A REVIEW OF SOLUTIONS TO PREVENT THE TEMPORARY SPREAD AND TREATMENT TECHNOLOGIES FOR AGENT ORANGE/ DIOXIN CONTAMINATED SOILS AT HOTSPOTS IN VIETNAM

ABSTRACT

During Vietnam war, from 1961 to 1671, American armed forces sprayed more than 74 million liters of herbicides including nearly 50 million liters of Agent Orange that contains Tetrachlorodibenzodioxin (2,3,7,8-TCDD) with high toxicity over the south of Vietnam. Although the war ended nearly 40 years ago, the effect of the chemical war is still serious as the toxic chemical residue has seriously polluted the environment and affected the health of the community surrounding the contaminated areas [1]. Being aware of the horrible impact of Agent Orange/Dioxin on the human health and the environment, Vietnamese government has taken many actions, including investigations, collection and treatment of Agent Orange/Dioxin to timely prevent and minimize its impact on the people and the environment. The State of Vietnam and Ministry of Defense have deployed many projects to evaluate the contamination level at hotspots, mainly at Bien Hoa, Da Nang and Phu Cat airbases and proposed solutions to minimize the pollution.

With the effort of the government, Ministry ot Defense and international organizations (UNDP - United Nations Development Programme; USAID - United States Agency for International Development; The Ford Foundation, CZECH international development co-operation, etc...) and from different financial support, until June, 2016, Vietnam has securely isolated more than 150000 cubic meters of the contaminated soil from Bien Hoa and Phu Cat airports in the landfill [4], completely treated more than 40000 cubic meters of the contaminated soil at Da Nang airport by In-Pile Thermal Desorption [5], and temporarily prevented the spread of Orange/Dioxin into the environment effectively for many contaminated areas [6]. Above actions are only initial and temporary solutions to effectively prevent the spread of Agent Orange/Dioxin into the surrounding environment. In the future, all contaminated areas should be completely treated by a suitable technology.

A lot of studies and experiments have been done to select a suitable technology for the complete treatment of Agent Orange/Dioxin, including: isolated landfilling combined with biotechnology, ball milling technology (MCDÔ), developed in 2012 by EDL company of New Zealand, bio-chemical technology (HPC EnvirotecÔ), developed in 2014 by HPC group, thermal desorption technology (MCSÔ), developed in 2013 by Thermodyne Technologies, Thermal desorption combined with using catalysts and soil washing technology (introduced by Japanese companies) and an integrated technology for the complete treatment of Agent Orange/Dioxin in the nation project of KHCN 33/11-15. However, studies and trials are continued to be improved and implemented on large scale on site to be feasible in Vietnam. Therefore, selection of a suitable technology to meet the requirements: complete treatment, application on large scale (over 500 000 cubic meters of the contaminated soil) [6], no secondary contamination, easy operation with reasonable cost in Vietnam\'s conditions is one of urgent issues. In this paper, a review of solutions, which have been implemented and will be developed

in the incoming years to prevent the temporary spread and treatment technologies for agent orange/dioxin contaminated soils at hotspots in Vietnam will be reported.

INTRODUCTION

During Vietnam war, from 1961 to 1671, American armed forces sprayed more than 74 million liters of herbicides including nearly 50 million liters of Agent Orange that contains Tetrachlorodibenzodioxin (2,3,7,8-TCDD) with high toxicity over the south of Vietnam. Although the war ended nearly 40 years ago, the effect of the chemical war is still serious as the toxic chemical residue has seriously polluted the environment and affected the health of the community surrounding the contaminated areas [1]. Being aware of the horrible impact of Agent Orange/Dioxin on the human health and the environment, Vietnamese government has taken many actions, including investigations, collection and treatment of Agent Orange/Dioxin to timely prevent and minimize its impact on the people and the environment. The State of Vietnam and Ministry of Defense have deployed many projects to evaluate the contamination level at hotspots, mainly at Bien Hoa, Da Nang and Phu Cat airbases and proposed solutions to minimize the pollution.

