8. Stabilization And Solidification x6l5c

HAZARDOUS WASTE MANAGEMENT TECHNOLOGY Stabilization and Solidification (S/S)

Lecturer: Dr Tan Kok Weng Email: [email protected]

Introduction  Stabilization and solidification (S/S) have been widely used in management of hazardous wastes.  These technologies are being applied to: i. Treatment of industrial wastes ii. Treatment of wastes prior to secure landfill disposal iii. Treatment of contaminated land where large quantities of soil containing hazardous substances are encountered.

 Stabilization – process where additives are mixed with waste - Contaminants are fully or partially bound by the addition of ing media, binders or other modifiers  to minimize the rate of contaminant migration from the waste  to reduce the toxicity of the waste.

 Solidification –process employing additives – Liquid or semi liquid to solid form  Objective of S/S would encom both in reduction in waste toxicity and mobility as well as improvement in the engineering properties of the stabilized material.

 Definitions  Stabilization is a process employing additives (reagents) to reduce the hazardous nature of a waste by converting the waste and its hazardous constituents into a form that minimizes the rate of contaminant migration into environment, or reduces to level of toxicity.  The purpose of addition of reagents:  Improve the handling and physical characteristic of the waste  Decrease the surface area across which transfer or loss of contaminants can occur  Limit the solubility of any pollutants contained in the waste  Reduce the toxicity of the contaminants

 Definitions  Solidification is described as a process by which sufficient quantities of solidifying material, including solids, are added to the hazardous materials to result in a solidified mass of material.  Solidifying the mass is accomplished through the addition of reagents e.g Pozzolan - increase the strength, decrease the compressibility and decrease the permeability of the waste.  3 major areas of application for stabilization technologies:  Land Disposal – the stabilization of waste prior to secure landfill disposal  Site Remediation – The remediation of contaminated sites  Solidification of industrial wastes – the solidification of nonhazardous, unstable wastes, such as sludge

 Fundamental physical and chemical mechanisms that control the effectiveness of the stabilization reagent  R&D is important - New reagents are developed or existing reagent are modified and adapted to new and different wastes.

 Successful stabilization employs one or more of the following mechanisms. 1. Macro-encapsulation 2. Micro-encapsulation 3. Absorption 4. Precipitation

1. Macro encapsulation (waste block)  Hazardous waste constituents are physically entrapped in a larger structural matrix  That is, hazardous waste constituents are held in discontinuous pores within the stabilizing materials  The entrapped materials are free to migrate  The stabilized mass may breakdown over time due to imposed environment stresses including     

Repeated cycles of wetting and drying Freezing Thawing Introduction of percolating fluids Physical loading stresses

2. Micro-encapsulation (waste particle)  Hazardous waste constituents are entrapped within the crystalline structure of the solidified matrix at a microscopic level  As a result, even if the stabilized materials degrade into relatively small particle sizes, most of the stabilized hazardous wastes remains entrapped.  Microencapsulation is more effective because it entrapped the hazardous waste within the crystalline structure of the solidified matrix at microscopic level.  As a results, even if the stabilized materials degrade into relatively small particle size, most of the stabilized hazardous wastes remains entrapped.

3. Absorption  Contaminants are taken into the sorbent in very much the same way a sponge takes on water  Adsorption requires the addition of solid material (sorbent) to soak up or absorb the free liquids in the waste  This process is primarily employed to remove free liquid to improve the waste-handling characteristics  Most common absorbent:      

Soil Fly ash Cement kiln dust Lime kiln dust Sawdust Clay minerals including bentonite, kaolinite, vermiculite and zoelite

Organic waste adsorbed to an organophilic clay

4. Precipitation  Precipitation process will precipitate contaminants from waste, resulting in a more stable form of the constituents within the waste  Precipitates such as     

Hydroxides Sulfides Silicates Carbonates phosphates

 Are then contained within the stabilized mass as part of the material structure

 Binder – used to denote a reagent that contributes to the strength gain associated with stabilization • The factors that affect the reagent: – Concentration of the contaminants – Quantity of the reagent – Synergistic effects of multiple contaminants and reagents 1. Portland Cement (Cementitious S/S ) 2. Pozzolan (Cementitious S/S ) 3. Thermosetting Organic polymer (Polymer S/S) 4. Thermoplastic (Polymer S/S)

1. Cement  Portland cement, made by firing a mixture of limestone and clay (or silicate) in a kiln at high temperatures  The kiln produce a clinker, which is ground to a powder that is a mixture of Ca, Si, Al and iron oxides  Waste materials are mixed with cement followed by the addition of waster for hydration (if necessary when water does not presence)  The hydration of the cement forms a crystalline structure, consisting of calcium aluminosilicate  This result in a rock-like, monolithic and hardness mass  Concrete – particulate composite consisting of hydrated cement and aggregate

 Cement-based stabilization is best suited for inorganic wastes, especially those containing heavy metals  Cause the high pH of the cement is able to retain the heavy metal in the form of insoluble hydroxide or carbonate salts within the hardened structure.  Lead, copper, zinc, tin and cium are likely bound in the matrix by chemical fixation, forming insoluble compounds  Mercury is predominantly held by physical microencapsulation

2. Pozzolans  The reaction of aluminosilicious material, lime and water results in the formation of Pozzolanic concrete  Fly ash, ground blast furnace slag and cement kiln dust  The theory of stabilization is similar to cement.  High pH environment is well suited to treat heavy metal.

