homework

I) What is Arsenic?

Arsenic is a widely distributed element in the earth's crust and is recognized as a toxic and carcinogenic substance. Arsenic is widely used as a pesticide, herbicide, wood preservative, semiconductor material, and feed additive. These anthropogenic pathways have introduced large amounts of arsenic into the environment, increasing the concentration and distribution of arsenic in environmental water bodies. In recent years, in some countries, especially Bangladesh, China, and Mongolia drinking water sources are found in concentrations that can lead to acute and chronic human poisoning of arsenic. Therefore, the arsenic in drinking water has caused great concern. Given the great danger of arsenic to human health and the increasing severity of arsenic pollution, in 1993, the WHO took the lead in the indicator value of arsenic in drinking water from 50 μg / L to 10 μg / L. Subsequently, the European Union, Japan, the United States, respectively, their drinking water arsenic standards for 10 μg / L.

1. Chemical properties of arsenic in water bodies

In the aqueous environment, the two common oxidation states of arsenic are As(V) and As (III). (As(V) is oxygenated surface water and As (III)is the main form of arsenic in groundwater, while As(III) is the form of arsenic in anoxic groundwater. When the pH was in the neutral range, As(III) was mainly present in the form of H3 AsO3, while As(V) was present in the form of H2 AsO4 – and HAsSO4 2-. Therefore, in the typical pH range of water (pH = 5 to 8), As(V) exists in the form of anions, while As (III) exists in the form of neutral molecules. Therefore, the drinking water arsenic removal technology will involve the removal of arsenic in 2 different

vale nice states and the presence of forms.

2. Research progress of the arsenic removal process

2.1 Coagulation and flocculation method

Coagulation and precipitation method because of its easy to use, easy to grasp, and accept and become the most widely used, the most widely used arsenic drinking water treatment method. The most common coagulants are iron salts and aluminum salts. Many studies have shown that the coagulation and precipitation method in addition to the arsenic effect and the oxidation state of arsenic in water, the initial concentration of arsenic, the type and dose of coagulant, water quality conditions, and other factors. as (Ⅲ) removal effect is poor As (V) removal rate is higher. The oxidation of As (Ⅲ) to As (V) can improve the removal rate of arsenic. When the initial concentration of As (Ⅲ) <0∙8 mg/L, sodium hypochlorite 1∙25 mg/L can effectively oxidize As (Ⅲ) into As (V) to achieve the same removal effect as As (V). (1) If the use of perchlorate coagulant, it can replace the sodium hypochlorite and iron salt 2 reagents to simplify the treatment method and perchlorate oxidation capacity than sodium hypochlorite, potassium permanganate, etc. stronger, in the oxidation process will not produce secondary pollution. (2) Taoyuan etc. (3) discovered suitable filtration measures such as sand filtration can greatly improve the coagulant efficiency of arsenic removal, which may be related to the adsorption of arsenic by sand particles. But the main reason is that the solid-liquid separation effect of sand filtration is obviously

This may be related to the adsorption of arsenic by sand particles, but the main reason is that the solid-liquid separation effect of sand filtration is significantly better than that of sedimentation, which allows the tiny flocs to be better separated from the water, resulting in lower arsenic concentration in the effluent.

Meng and some other scientists (4) also found that sand filtration could improve the removal of arsenic. Another way to improve the efficiency of arsenic removal is to increase the particle size of arsenic-containing flocs.

2.2Adsorption method

The adsorption method is a simple and easy technique, suitable for large quantities and low arsenic concentrations in water treatment systems. The method uses high surface area, and insoluble solid materials as an adsorbent agent, physical adsorption, chemical adsorption, and other effects of dissolved arsenic in the water will be fixed on its surface. The adsorbent mainly includes activated alumina. The adsorbent mainly includes activated alumina, activated carbon, bone carbon, zeolite, Natural or synthetic metal oxides, and their hydrated oxides, etc.

According to the principle of the adsorption method, the larger the surface area of the adsorbent, the stronger the adsorption capacity. Mohan and other scientists (5) found the results of arsenic removal studies with common adsorbents showed that low-cost adsorbents (e.g., treated furnace agents, treated slag, activated carbon developed from agricultural waste, needle iron ore, etc.) were found to have good arsenic removal effects. In recent years, the improvement of traditional adsorbents and the development of new and efficient arsenic removal adsorbents have been more active. The results showed that the removal of As(III) and As(V) could reach 83.4% and 37.4% with Ca(OH)2 modification of waste wheat barley, which was higher than the used NaOH to remove arsenic.

Natural iron, manganese ore, and manganese adsorbent are also used to remove arsenic from drinking water. Iron cations in iron oxides and hydroxyl groups composed of surface functional groups (Fe-OH) can be positively charged through proton association and dissociation, thus adsorbing arsenic in the form of anions. Arsenic is in the form of ions [6]. Zero-valent iron is an efficient adsorbent for pre-oxidizable arsenic agents. In the presence of oxygen, zero-valent iron is rapidly oxidized in water to iron hydroxide, which adsorbs arsenic from water. Therefore, the removal rate of arsenic by zero-valent iron is related to the content of iron hydroxide in water and the pH value of water, and the removal rate of As(V) is higher than As(III). (7) Berna et al confirmed that higher dissolved oxygen (DO) and lower pH could accelerate the rate of zero-valent iron corrosion and removal of arsenic by zero-valent iron.

