the possibility of leaching into ground water and plant uptake and subsequent transfer to humans could be limited.
Over the years there has been great concern about the need to reduce the amount of contaminants reaching humans through the food chain. A number of remediation measures such as excavation to landfill, soil washing, phytoremediation, solidification, and stabilization/immobilization are being assessed and applied to contaminated sites with a view to reduce the amount of contaminants that leach into the soil and ground water.
Many options are available for the remediation of contaminated sites. The different remediation options come with their own specificity and limitations. To deal effectively with each contaminant, it is important to have a correct understanding of the mechanisms and the factors controlling its reactions in the soil environment. This way, the problems associated with any contaminant can effectively be controlled.
TECHNOLOGIES FOR THE REMEDIATION OF As CONTAMINATED SOILS
The section below presents a brief over view of some of the commonly used technologies for the remediation of arsenic contaminated soils. The technologies covered include: excavation to landfill, soil washing, phytoremediation, solidification and stabilization/immobilization.
Excavation to landfill followed by land filling
This process involves the digging and removal of contaminated soil for disposal in landfill and the soil replaced with clean soils. This procedure is very demanding and expensive and distorts the soil strata.
Soil washing
Soil washing has been applied to remove metals and other contaminants from contaminated soils. It can be used alone or in combination with other procedures such as surfactants, adsorption and precipitation (Torres, 2011). The practice of using acid or alkaline solution for soil washing presents the problem of extreme pH which has implication for soil properties.
Phytoremediation
Phytoremediation is the use of rare plants to clean up soil, sediment and water contaminated with inorganic and organic contaminants (Chaney et al., 1997 and Schnoor, 2002). Prior to the emergence of phytoremediation, efforts at remediating contaminated soils concentrated on incineration, soil excavation, soil washing and land filling. These methods are more expensive and less environmentally friendly (Vangronsveid and Cunningham, 1998 and Brady and Weil, 1999).
As a natural process through which organic and inorganic contaminants are cycled, the success of phytoremediation depends on the ability of the plants to sequester, degrade, immobilize or metabolize the contaminants in situ (Schnoor, 2002). This way, the soil is not distorted as in earlier procedures such as excavation and land filling.
Phytoremediation is applied mainly in soil in the following forms: Phytotransformation, Rhizosphere Biodegradation, Phytostabilization, and phytoextraction; while in water it is applied in the forms of Rhizofiltration, Hydraulic control plume capture, Phytovolatilization, Vegetative caps and Constructed wetlands (Schnoor, 2002).
Phytoextraction technology uses hyper accumulating plants to reduce contaminant concentration. This technology has been applied to solve As related pollution problems. Several plant species such as Canna glauca L., Colocasia esculenta L. Schott, Cyperus papyrus L. and Typha angustifolia L are being assessed and used for the remediation of As contaminated soils (Jomjun et al., 2011).
Past studies reveal that success in phytoremediation is tied to the quality of interactions between plant, soil and contaminants. This plant-soil-contaminant interaction is specific (Chekol et al., 2002). Chaney et al., (1997) noted that soil and plant chemistry are unique and should therefore be given individual consideration in the design and application of each phytoremediation programme. This implies that each soil, plant and contaminant should be treated separately and as unique factors capable of influencing the result of each experiment. This provides a basis for the assessment of each remediation scheme on different soils and with different amendments.
Solidification
This involves the use of inorganic materials such as cement and lime items which have binding properties that can inhibit the movement of As in the soil. This technology limits toxicity or water solubility of the contaminants and it is therefore considered by the US EPA as a potential applicable technology for As remediation Akhter, et al., (1997).
Stabilization/immobilization
Stabilization is a chemical means of reducing the mobility of contaminants through the use of amendments which can adsorb, form complexes or co-precipitate the contaminant. This technology limits the spread of contaminants in the soil by reducing leaching and the bio-availability. Even though the total content is not reduced, the available or labile fraction in the system is reduced by immobilization technique (Kumpiene et al., 2008).
The use of amendments to control the mobility of contaminants in the soil system is environmentally friendly remediation technology capable of limiting the movement of contaminant while keeping the soil strata intact without distortion. The world over, there is a need for a technology that is reliable, safe and sustainable. This accounts for the choice of this procedure in many as well as in this remediation experiment.
WHAT ARE AMENDMENTS?
