How To Clean Up Arsenic

In areas where drinking water contains unsafe amounts of arsenic, the immediate concern is finding safe water. We have two main options: Find a new safe source or remove arsenic from the pollution source. If safe water cannot be found, the short-term goal should be to reduce arsenic levels. There are several methods available to remove arsenic from water, this article will introduce several methods: Oxidation; Coagulation, precipitation and filtration; Adsorption (sorptive filtration); Ion exchange; Membrane engineering.

Arsenic origin and distribution in nature

Arsenic in nature can exist in the components of the soil, water, air, biological environment ... and is closely related to geological, geochemical and biochemical processes. These processes will cause the primary arsenic present in some geological formations (stratigraphic units, manganese, hydrothermal changes and arsenic sulfide ores) to continue to disperse or concentrate, causing pollution. habitat.

In the world, many countries have researched and determined the arsenic content in rocks and ores, in weathered soil and crust, in water, in the air ... In Vietnam, some scientists of Geology and Hydrology have studied on arsenic existence in rock, ore, weathered soil and crust as well as in loose sediments, seawater, rainwater, underground water ... Thanks to modern tools and analytical methods as necessary. Being activated by neutrons and atomic absorption spectroscopy, in about 10 years, Vietnamese scientists have analyzed arsenic content in natural environment components.

Arsenic pollution mechanism and existence of arsenic in water

Arsenic is released into water by oxidation of sulfide minerals or reduction of arsenic-rich hydroxide minerals. So far, there have been many different assumptions about the mechanism of heavy metal contamination, including arsenic, but not yet agreed.

Through geochemical and biochemical processes, hydrogeological conditions that arsenic can enter the aquatic environment.

Arsenic content in groundwater depends on the nature and state of the geochemical environment. Arsenic exists in groundwater in the form of H3AsO4-1 (in acidic to near neutral pH environment), HAsO4-2 (in alkaline environment). H3AsO3 compounds are formed mainly in weak oxidation-reduction media. Arsenic compounds with Na are very soluble. Arsenic salts with Ca, Mg and organic arsenic compounds in the near-neutral pH environment, poor Ca, are less soluble than organic compounds, especially arsenic-fulvic acid is very stable. the tendency to increase with the ratio of arsenic to fulvic acid. As5 + compounds are formed in this manner.

As we all know, arsenic is a trace element, essential for the growth and development of humans and organisms. Arsenic plays a role in nuclein metabolism, protit synthesis and hemoglobin. The role of arsenic and the origin of arsenic pollution due to development activities

Arsenic is an element present in many chemicals used in many different industries such as: chemicals, fertilizers (phosphates - phosphates, nitrogen - nitrogen), pesticides, paper, textiles ...
Many industries use fossil fuels such as cement industry, thermal power, ... Solid waste combustion technology is also a source of air and water pollution by arsenic.

Mining and processing industries of ores, especially sulphide ores, and metallurgy, create arsenic pollution sources. Mining in primary mines has exposed sulfide ores, increasing weathering, erosion and creating a large amount of arsenic-contaminated waste rock in the vicinity of the quarry. In ore sorting plants, arsenopyrite is separated from beneficial minerals and exposed to air. Arsenite is washed away by driftwood, resulting in a large amount of arsenic being introduced into the surrounding environment.

The free miners add sulfuric acid, petroleum, and detergent. Arsenopyrite, after being separated from the ore, will become waste and are piled up outdoors and drifted into rivers and streams, causing widespread pollution.
These are arsenic emission sources that pollute water, soil and air.

Impacts of Arsenic on human and biological health

Arsenic is a very toxic substance that can cause 19 different diseases, including terminal diseases such as skin and lung cancer.

Arsenic affects plants as a substance that inhibits metabolism, reducing crop yield.
Arsenic poisoning is called arsenicosis. It is an environmental disaster for human health. The manifestations of arsenic poisoning are darkening of the skin (melanosis), thickening the epidermis (kerarosis), which in turn leads to gangrene or skin cancer, tooth inflammation, and joints ... There is currently no effective method in the world. cure arsenic poisoning.