With the effort of the government, Ministry ot Defense and international organizations (UNDP - United Nations Development Programme; USAID - United States Agency for International Development; The Ford Foundation, CZECH international development co-operation, etc...) and from different financial support, until June, 2016, Vietnam has securely isolated more than 150000 cubic meters of the contaminated soil from Bien Hoa and Phu Cat airports in the landfill [4], completely treated more than 40000 cubic meters of the contaminated soil at Da Nang airport by In-Pile Thermal Desorption [5], and temporarily prevented the spread of Orange/Dioxin into the environment effectively for many contaminated areas [6]. Above actions are only initial and temporary solutions to effectively prevent the spread of Agent Orange/Dioxin into the surrounding environment. In the future, all contaminated areas should be completely treated by a suitable technology.

A lot of studies and experiments have been done to select a suitable technology for the complete treatment of Agent Orange/Dioxin, including: isolated landfilling combined with biotechnology, ball milling technology (MCDÔ), developed in 2012 by EDL company of New Zealand, bio-chemical technology (HPC EnvirotecÔ), developed in 2014 by HPC group, thermal desorption technology (MCSÔ), developed in 2013 by Thermodyne Technologies, Thermal desorption combined with using catalysts and soil washing technology (introduced by Japanese companies) and an integrated technology for the complete treatment of Agent Orange/Dioxin in the nation project of KHCN 33/11-15. However, studies and trials are continued to be improved and implemented on large scale on site to be feasible in Vietnam. Therefore, selection of a suitable technology to meet the requirements: complete treatment, application on large scale (over 500 000 cubic meters of the contaminated soil) [6], no secondary contamination, easy operation with reasonable cost in Vietnam\'s conditions is one of urgent issues. In this paper, a review of solutions, which have been implemented and will be developed

in the incoming years to prevent the temporary spread and treatment technologies for agent orange/dioxin contaminated soils at hotspots in Vietnam will be reported. In the table 1, there is a list of studies to prevent the temporary spread and treatment technologies or Agent Orange/Dioxin at hotspots in Vietnam since 1994.

Table 1 List of studies to prevent the temporary spread and treatment technologies of Agent Orange/Dioxin at hotspots in Vietiam since 1994

Besides, a lot of studies and experiments have been done to select a suitable technology for the complete treatment of Agent Orange/Dioxin, including: isolated landfilling combined with biotechnology, ball milling technology (MCDTM), developed in 2012 by EDL company of New Zealand, bio-chemical technology (HPC EnvirotecTM), developed in 2014 by HPC group, thermal desorption technology (MCSTM), developed in 2013 by Thermodyne Technologies, Thermal desorption combined with using catalysts and soil washing technology (introduced by Japanese companies) and an integrated technology for the complete treatment of Agent Orange/Dioxin in the nation project of KHCN 33/11-15.

However, studies and trials are continued to be improved and implemented on large scale on site to be feasible in Vietnam [6). Therefore, selection of a suitable technology to meet the requirements: complete treatment, application on large scale (over 500 000 cubic meters of the contaminated soil) [7], no secondary contamination, easy operation with reasonable cost in Vietnam\'s conditions is one of urgent issues

2. CURRENT STATUS OF AGENT ORANGE/DIOXIN CONTAMINATION IN SOILS AND SEDIMENTS AT HOTSPOTS IN VIETNAM

Based on the data from American ministry of Defense, during Vietnam war, the main storage of herbicides was mainly located at Tan Son Nhat, Bien Hoa and Da Nang airbases. The spraying operation was carried out at Bien Hoa, Da Nang and Phu Cat airbases, therefore, Agent Orange/Dioxin contamination primarily happened at these hotspots.