3. Thermosetting Organic Polymers  Hazardous wastes may be stabilized through an organic polymer process that involves mixing of a monomer , such as urea-formaldehyde, that acts as a catalyst to form a polymeric material  A sponge like mass is thereby formed, trapping solid particles of hazardous waste within matrix  Micro-capsulation

 However, it will leave particularly liquid wastes

some

un-trapped,

• E.g Reactive monomers urea-form-aldehyde, phenolic, polyesters, epoxides, and vinyl, which form a polymerized material when mixed with a catalyst

4. Thermoplastic materials  Hazardous waste may be stabilized by blending molten thermoplastic materials with wastes at high temperatures  When cooled down, the molten will solidified, the waste will thermo-plastically coated and typically containerized (put in drums) for ultimate disposal  The used of thermoplastic has received attention for mixed waste, that is, waste that is both hazardous and radioactive  Type of molten thermoplastic material:  Asphalt, paraffin, bitumen, polyethylene, polypropylene and sulfur

Technologies Traditional in-situ vitrification • The traditional in-situ vitrification process employs an array of electrodes placed vertically into waste or contaminated soil, and an electric current is ed through the soil between the electrodes. • The heat generated from the resistance of the soil to the age of the current is referred to as Joule heating.

• As the heated soil melts progressively downwards, the electrodes are allowed to sink through the melted soil, enabling melting depths of 7 m or more. • An off-gas hood covers the entire melt and some distance around the outside edge to control release of gases and airborne particles generated within or near the melt.

• The off-gases are drawn into the hood by the negative pressure created by a fan, then treated in a process train before being discharged to the atmosphere. • When the melting has progressed to the desired depth, the power to the electrodes is shut off and the melt is allowed to cool. • The electrodes are left in place in the melt and are sawn off at the ground surface. New electrodes are installed at each new melt location. • The final melt is smaller in volume than the original waste and associated soil due to: – Removal of volatile contaminants; – Reduced void space; – Higher density of glass relative to waste materials.

• Each melting produces a single block shaped monolith of glass. Most vitrification projects require multiple, overlapping melts to cover the area and the volume of the contaminated site.

Technologies Planar in-situ vitrification • Like traditional in-situ vitrification, planar in-situ vitrification employs the same Joule heating principle but differs in the application of electric current and in the starter path configuration. • In planar in-situ vitrification, the current travels between pairs of electrodes, causing two parallel planar melts to form. • As the melts grow downwards and spread laterally, they eventually meet in the centre of the electrode array and fuse together into one melt. The final planar melt has the same size and shape as a traditional in-situ vitrificatn melt.

Technologies (In-situ) Plasma arc in-situ vitrification • Latest and less tested technique based on established plasma arc technology. • Bottom-up’ in-situ vitrification process • Electrical energy is applied as direct current between two electrodes within a torch, creating a plasma of highly ionized gases at very high temperatures. • The resistance to the flow of current between the two electrodes generates the plasma. • The operation involves lowering the torch into a pre-drilled borehole of any depth and heating the wastes and soil as the torch is gradually raised.

•In-situ

Plasma Vitrification (Circeo and Martin, 1997)

• The organic fraction of the wastes is pyrolysed and the inorganic fraction is vitrified, thus converting a mass of soil and or waste into a highly stable, leach resistant slag column. • Advantage - gases and vapours escape from the subsurface above the melt zone rather than being trapped beneath it the likelihood of melt expulsions is reduced.

• The in-situ vitrification process can immobilize extremely hazardous materials and radionuclides that may be difficult to treat.

Technology Selection • Waste characteristics (pretreatment is required?) • Process type/processing requirements • Management Objectives and Regulatory requirements • Budget

Sharma (2004)

S/S technology – The advantages: – Low cost because the reagents are widely available and inexpensive – Can be used on a large variety of contaminants – Can be applied to different types of soils – Equipment simple compare to other treatment process – High effectiveness

Sharma (2004)

S/S technology – The disadvantages: • Contaminants are not destroyed or removed • Large volume of the treated waste • Volatile organic compounds and some particulates may come out during treatment process • Delivering reagents deep into the wastes and mixing them evenly is difficult • In situ S/S site may not be redeveloped • Long-term efficiency of S/S is still uncertain Sharma (2004)

References: • LaGrega, M.D., Buckingham, P.L., Evans, J.C. 2001, Hazardous Waste Management, 2nd edi, McGraw-Hill. Additional references • Sharma, H. D., and Lewis, S. P. 1994. Waste Containment Systems, Waste Stabilization, and Landfills: Design and Evaluation. Wiley, New York. • Lizzie Grobbel and Zhijie Wang . A Review of stabilization/Solidification (S/S) Technology for Waste Soil Remediation. [Online source]

Key : Term

Definition

Stabilization

process where additives are mixed with waste to minimize the rate of contaminant migration from the waste and to reduce the toxicity of the waste.

Solidification

process employing additives by which the physical nature of the waste is altered during the process