Amendments are materials used to improve soil fertility, long-term chemical or physical status or the stability of the soil (Park, 2007, EPA, 2007) and for the remediation of contaminated soils (Kumpiene et al., 2008). Amendments are classified depending on the purpose and the nature of the material that is used. Thus we have organic, mineral and pH amendments. Table 1 below shows the different kind of amendments.
Table 1. Types of amendments in use; their uses and description. Adapted from EPA (2007)
Organic Amendments
Description
Uses
Biosolids
Organic solid byproduct produced by municipal wastewater treatment processes
Soil amendment, source of nutrients, liming and sorbent materials.
Farm yard manure
Product of livestock farm
Used as nutrient and organic matter amendments material
Compost
Products that results from aerobically decomposing raw organic materials
reduce pathogens in organic waste, source of nutrient and sorbent material
Digestates
A general category for organic wastes that have been partially treated through anaerobic digestion.
high-organic-matter semi-solid material that can have a relatively high nutrient content
Pulp sludge
Papermill (pulp) sludges. Some may combine other residues such as waste lime, fly ash, or kaolin with their pulp sludges,
use as soil amendments on disturbed lands
Yard/wood wastes
Yard waste (lawn, garden, shrub/tree trimmings, etc.) made available for local reuse
Organic matter amendments or mulching materials.
Ethanol production byproducts
Byproduct from ethanol
Soil amendment
pH Amendment
Lime
Amendments such as CaCO3, CaO, or burnt lime; Ca(OH)2, or hydrated lime; and industrial waste products, such as cement kiln dust and sugar beet precipitated calcium carbonate.
pH-neutralizing amendments
Wood ash
Products obtained after burning wood
Source of k and other micronutrients
Fly ash
coal fly ash which are used as lime sludge
Source of material for soil aggregate stability
Sugar beet
Sugar beet lime, the limestone byproduct of this process, is available wherever sugar is produced or packaged
Lime material, source of organic matter and contribute to the CEC of the soil
Cement kiln dust
byproduct of the cement industry
Highly soluble and reactive lime material
Red mud
Red mud is a highly alkaline byproduct of the aluminum industry
alkaline byproduct of the aluminum industry used on acid drain soil
Lime stabilized biosolids
Product of secondary treatment of biosolids via addition of CaO or other lime (alkaline)-based reactive products.
Lime material and contribute CEC to the soil
Mineral Amendments
Foundry sand
A byproduct of the metal casting industry
Basically used as soil conditioner to improve texture
Steel slag
Steel slag is available locally in moderate quantities.
Combined alkaline soil amendment, sorbent, and micronutrient source.
Dredged material
M#ay contain organic contaminants, including herbicides.
used to modify surface soil texture or, in thicker lifts
Gypsum
Byproducts of P fertilizers, titanium pigment production, and other industries that neutralize sulfuric acid extracts
Gypsum is used to enhance soil aggregation, offset aluminum (Al) toxicity, and ameliorate sodic soil conditions.
Water treatment residue
Alum and other compounds used for treating water
Used in drinking water plants to flocculate or precipitate P, fine clays, silts, and organics. Sludges as soil conditioner to improve texture or as a sorbent for excess P or other contaminants of concern.
Coal combustion products
Product of coal
Used as liming alternatives for ameliorating acidic soil and for metal-sorption
Fertilizers
NPK carriers and micronutrient sources
Source of nutrient and sorbent materials
Why use amendments?
We are surrounded by many abandoned contaminated sites which need to be profitably put back into use. Contaminant immobilizing amendments are vital tools for the restoration of abandoned lands. Mobile and bioavailable contaminants pose serious health risk to animals and humans who may be exposed to contaminated sites and other receptors. Immobilization helps to reduce the mobility and bioavailability of contaminants and thus protect man, other living organisms and the environment from the dangers associated with exposure to potentially toxic elements. Chemical immobilization is a low cost and environmentally friendly technology that can be applied to effectively deal with most of the problems associated with contaminated sites such as mining, landfills, refineries, smelters, plating facilities (EPA, 2007).
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Implications for using amendments
Application of amendment to soil triggers varying results depending on the characteristics of the soil, the nature of the amendment and the contaminant in question. Madrid et al., (2007) reported that the application of municipal solid waste compost over 3 consecutive years resulted in mobilization of Zn, Pb, Cu and Ni in the 0-25 deep soil in the amended plot compared with the control, whereas the metal content of the subsoil was not affected. The content of ZN and Pb were significantly higher than the other elements after the period of the experiment. The difference in metal concentration at the different soil depth was attributed to the sandy nature of the soil and the amount of water added. The above observation shows the specificity associated with different elements and their response to the same kind of treatment. Result for the use of amendment should therefore be specific to soil and the type and the properties of the amendment and the conditions under which it the application was made.