2.1. Da Nang airbase

During the war, Da Nang airbase is the storage place of herbicides for the Ranch Hand campaign and 52 700 tanks of Agent Orange, 29 000 Agent White and 5000 Agent Green were stored and consumed here. It was also the service place for Pacer Ivy campaign to collect 8200 tanks of Agent Orange [8]. Analysis showed that highest concentration of Agent Orange/Dioxin was found at storage, feeding and washing areas.

The highest concentration at feeding and washing area is 365000 ppt TEQ that is 365 higher than the acceptable level. At storage area, it is about 20600 ppt TEQ [3]. Dioxin spread into underground water with the concentration of 0.86 ppt TEQ.

Currently, USAID is using IPTD (in-pile thermal desorption) technology, developed by Terra Therm to completely remediate the contaminated soil. Based on the plan, 80000 cubic meters of the contaminated soil will be treated by IPTD technology, the remaining volume will be treated by isolated landfill on site.

2.2 Phu Cat airbase

Phu Catairbase served the Ranch Hand campaigntrom 1968 to 1970 with the storedand sprayed volume of herbicides of 17000 tanks of Agent Orange, 9000 of Agent White and 2900 tanks of Agent Green. The contamination level of the storage area at Phu Cat airbase is 11400 ppt TEQ on average and 49500 ppt TEQ as the highest concentration [4]. The whole volume of the contaminated soil at Phu Catiairbase (over 7500 cubic meters) was treated by landfill under the project, funded by UNDP and GEF (global environment facility) in 2012.

2.3 Bien Hoa airport

Bien Hoa airport is the main base for Ranch Hand campaign düring Vietnam war with the stored and sprayed volume of 98000 tanks of Agent Orange, 45000 tanks of Agent white and 16000 tank Green [5]. Analysis showed that the concentration of Dioxin is 35865 ppt TEQ on average, 409818 ppt TEQ as high level and 11.10⁶-1.5.10⁶ ppt TEQ as exceptionally high level. Some areas at Bien Hoa airbase such as the western south runway, etc had retatively high concentration of Agent Orange/Dioxin with the estimated volume of contaminated soil of nearly 500000 cubic meters [7].

From 2005 untit now, Chemical division of people\'s army of Vietnam has treated over 150000 cubic meters of the highly contaminated soil by landfilling, including 3500 cubic meters by active landfill (isolated filing, combined with biotechnology). The remain area at Bien Hoa airbase was only separated and taken actions to prevent the spread of contaminants into the surrounding areas, as well as installed warning signs of contamination to community.

3. SOLUTIONS TO PREVENT THE TEMPORARY SPREAD AND TREATMENT TECHNOLOGIES FOR AGENT ORAN DIOXIN CONTAMINATED SOILS AT HOTSPOTS IN VIETNAM

3.1 Solutions to prevent the temporary spread of Agent Orange/Dioxin

Due to the severe effect of Agent Orange/Dioxin on human health and the environment, it is indispensable to isolated contaminated areas and take actions to minimize the pollution, then, to step by step collect and completely treat the contamination sources. While in Vietnam, a complete remediation technology has not been found, yet and there are not enough financial sources for the complete remediation, solutions and actions to prevent the spread of contaminants and minimize the impact of Agent Orange/Dioxin on the environment and community should be applied. There are the following solutions:

-Cement the surface of the highly contaminated area.

- Choose the suitable group of plants for the contaminated area.

- With the contaminated area of huge size and complex terrain, it is necessary to build isolating canals and sedimentation/fiter system.

Some projects to prevent the spread of Agent Drange/Dioxin into the environment at Bien Hoa airbase (a) and Da Nang airbase (b) are shown on the figure 1:

Figure 1: Projects to prevent the spread of Agent Orange/Dioxin into the environment at Bien Hoa airbase (a), Da Nang airbase (b) and the western south of the runway at Bien Hoa airbase

3.2 Treatment technoiogies for Agent Orange/Dioxin contaminated soils at hotspots in Vietnam

3.2.1 Isolated landfill

Isolated landfill has been widely used for domestic and hazardous wastes in almost all countries in the world [13]. This method was applied in Italia after Seveso disaster in 1976 and in Taiwan from 2003 to 2008 to treat contaminated soils.