Nanomaterials have particle diameters of 1 to 100 nm, and as a new type of adsorbent, they have special physicochemical characteristics and special properties that are superior to traditional materials. Sabbatini et al [8] used iron oxide nanoparticles for the adsorption of arsenic removal and found them to be cost-effective and effective in removing arsenic. The disadvantage of the adsorption method is that it is difficult to recover, and not easy to regenerate, and the adsorption efficiency decreases after regeneration. When some common ions in water (such as phosphate, sulfate, chloride, fluoride, etc.), these substances compete with arsenic for adsorption, thus reducing the efficiency of arsenic removal.

2.3 Arsenic removal by ion exchange

Ion exchange has a good effect on the removal of As(V), while As(Ⅲ) exists in the form of neutral molecules in the water body so As(Ⅲ) is usually easy to penetrates the ion exchange. The ability of ion exchange to remove As(V) depends mainly on the spatial separation of adjacent charges in the resin, the mobility, the extensibility of functional groups, and hydrophilicity.

The pH value was found to have a strong influence on the removal of As(V). This is because as the pH value increases, As(V) was converted from H2AsO4- to HAsO4 2- and the selectivity for the strong alkali-type resin is more on divalent anions than monovalent anions. In addition, the high concentration of SO4 in the water, NO3-and Cl- and TDS (greater than 1,000 mg/L) can also compete with As(V) then lead to ion exchange failure. Therefore, the ion exchange technique is more suitable for cleaner water bodies with less ion strength.

2.4 New arsenic removal technology by membrane

2.4.1 Nanofiltration membrane

Nanofiltration is one of the promising arsenic removal technologies, which could create higher water yield with lower energy consumption and does not require any chemical technology, so it is super suitable for small hydrology factories. The removal mechanism of nanofiltration includes (1) the spatial rejection of uncharged nanoscale components in the membrane; (2) the repulsion effect of solution (same ion) and membrane charge. Therefore, the retention of ions by NF membranes is highly dependent on the membrane properties. Vrijenhoeka et al [9] used NF-45 polyamide nanofiltration membrane to study the effect of arsenic removal. The results showed that when the mass concentration of arsenic was between 10 and 316 μg/L, 60-90% of As(V) will be reversed. However, the removal rate of As(III) was much lower than that of As(V), and the removal rate decreased with the increase of arsenic concentration in the influent water. In the presence of 0.01 mol/L NaCl, the removal rate of As(V) was significantly increased, especially when the concentration of arsenic in the influent water was small.

However, when Seidel et al. [10] repeated the above experiment with the BQ01 type of sulfonated polysulfide nanofiltration membrane, they found that the removal of As(V) was reduced by about 5% in the presence of 0.01 mol/L NaCl. This indicates that NaCl has a significant effect on the removal of As(V) and it is determined by the membrane properties. The effect of pH on the removal rate of arsenic by NF- 45 membrane showed that as the pH of the solution increased, the removal rate of As(V) will increase at the same time.

2.5 Pre-oxidation process

Many studies confirmed that the toxicity, solubility, and mobility of As(III) are much greater than those of As(V). Because As(III) usually exists in molecular form so the removal rate of As(III) by various processes is much lower than that of As(V). So when we remove arsenic from the groundwater, we need pre-oxidized As(III) to As(V).

2.5.1 Pure pre-oxidation process

The redox unit of the As(III) - As(V) system is 0.560V, therefore, neither aeration nor the addition of pure oxygen can rapidly and effectively oxidize As(Ⅲ) to As(V), then the addition of chemical oxidant is required. Due to the different redox potentials (see Table 1) and the mechanism of oxidation reaction, the oxidant in various water treatment

Due to the different redox potentials (see Table 1) and oxidation mechanisms, the oxidation degree, and rate of As(III) oxidation are different in water treatment.

Table 1.

In the range of pH = 6.3 to 8.3, both Cl2 and KMnO4 were able to rapidly oxidize As(III) to As(V) within 40 s. Although the presence of dissolved Mn2+, Fe2+, and sulfides in water and TOC will slow down the oxidation rate, the complete oxidation could be completed within 1 min. O3 indirectly oxidizes As(III) by hydrolysis to produce -OH, so the oxidation rate is very fast. However, the natural organic matter (NOM) in the water can greatly slow down the rate of oxidation by trapping -OH. Therefore, O3 is not suitable for the oxidation of As(III) in heavily organically polluted waters; ClO2 can only limit oxidize As(III); NH2Cl has almost no effect on the oxidation of As(III) [11].