 

Regarding the determination and evaluation of arsenic's effects on the body, in recent years, there have been studies of hair and blood analysis to determine the arsenic content. Analysis of arsenic content in hair showed similarity between areas contaminated with groundwater by arsenic. Analytical data in Thuong Cat (control point with water not contaminated with arsenic, As <10 µg / l) and Van Phuc, Son Dong (study site with high arsenic-contaminated water, As> 50 µg / l) showed: Arsenic value in human hair in the control sample was only 0.27 mg / kg (in the range of 0.04 to 0.84 mg / kg), while in study sample contaminated with arsenic was 0 , 79 mg / kg (0.01 - 3.3mg / kg) and 1.61 mg / kg (0.16 -10.36mg / kg).

In Son Dong, 70% of samples had arsenic concentration in hair greater than 1 mg / kg, some samples up to 10 mg / kg. This result is comparable to the study in West Bengan India, where there is a heavy arsenic contamination with arsenic content in people's hair about 3-10mg / kg. The WHO standard value is 0.02-0.2 mg / kg. The study results in Ha Nam in 2004 also showed that in Sin Hu and Bang Mon, located in the tropical changing zone with high arsenic content, there were signs of chronic poisoning, which increased significantly with some diseases such as malaria. digestive, psychiatric, osteoarthritis, cardiovascular disease, pneumonia.

How to clean up arsenic pollution in domestic water

When researching and proposing treatment technology, arsenic removal must firstly base on its existence state, its level or concentration in water, local factors and conditions ...

It should be emphasized that the permitted content of arsenic in drinking water according to TC 505 / BYT in 1992 was 0.05 mg / l or 50 µg / l, but due to the high toxicity of arsenic, it was regulated according to TC 1329 / BYT. in 2002 it was 0.01 mg / l or 10 µg / l ie as defined by the World Health Organization (WHO), USA (US EPA), European Community (EU). Here are some technologies to treat arsenic pollution water in countries and Vietnam.

Technologies for removing arsenic from water, mostly underground water, are classified into the following main methods:

Flocculation, precipitate - Clear or Add flocculation - precipitate - settle; Oxidation; Use sunlight or photochemical oxidation; Adsorption; Ion exchange; Filter through the filter material layer, Filter the membrane; Biological methods and plants; Use a combination of the above methods.

Flocculation - Precipitation

Co-precipitation - sedimentation - filtration simultaneously with the treatment of iron and / or manganese is available in natural groundwater. This is the simplest treatment method, by pumping groundwater from a well, then ventilating to oxidize iron, manganese, creating iron hydroxyides and precipitated manganese. Arsenic (III) is oxidized at the same time to As (V), which is able to adsorb onto the surface of the Iron or Manganese hydroxy floc that form and settle to the bottom of the tank, or adsorb and be trapped on the surface. grains of sand in the filter tank. Research by Center for Investment and Industrial Park (CEETIA), University of Civil Engineering and Center for Environment and Sustainable Development (CETASD), University of Science in 2000 - 2002 showed that modern technology is available at water plants in Hanoi, mainly to Treatment of iron and manganese, allowing the removal of 50-80% of arsenic present in deep-circuit groundwater in Hanoi area. Recent research by CETASD and the Swiss Federal Institute of Environmental Technology shows that for households using a single well, where there is a high iron content in groundwater, a pattern of aeration of groundwater by spraying rain on the surface of sand filters (slow filtration), common in households today, allows the removal of up to 80% arsenic in groundwater along with the removal of iron and manganese. These studies have also shown that arsenic content in water after treatment by the above method depends heavily on the composition of other compounds in the source water and in most cases, it is not allowed to reach lower arsenic concentrations. standard, so it is necessary to continue to process by other methods.

Chemical flocculation

The simplest flocculation method is to use burnt lime (CaO) or slaked lime (Ca (OH) 2) to remove arsenic. Efficiency is about 40-70%. The high efficiency of lime flocculation with pH above 10.5 allows to achieve high arsenic reduction efficiency, with initial arsenic concentration of about 50 µg / l. Can be used for arsenic removal in combination with water softener. However, this method is difficult to allow arsenic concentration in treated water down to 10 mg / l. One limitation of the lime method is the creation of a large amount of residue after treatment.
Also can use flocculation method, precipitation with aluminum sulfate or iron chloride.
Oxidation

Oxidizing by strong oxidizing agents: The oxidizing agents are allowed to use in water supply such as chlorine, KMnO4, H2O2, ozone.