Benson and Othman (1993) assessed the effects of repeated application of municipal solid waste compost in comparison to farmyard manure on the accumulation and distribution of trace metals and found that the application of the two organic amendments significantly increased the organic carbon and nitrogen contents of the soil. It was also found that the municipal solid waste compost significantly increased the Cd, Cu, Pb and Zn in the soil. This is another case of mobilization of metals induced by amendment.
While it is generally accepted that organic material in the form of compost helps in the immobilization of inorganic contaminants, it has also been established that not all organic amendments are suitable for the immobilization of metals in soil. For instance, van Herwijnen et al., (2007) examined the use of two composts derived from green waste and sewage sludge amended with minerals (clinoptilolite or bentonite) which were used for the remediation of metal-contaminated brownfield sites to create a green space. Their results showed that the green waste compost reduced the leaching of Cd and Zn from the soil by up to 48% whereas the composted sewage sludge doubled the leachate concentration of Zn. However, the same soil amended with composted sewage sludge showed an efficient reduction in plant concentrations of Cd, Cu, Pb or Zn by as much as 80%. A possible explanation for this disturbing observation is the difference in the properties of the organic matter and in the amendments being used. Municipal sewage sludge appears to be generally richer in metal content than green waste compost and released the metals upon addition to the soil. Metal availability from compost is associated with compost maturity and level of humification. The humic materials in compost tend to increase during maturation and fix metals, thus decreasing their availability. Achiba et al., (2009) worked on compost and farmyard manure and reported that the yearly application during 5 successive years to a Tunisian calcareous soil improved its fertility, but observed that application of municipal solid waste compost led to a significant increase in the concentration of heavy metals in the top 20 cm layer of the treated soil. These findings which are in agreement with an early observation by Benson and Othman (1993) and van Herwijnen et al. (2007b) suggest the need to separate metal-rich waste from the municipal waste prior to composting.
van Herwijnen et al., (2007b) investigated earlier observations that some composts reduce leaching of metals from contaminated soil while others increase the leaching of soil bound metals from the same soil. They concluded that metal mobility differs between amendments and between metals within an amendment/soil mixture. This observation is an indication that the ability of a compost material to increase or decrease metal leaching is dependent on the metal in question. Very interestingly, Chaney et al., (1997) had earlier observed that effective development of phytoremediation requires that each element being considered for remediation be treated separately because of the specificity of soil and plant chemistry associated with each element.
The above findings can be summed as follows; amendments can be used to immobilize contaminants. Success in the use of amendment to immobilize contaminants depends on intelligent and careful selection and combination of the right amendment for the right soil under a particular environmental condition.
AMENDMENTS FOR IMMOBILIZING As IN SOIL
The various amendments used over time for immobilizing As in the soil includes, silica, phyllosilicates, organic matter, clays, amorphous Al and Mn oxides and iron rich materials. A summary of major reports about the effectiveness of each of them is presented in the section below.
Silica
…………………………………
Phyllosilicates
……………………………………
Organic matter
Organic matter content of the soil or amendment does influence As sorption in the soil. Kumpiene et al., (2008) investigated the effect of organic matter on As mobility and reported that it has varying effects depending on the pH of the soil and the degree of humification of the organic matter. This is line with earlier reports by Maddrid et al., 2007 and Udom et al., 2004 that organic matter enhances immobilization of metals through the formation of stable complexes which have the capacity to decrease the lability and consequently phytotoxicity.
The effectiveness of organic matter on contaminant mobility is influenced by pH and the degree of humification which in turn influences other reactions. For instance, fresh organic matter increases mobility than retention. Closely related to the issue of age or maturity of organic matter is the case of quantity. In an experiment comparing three different soils of varying organic matter contents stored under anaerobic condition, Kumpiene, et al., 2007, found that the soil which had higher amount of organic matter mobilized As while the one that had low organic matter content did not.
These observations all points to the fact that organic matter exerts significant influence on As adsorption and mobility in the soil environment and therefore should not be neglected in any experiment designed to assess the stability of the element in an amended soil.
Clays
Clay minerals can be applied to control As mobility in contaminated soils. García-Sánchez et al., (2002) reported 80 and 50% efficiency respectively for two bentonite and limonite applied at the rate of 10% in soil of pH 4.6.