In Vietnam, from 2005 until now, more than 150000 cubic meters of Agent Orange/Dioxin contaminated soils have been treated by the isolated landfill at Bien Hoa and Phu Cat airbases. HDPE film, bentonite and Enviromat filtration material are mainly materials for the isolated landfill

1. Scientific background of landfill

Landfill is based on the anti-permeability and isolating capability of materials, including: reinforcement, geoengineering fabric, HDPE film with high anti-permeability (~5.10^-11, m/s) and adsorption materials (Environmat, Bentofix, Betomat etc...) with high swelling capacity and degradability of toxic chemicals. The contaminated soil could be kept from 50 to 100 years, allowing Agent Orange/Dioxin to spontaneously decompose with the half-time.

1. Technical requirements for the landfill

The land fill site was chosen to meet Vietnam\'s construction standards for the landfill of solid wastes (TCXDVN 261:2001). The landfill has the structures as follows:

+ Reinforcement layer: it is the mixture of soild and sand with the addition of 4% to 10% of bentonite. Its thickness is from 25 to 40 centimeters to replace the clay layer, based on the Decision 60/QĐ-BKHCNMT.

+ Geoengineering fabric layer: it is to improve shear strength and reduce washing and corrosion.

+ Isolating film: it is made from HDPE with the thickness of 2 milimeters with high durability, anti-permeability (~5.10^-11, m/s), physical properties and chemical resistance.

+ Adsorption materials: they are named as Environmat, Bentofix or Betomat with alkaline property with adsorption capability and degradability of some toxic chemicals. They have high swelling to seal other structures to prevent the leak of pollutants.

+ Water adsorption layer: it is made from sands and pebbles with reasonable size.

+ Cover layer: It is on the surface of the landfill and it is made from the clean soil with the thickness from 0.5 to 1 meters to grow plants.

The order of layers is displayed on the Figure 2:

Figure 2: The structure of the landfill and construction work in progress at Bien Hoa airbase

The landfill does not require an advanced level of the technolagy. It is also easy to deploy with low cost (30 to 50 USD per cubic meter), and feasibie with the huge quantity of contaminated soils and sediments. However, it is only the isolation from the erivironment, not the complete remediation.

3.2 2 Thermal desorption technology

Thermal desorption technology utilize high temperature to desorp contaminants from soils and sediments. In 2012, in Vietnam, USAID coordinated with Ministry of Defense to carry out the project to remediate 76000 cubic meters of Agent Orange/Dioxin contaminated soil by In-pile thermal desorption (IPTD, developed by Terra Therm-USA) at Da Nang airport. IPTD is an advanced technology that allows to treat Agent Orange/Dioxin in the contaminatere with high efficiency.

However, at the moment, it is still under the phase 2 at Da Nang airbase, therefore, a general report on the economic effectiveness for the huge volume of the contaminated soil with high concentration of contaminants has not been concluded.

1. Scientific background of IPTD

Thermal desorption technology includes two steps:

+ Supplying heat to volatilize contaminants from soils and sediments.

+ Collecting contaminants in the vapour phase to treat in the following process.

1. Technical requirement of IPTD

In In-pile thermal desorption, the contaminated soil is excavated and collected into one ore more piles. The desorption and hydrolysis of contaminants are driven by the heat transferred from heating wells in many days. The volatilization of contaminants depends the temperature and technology models.

In IPTD, soils between heat wells are heated to 335°C. Then, contaminants will be volatilized and/or decomposed by some mechanisms: evaporation, steam distillation, vaporization, oxidation and thermal decomposition (without oxygen). Water and Agent Orange are volatilized, then, collected into vacuum heat wells, then, moved through t cooling system, condensated and discharged through an emission treatment system.