Electrochemical oxidation: Arsenic containing water can be treated by alloying electrodes and applied to small-scale water users.

Photochemical oxidation: A group of Australian scientists have invented the technology to remove arsenite (As (III)) and other solutes such as Iron, Phosphorus, Sulfur, ... from water by removing oxidants and substances. photochemical adsorption: (shining ultraviolet rays into water and then settling). Oxidants can be either pure oxygen or aerated.

Photochemical adsorbents can be Fe (II), Fe (III), Ca (II). Sunlight can be used as a source of ultraviolet rays. The reaction can take place at room temperature and low light, without requiring complicated equipment. Since As (III) is oxidized to As (V) at a very slow rate, strong oxidizing agents such as Cl2, H2O2 or O3 can be used. Most of the treatment cost is these oxidants.

Adsorption

Activated aluminum adsorption: Activated aluminum is effectively used to treat water with a high content of dissolved solids. However, if there are high levels of compounds of Selenium, Fluoride, Chloride, and Sunffate in water, they can compete for adsorption. Activated aluminum is highly selective for As (V), so each treatment can reduce its adsorption capacity by 5 - 10%. It is necessary to revert and replace the filter material when used.

Adsorption filter column with activated aluminum for hand pump drilled wells designed by Indian scientists. Experts have chosen Activated Aluminum as the adsorbent, based on their selected properties and high adsorption capacity for arsenic, its reconstitutionability, availability of supplies, and bypassing the chemical requirements. .

This method is relatively convenient, especially for poor rural areas. Just pour the well water to be treated through the filter material layer. The working time of the equipment depends on water quality and iron content in source water. The higher the iron content in the source water, the higher the arsenic reduction efficiency and the higher the duty cycle before reconstitution.

Activated aluminum oxide adsorption: Project Earth Industries (PEI Inc.) has created an inexpensive, aluminum-derived adsorbent capable of separating arsenic in two popular forms. in the country are As (III) and As (V). This adsorbent has high chemical properties, surface area and porosity, and has 10 times higher adsorption capacity than conventional materials in the presence of competing ions.

Adsorption intensity is fast, allowing to achieve high efficiency, the amount of arsenic after treatment reaches below the limit found by the analytical equipment in the laboratory. This material has also been tested for non-toxicity according to the standards of the US Environmental Protection Agency and has been tested in India and Bangladesh (1998, 1999). PEI's arsenic removal equipment was installed in Lalpur, Chakdah, West Bengal. With a capacity of 1,000 l / day, the experimental equipment allows to reduce arsenic from an initial average value of 340 ppb to below 50 ppb (Bangladesh drinking water standard).

Laterite: Laterite is an acidic soil with red color, very popular in tropical regions. The main components of Laterite are Iron and Aluminum Hydroxides, or their hydrated oxides, and small amounts of Manganese and Titanium compounds. Under natural conditions, this clay has a positive surface charge, and is able to adsorb impurities with negative charges such as arsenic. Laterite can be put directly into the water to be treated as an adsorbent, then allowed to settle, or it can be used as adsorbent in filter tanks. In India, people have conducted experimental studies to treat arsenic with high concentrations in groundwater with laterite at the rate of 5 g laterite / 100 ml of water. Processing efficiency reaches 50 - 90%. Efficiency can be higher when the laterite treatment with 0.01 M HNO3 solution.

In addition, there are many other adsorbents that have been studied and applied to remove arsenic in groundwater.

Use chlorine iron tablets: When you put iron tablets into water, chlorine acts as an oxidizing agent, converting As (III) into As (V). Then As (V) will be adsorbed onto the formed Iron hydroxide cotton. Then stir, to settle and then decant the water or filter through a filter tube. Arsenic sediment is discharged to the landfill. The arsenic here converts to the volatile AsH3 and diffuses into the air.

Compare the arsenic reduction efficiency by flocculation device - deposition (Jar Test) with 3 different types of flocculation alum: FeCl3, FeSO4, Al2 (SO4) 3. The results showed that FeCl3 allows to achieve the highest arsenic reduction efficiency: more than 90%.