The main limitation with the use of clay minerals for the remediation of As contaminated soil is that its effectiveness is influenced by the pH of the soil. The lower the pH, the lower the amount of As immobilized.
Amorphous Al and Mn oxides
Attempts have been made to control As mobility in soil with the use of amorphous Al and Mn oxides. It has been demonstrated that Al oxide (García-Sánchez et al., 2002) can contribute to As immobilization in soil. Similarly, reports by Chiu and Hering, 2000 and Mench, et al., 2000, showed that Mn oxides have the capacity to adsorb high amount of As. The main advantage with the use of Mn oxide is its high sorption capacity and the reduction in As toxicity as it is capable of oxidizing As(III) to As(V).
Mn oxide is however easily reduced at high Eh compared to iron oxide. This makes iron oxides better amendment material for As immobilization than Mn oxides when long term stability is the focus. That is one on the reasons for going for iron based materials in this current work.
Iron-rich materials
The use of iron based amendments for As immobilization has attracted enormous attention (Kumpiene, et al., 2008). A major attraction for these materials in stabilization studies is the fact that they provide enough surface area for sorption of cations as well as anions (Cornell and Schwertmann, 2003) and because As immobilization is highly dependent on adsorption and desorption reactions, iron provides excellent opportunity for this.
The effectiveness of iron oxide in reducing As in the soil is associated with its point of zero charge (PZC). Lee et al., 2011, reported that soils amended with iron oxide have their pH modified to a range of 7-10 which enhances adsorption of arsenate. Furthermore, the authors opined that when large amount of calcium (Ca) is present in available form, it facilitates the formation of Ca-As precipitates which have low solubility and low soil pH and therefore could contribute to the immobilization of arsenic in iron amended soil. This is line with the report by Bothe and Brown, (1999) that lime amendment reduced the mobility of As due to the formation of low Ca-As precipitates.
Current effort to use ochre for remediation is in line with the principles of tapping the efficiency of iron based materials for As sorption. Application of ochre to control As mobility is a welcome remediation measures which limits the mobility of the element (Doi et al., 2005) and represents a beneficial use of industrial by-products (Kumpiene et al., 2007). This is the bases for this current study which explores the possibility of using abandoned waste product (ochre) to reduce the available As species in some contaminated soil the UK.
FACTORS INFLUENCING THE PERFORMANCE OF IRON OYHYDROXIDES ON As
The use of iron oxyhydroxide to immobilize arsenic has been recognized a s apotential technology for the remediation of As contaminated soils. It is important to state that the effectiveness of this material depends on some factors. Key among the factors are, soil characteristics, sorption capacity and the prevailing environmental conditions. A brief outline is provided below about the effect of each of the factors.
Soil characteristics
Soil characteristics such as pH, organic matter content and type (Kumpiene, et al., 2008) affect the effectiveness of iron oxide on As immobilization. The role of pH on adsorption reaction is a major one because of its effect on several soils reactions.
Sorption capacity
Sorption is an important characteristic of any sorbent and can be influenced by several factors. OPPTS, (2008) listed “organic carbon, pH, textural class, effective cation exchange capacity (ECEC), surface area, iron and aluminum oxides content” as important factors influencing the properties of any sorbent. Iron oxyhydroxides are product of mineral weathering and are rightly called secondary minerals (Brown, 2005) and present large surface areas for As sorption.
High sorptive property has been reported for Fe-oxyhydroxides (Krawczyk-Bärsch et al., 2004, Harrison and Berkheiser, (1982). This property is attributed to the large surface areas for sorption and complexation (Garci´a- Sanchez et al., 2002 and Krawczyk-Bärsch et al., 2004). Gim´enez et al., (2007) compared the sorption capacity of natural and synthetic materials commonly used for sorption reactions and found that the natural materials have lower values than synthetic materials implying that comparatively, natural materials have less available sites for sorption and should be expected to adsorb less contaminants. This may be attributed to the fact that natural materials are usually associated with some impurities. Among the natural materials there is also difference in demonstrated capacity for sorption of contaminants. In the investigation by Gim´enez et al., (2007) it was concluded that hematite was a better material for As sorption than goethite and magnetite especially under acidic conditions. This difference in sorption capacity can also be related to available surface area for sorption.