In the project at Da Nang airbase, the pile size is 105 meters long, 70 meters wide and 6 meters deep. There are 8 modules with the total volume of 44100 cubic meters and heated by 1254 heating electrodes. The heating process is started by the thermal flow, from low electrical current, then gradually increased to the maximum current within 3 to 7 days. When the pile gets heated, the soil becomes dry and water will evaporate. Vapor will be colected then, it and condensated water will be treated by a suitable technology to meet environment standards before discharged into the environment.

IPTD has high decomposition efficiency, from 95 to 99.8%. It is the complete remediation and feasible to app for different persistent organic pollutants at the same time. However, the treatment cost is high (estimated from 840 to 1000 US dollars for one cubic meter) and it still releases a considerable volume of products, containing Agent Orange/Dioxin.

Recently, some researchers have proposed to apply thermal desorption technology in the presence of catalysts. The oxide form of transitional metals play a considerable role in the complete decomposition of Dioxin/Furan compounds in the volatile state as transitional metals own the incomplete electron sub-shell d¹S² ÷ d¹⁰s¹, making them having the outmost shell as s²p⁴, being rich in electron density, to form crystalline forms with different level of energy.

Transitional metals normally exist as p-semiconductor, then, forming holes in the valence band are positively charged and attact negativeiy charged substances. During the treatment process, the molecular structure of contaminants is destroyed and converted to CO₂ and H₂0. Some metal oxides such as of Mn, Fe, NI, Mo, Co, V and Cu at high density of electrons have stronger catalysis than noble metals [10,13,91]. Iron Oxide Fe₃O₄, with the cubic crystalline structure, black colour, metallic light, brittleness, high semiconductivity, existing as manhetit ore is the most attentive one.

Authors Minghui and Shijing reported that nano materials like Fe³O⁴ ; α-Fe²O³/Fe³O⁴, composite and α-Fe²O³ decomposed hexachlorobenzene (HCB) at temperature of 300°C. The addition of Calcium oxide will aiso increase the stability and effectiveness of the catalyst system.

3.2.2 Ball milling technology or mechanochemical decomposition- MCDÔ

a. Scientific background of MCDÔ technology

Dehalogenation process is carried out by milling the contaminated soil with chemicals in the milling device. Energy that releases from the collision between soil particles and balls will initiate the decomposition reactions of chlorinated organic compounds to chlorinated inorganic compounds.

The treatment mechanism of MCDÔ technology is displayed in the Figure 4.

This method is still in trial process with the efficiency from 20 to 99%. It has been applied in some countries like United States of America, Japan and New Zealand.

Figure 4: The decomposition mechanism and trials of MCDÔ for the complete remediation of Agent Orange/Dioxin at Bien Hoa airbase in 2012

1. Technical requirement

The system includes:

1. Vibrating sieve to screen soil particies less than 10 milimeters. The aieve is located on the feeding hopper to eliminate particles over 10 milimeters. Only particles less than 10 milimeters will fall into the hopper.

2. Grinding equipment to break particles over 10 milimeters into ones less than 10 milimeters.

3. Indirect drying machine: the contaminated soil is fed into the drum roiling dryer. The effectiveness of MCDÔ depends on the humidity of the contaminated soil (less than 2%), The contaminated soil is heated at temperature in the range from 95 to 98°C. The dried contaminated soil is fed into MCDÔ reactors.

4. MCDÔ reactors: reactors are instailed in series. The dried soil is fed into the reactors based at a specific rate, based on the gravity. The treatment efficiency depends on rolling speed, ratio of balls, and residual time in reactors.

5. Mixers and humidifiers: The treated soil, discharged from MCDÔ reactors will fall into mixers. After being mixed, it will be discharged from mixers and the treatment process finishes.