Using iron filings combined with sand: This technology is made by experts from Connecticut University, USA. People use filter column with adsorbent material with iron mixed with quartz sand. Groundwater is mixed with barium sulfate and filtered through filter column. Iron filings are 0 valence iron ions, which reduce inorganic arsenic to precipitate with iron, precipitate mixture, or combine with sulfate to create arsenic pyrite. This method can be used to install a separate water treatment facility, or to install it as a part of a well water treatment facility. Arsenic in water after treatment reaches below 27 mg / l.

Iron hydroxyt: Granular iron hydroxide is used in the adsorption column. This technology combines the advantages of the flocculation-filtration method, which has high processing efficiency and low residue generation, with the activated aluminum method, with the advantages of simplicity. Iron hydroxide particles are produced from FeCl3 solution by reacting with NaOH solution. The resulting precipitate is washed, water separated by centrifugation and granulation under high pressure. This material has a high adsorption capacity. Arsenic concentration in pre-treatment water is 100-180 mg / l, and <10 mg / l after treatment.

Combination of oxidation, adsorption - filtration with planting or oxidation with sand filtration and planting. Some plants such as water bamboo (Cyperus Alternifolius or Thalia dealbata) or the sweet potato Colocasia Esculenta are also effective in removing arsenic from water.

Ion exchange

This is the process of exchanging ions in the solid and liquid phases, without changing the structure of the solid. Arsenate (As (V)) ions can be removed from water by ion exchange with strong acid (Cl-) based anion exchange material. This ion exchange material has the advantage of being able to use concentrated salt solution NaCl to reconstitute the saturated ion exchange particles. Arsenic concentrations after treatment can be lowered to less than 2 ppb. However, the ion exchange technology is relatively complex, less likely to apply for each individual household.

Filter technology by water filtration system

Filtering technology through the sand filter material layer: Arsenic is removed from the water in the sand filter tank by co-precipitation with Fe (III) on the surface of the sand particles and the space between the pores in the sand layer. Fe (II), in a water-soluble form, will be oxidized by the oxygen of the air to form Fe (III). Fe (III) hydroxide will be adsorbed on the surface of sand grains and form a thin adsorption layer. Arsenic (V) and arsenic (III) in water will adsorb to that Fe (OH) 3 layer and be trapped in the filter material layer. As a result, water out of the filter tank was released from iron and arsenic.

Membrane filtration technology: Use semi-permeable membranes, which allow only water and some dissolved substances to pass through, to purify the water. Membrane filtration technology makes it possible to separate any type of dissolved solids from water, including arsenic. However, this method is often very expensive and therefore is often used in necessary cases, where it is difficult to apply other methods such as desalination, removal of some ions such as arsenic ... There are many types of membranes. Used as microfiltration, reverse osmosis, electrolysis, ultrafiltration and nanofiltration.

The efficiency and cost of the membrane filtration process depends on the source water quality and the water quality requirements after treatment. Normally, the more polluted the source water is, the higher the water quality required after treatment, the more easily the filter will be clogged with impurities, sediment and microorganisms (algae, moss, microorganisms ... ).

Conclusion: Harm and solutions for arsenic in domestic water

In order to avoid arsenic poisoning, comprehensive measures, from planning, management, to the development of appropriate pollution treatment and production technologies, to communication, education, and solutions should be taken. medical, community health care, ...

It is necessary to classify, zoning according to the type of pollution and classify according to the degree or concentration of contamination by arsenic.

Based on the specific conditions of each locality, it is necessary to select an appropriate water treatment technology for arsenic. With groundwater with a high iron and manganese content, iron and manganese treatment by traditional aeration and filtration methods also allows the majority of arsenic to be removed, significantly reducing the risk of arsenic contamination, however in many fields.

The concentration of arsenic below standards is not allowed, at the same time, it also depends on the operating mode of the water extraction, treatment and supply system. With the Hanoi area groundwater having high iron, manganese and ammonium content, it is necessary to continue to develop solutions to treat arsenic with removal of iron, manganese, and ammonium. Based on specific conditions of each locality, appropriate technology solutions must be selected. Therefore, it is necessary to have a thorough study and an interdisciplinary solution, with the participation of many sectors, towards achieving a suitable and sustainable solution.

Source: https://blebees.com

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