Lee et al., (2011) attributed the reduction of arsenic in amended soil to controlled mobility through sorption at the inner sphere complexation on the reactive surfaces of iron oxides. Iron oxide is one of the materials with greatest potential for the reduction of mobile species in the soil (Kumpiene et al., 2008). Jang et al., (2008) reported that adsorption capacity for As was related to the form of iron hydroxide for a given iron content. This capability is related to large surface areas presented by iron oxides for adsorption as different forms of iron oxides possess different surface area for reaction. This is in line with the findings of Gim´enez et al., (2007) that hematite was a better material than goethite and magnetite for As sorption especially under acidic conditions.
Sorption reactions especially with As is highly pH dependent (Gim´enez et al., 2007and Clara and Magalhaes (2002) Bigham et al., (1990) reported that “iron can be precipitated at low pH to yield fertihydrites and ferric oxyhydroxide sulphate”. In less acidic solutions arsenic is not easily adsorbed by Fe oxyhydroxide as a result of precipitation of metal. This explains why Fe-oxyhydroxides demonstrate high sorption capacity for As at low pH values. Controlling the solution pH can influence the extent of oxidation and iron hydrolysis which in turn affects the distribution of As.
These reports all suggest the importance of adsorptive sites and surfaces in adsorption of metals and other contaminants. The extent of adsorption depends on initial concentration of the sorptive media, pH and other competing ions in solution and the concentration of arsenic (Clara and Magalhaes 2002).
Environmental conditions
The prevailing environmental factor acts as modifier for many reactions in the soil system. The role of environmental factors in controlling metal release was studied by Grybos et al., (2007). These authors found that OM release and iron oxyhydroxides reduction controls metal release. Kumpiene, et al., (2007) reported that arsenic remobilization was controlled by liquid/solid ratio, microbial activities and redox potential. Gim´enez et al., (2007) reported that As sorption on goethite, hematite and magnetite decreased with alkaline pH. These all implies that under fluctuating conditions of Eh, pH and available soluble As release can be expected (WHO, 2001). These findings suggest that studies designed to understand ionic stability in ochre amended soils should take into consideration, OM, pH, Eh.
It has been observed that no single study was found in literature that investigated the combined effect of these factors therefore, this present researcher thought it necessary to incorporate all these important factors in this present studies.
MECHANISM OF ION MOBILITY AND IMMOBILIZATION
Different explanations have been advanced for the mobility and immobilization of metals and metalloids with different amendments. According to Achiba et al., (2009) metal mobility is influenced by factors such as the nature of the element, the soil properties (e.g. pH, Eh, clay content, organic matter rate) and the type and quality of soil amendments.
Hodson and Valsami-Jones (2000) used bone meal as an amendment and observed that metal immobilization and soil pH increased with increasing bone meal concentration and decreasing grain size. This observation suggests that metal immobilization could be attributed to pH rise which results in precipitation of insoluble metal phases and also the precipitation of metal phosphate following phosphate release from the dissolving bone meal and adsorption of metal ions onto the bone meal surface.
In a preliminary investigation into the use of ochre as a remedial amendment in arsenic contaminated soils, immobilization was attributed to sorption (Doi et al. 2005). In a related trial by Warren and Alloway, (2003) sorption or co-precipitation was advanced for the immobilization of arsenic. Kumpiene, et al., (2008) attributed immobilization by iron to adsorption by replacement of OH- groups with As ions and the formation of amorphous arsenate.
Contaminant sorption and immobilization may occur through adsorption, formation of stable complexes such as organo-complexes, surface precipitation and ion exchange (Kumpiene et al., 2008). These mechanisms are available as means by which contaminants are immobilized. For instance, immobilization of As occurs through adsorption on Fe oxides by replacing the surface hydroxyl groups with the As ions and by the formation of amorphous Fe(III) arsenates. Common remediation techniques are based on precipitation and adsorption (Clara and Magalhaes, 2002). Doi et al., (2005) showed that ochre removes As from solution by sorption. Cances et al. (2005) reported that the “precipitation of arsenic bearing minerals and complexation with iron oxides surfaces can or do mostly occur and are responsibly for decreases in arsenic mobility and potential bioavailability.
One important deduction that can be made from the issues raised above is that immobilization cannot be attributed to just one mechanism. The contributing factors are adsorption, precipitation/co-precipitation and complexation with the contaminant, the nature of the amendment and the properties of the soil and prevailing environmental conditions as important factors.
POINT OF ZERO CHARGE AND As ADSORPTION
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