6. Environment treatment equipment: the emission goes through the filter, made from temperature resistance fabrics (100°C). Moisture and suspended particles go through the activated carbon filter. The final discharged air does not pollute the surrounding environment, with the noise less than 65 dB in the distance of less than 5 meters, and no unpleasand ordour.

The pilot process of MCDÔ was not classified on a single organic or inorganic substance. With low concentration of contaminants, the efficiency is high. It is feasible to adjust the quantity of balls to achieve the desired efficiency. It is also easy to arrange electricity, water, labour, excavation equipment, transportation and monitoring. However the cost is about 380 USD per one cubic meter of the contaminated soil. It is also difficult to adjust the humidity less than 2% in reality. There are also safety risks related to dust, emission, etc.

3.2.3 Chemical technology

The chemical method is applied an trial to remediate Agent Orange/Dioxin contaminated soi. However, due to the high cost and secondary contamination, this method is only used for the small volume of the contaminated soil, which has been concentrated by adsorbents and absorbents. The common chemical methods include:

+ Fenton\'s reactions and Fenton/UV.

+ Photocatalytic reactions.

+ Decomposition by nano Fe⁰ (nZVI).

Research shows that in case of not being affected by other factors, the effectiveness of nano Fe⁰ on RX compounds is rapid and it is able to decompose many types of RX compounds. This is the main basis for studying and applying Fe⁰ in the remediation of Agent Orange/Dioxin contaminated soil. The estimated cost is from 450 to 700 US dolla per one ton of the contaminated soil [10]. The treated soil is secondarily polluted by the chemical materials and causes environmental risks.

3.2.4 Soil washing technology.

Soil washing is a chemical and physical process to remove contaminants from the soil and sediment. The volume of contaminants is reduced significantly and treated in the following process. Soil washing has been applied in many projects with positive results, including Santa Maria, CA with the efficiency of 99% for organic compounds in the soil (oil, fuel, PCBs, PAHs) with the capacity of 30 to 65 tons per hour (EPA-May, 1992), BioTrol in New Brighton Minnesota with the efficiency of 91 to 94% tor the pentachlorophenol contaminated soil. Recently, SHIMIZU has introduced the washing technology, widely applied in Japan with the cost of 180 US dollars per ton [9] and feasible to be applied in Vietnam.

1. Scientific background of the soil washing .

The concentration of Agent Orange/Dioxin is reduced by three mechanisms: separation by particie sizes, phase transfer and removal by chemical and physicochemical methods.

The washing solution might be water, acidic or basic solution, surfactant solution, dissolving solvent or complex solution.

Surfactants with Agent Orange/Dioxin dissolving capacity, low adsorption on the soil and high biodegradability are normally selected for the washing technology. Agent Orange/Dioxin are hydrophobic compounds. At the beginning, a small amount of surfactants are attached on Orange/Dioxin molecules. With the growth of pulling force from hydrophilic heads, Orange/Dioxin molecules are gradually moved into the solution. Then, the contact area increases. The process takes place continuously and leads to the increase of the number of surfactants molecules, which are attached on Orange/Dioxin. Finally, the pulling force of hydrophilic heads is strong enough to separate Orange/Dioxin from the soil surface to form micelles with Orange/Dioxin as their core.

1. Technical requirements of the soil washing

The washed soil, separated from the particle-size screening equipment, it is washed again by the fresh surfactant solution, then, separated from the treatment system as the clean product. The solution after washing having fine soil particles with small sizes but much higher concentrations of contaminants. These fine soil particles need treated further by a suitable technology. The used solution, containing almost all quantity of contaminants will be treated on the line by normal effluent treatment system, then, recycled for the washing process. The soil washing is feasible for the contaminated soil with sands, accounting tor 60 to 70% of its composition and the volume of over 5000 cubic meters.

Figure 5: The soil washing line for the remediation of persistant organic compounds, built by Shimizu in Japan from 2002 to 2012.

The soil washing have some advantages, including its feasibility on the large scale, removal of contaminants with complex composition such as organic compounds, heavy metals, radiative agents, etc, without releasing pollutants into the environments. It is also possible to select solutions to remove completely contaminants and recyle the solution. The disadvantage is high volume of solution per one cubic meters of the contaminated soil (8-10 cubic meters of the solution per one cubic meter of the contaminated soil). The cost is about from 70 to 80 US dollars for one ton of the contaminated soil [109], depending on the capacity.

3.2.5 The integrated technology for the complete remediation of Agent Orange/Dioxin Công nghệ tích hợp xứ lý dất nhiễm dacam/dioxin

a. Scientific background of the integrated technology

Based on the implementation of remediation of the contaminated soil, it is difficult to find a single method to remediate Agent Orange/Dioxin contaminated soil completely with the low cost. Therefore, it is necessary to combine proven methods into an integrated technology for the complete treatment of Agent Orange/Dioxin to meet required targets.

Figure 6: Trend to combine proven technologies to remediate the contaminated soil in Superfund projects or US-EPA from 2009 to 2011

b.Technical requirements of the integrated technology

In the program KHCN-33.02 of Ministry of sciences and technolagies, Chemical division of People\'s army of vietnam has studied and developed a feasible integrated technology that combines four proven treatment methods:

• Soil washing, using surtactants
• Chemical and physicochemical treatment
• Thermal desorption in the presence of catalysts
• Sotiditication and stabilizing

Intergrated technology = Soil washing + Oxidation-Reduction

Dioxin is easy to combine irreversibly with organic compounds in the soll, especially humic acid, which is the main humus component of silt and clay. Research on the pedological composition of the contaminated soil show that course, medium and small sands with diameter larger than 0.067 mm accounts for up to 74% while silt and clay with diameter less than 0.067 mm accounts for only 26%. However, 89.31% of the total quantity of 2,3,7,8-TCD present in silt and clay.

Figure 7: A feasiole integrated technology for the complete remediation of Agent Orange/Diaxin contaminated soil in Vietnam\'s conditions

The distribution of Agent Orange/Dioxin is mainly in silt and clay, therefore, the first step of the integrated technology is to wash the contaminated soil by the surfactant solution and separate soil particles based on the sizes on the screening equipment.

After the washing process, it is possible to separate 70 to 75% of the volume (as course, medium and fine sand with the low concentration of Agent Orange/Dioxin (less than 1000 ppt TEQ), meeting Vietnam\'s standard QCVN 45:2012/BTNMT and no need for futher treatment. Agent Orange/Dioxin in the solution and silt/clay (diameter less than 0.067 milimeters) will be further treated by chemical agents and activated carbon to increase the treatment efficiency.

Activated carbon after adsorption and silt/clay with high concentration of Agent Orange/Dioxin will be collected by the floatation method, then, further treated nano Fe₃O₄/CaO. The treatment efficiency reaches 99.32% with the estimated cost of 150 to 170 US dollars.

4. CRITERIA FOR THE SELECTION OF THE INTEGRATED TECHNOLOGY FOR THE COMPLETE REMEDIATION OF AGENT ORANGE/DIOXIN CONTAMINATED SOIL IN VIETNAM

The criteria for selecting treatment technologies for persistent organic pollutants (POPs) have been proposed by United nations industrial development organization (UNIDO) and International center for advanced sciences and technologies. They should include: proven technologies, treatment cost, concentration limit after treatment remediation time, reliability and safety, development trend and other effects. Besides, political, legal and financial factors should be also considered. About political factor, it is about the full agreement of governmental organizations, the readiness of the community and other parties to accept the technologies and equipment. The legal system should be strong enough to ensure the satety for the community and environmental protection. Finance is aiso critical for the success of the treatment project. For the remediation of the contaminated soil, due to the huge volume, complex pedological composition, the selection of the technolooy should he cost-minimized and comply to current standards in term of concentration limits of contaminants.