2.5.2 Oxidation and adsorption techniques

In recent years, the oxidation and adsorption of As(III) have been combined, to greatly shorten the removal process. Zero-valent iron is easy to get, and it is inexpensive, non-toxic, and non-hazardous. The oxidation of As(III) has received much attention from researchers [12-16]. Because the mechanism of Fe(0) oxidation of As(III) is controversial, yet, studies [12-13] show that it can be broadly explained as follows:

RI is H2O2, OH-, O2- or Fe(VI) those intermediate products formed by the reaction of Fe(0), Fe(II), and dissolved oxygen.

1 Fe(0) + 1 /2 O2 + 2 H2O-(RI) → Fe(II) + H2O + 2 OH-,

2 Fe(II) + 1 /4 O2 + H2O-(RI) → Fe(III) + 1 /2 H2O + OH-

3 As(III) + RI → As(V) ;

4 Fe(III) + 3 H2O → Fe(OH) 3 + 3 H+ ;

5 adsorption of Fe(III) aggregates on As(III) and As(V) and co-precipitation of HFO with As(III) and As(V).

Leupin et al [17] studied the oxidation and removal of As(III) by Fe(0) in artificially groundwater with a mass concentration of 500 μg/L As(III). The results confirmed that dissolved Fe(II) at mass concentrations up to 8 mg/L was released to form HFO, and almost all of As(III) was oxidized to As(V) and adsorbed on the surface of HFO to be retained by the sand filter layer, which reduced the mass concentration of arsenic in the effluent to 50 μg/L. Bang et al[18] showed that DO and pH had a significant influence on the removal of arsenic from Fe(0). This is because higher DO and lower pH can increase the decay rate of Fe(0). Tyrovola et al[19] concluded that PO4 3- and NO3 - will slow down the removal rate of arsenic, and the temperature from 20 to 40 ℃will determined the removal rate of arsenic.

The new technology of zero-valent iron oxidation and adsorption is one of the most promising in recent years. Because it is especially suitable for developing countries, especially remote areas due to no chemical dosing required.

2.5.3 Biological oxidation technology

The bio-oxidation process has unparalleled advantages compared to the normal physical-chemical pre-oxidation process as it does not require the addition of chemical solutions it is more economical and environmentally friendly therefore this technology has a promising application for developing countries. Ioannis et al [20] found that some common microorganisms in groundwater, such as Gallionella ferrooxidans and Leptothrix ochraceus, could oxidize Fe2+ while simultaneously oxidation As(III). The formation of multiple complexes including Fe oxides, a significant amount of organic matter, and bacteria was deposited on the surface of the filter media to show unique retention of arsenic through unique adsorption and co-precipitation. When the mass concentration of As(III) in the influent water was 200-250 μg/L, Fe2+and dos were 2.8 and 3.7 mg/L. After 2,000 BVs, the As(III) removal capacity of the system was consistently higher than 95%. Biological oxidation provides a new idea for the development of arsenic removal technology.

II. Conclusion

Most of the methods are mainly used to remove As(V) from water and it’s less effective for the removal of As(III) in water. Therefore, the common practice in the process of arsenic removal is to pre-oxygenate As(III) to As(V) before removal. The methods of oxidation are chemical oxidation and biological oxidation and current chemical oxidants are chlorine, ozone, hydrogen peroxide, potassium permanganate, and other manganese compounds. Chemical oxidation is prone to the formation of residues and other byproducts, which will create secondary pollution and increase treatment costs. In recent years, many researchers research microbial pre-oxidation and attempt to promote the use of bio-oxidation [21].

Conclude, each method could use in different conditions of application and each of them has its advantages and disadvantages. In general, the adsorption method can be much more successful in the removal of arsenic. However, there are still many problems that need to be solved, such as most of the adsorbents can only effectively adsorb As (V) and the efficiency of As (III) adsorption is generally not high. Therefore, As (III) must be pre-oxidation to As (V) which makes the treatment process of arsenic becomes complicated. And the presence of phosphate, sulfate, silicate, and fluoride substances in drinking water will easily compete with the arsenic adsorption site then which will reduce the efficiency of the removal of arsenic. Therefore, these substances need to be removed before treatment. This will also increase the treatment steps. In addition, the strong adsorption between the adsorbent and arsenic will let adsorption to be difficult to regenerate, recover, and reuse. The adsorption that consists of arsenic is difficult to meet the environmental soundness requirements and the problem of subsequent treatment is not easy to solve.

Many new adsorbents have high adsorption capacities but are generally complex and costly to manufacture so there is still a considerable gap in the practical application. The problem with the ion exchange method is the amount of ion exchanger is generally large and the ion exchange capacity is not that high, and the practicality of the new ion exchange agent has yet to be verified. Although the biochemical method has been proven to be feasible in experiments, it has not been reported used in the real life. The efficiency of arsenic removal by electrocoagulation is high, but this method requires special equipment to operate the technical conditions are also high requirements for workers to use it.

The coagulation and precipitation method of arsenic removal is influenced by the efficiency of solid-liquid separation. The traditional precipitation process or simple sand filtration is difficult to make the effluent arsenic down to 10μｇ／so it is necessary to find new techniques to achieve good solid-liquid separation. The microfiltration membrane has good solid-liquid separation but because most of the arsenic in drinking water is in a dissolved state then the effect of arsenic removal by the membrane is not ideal. If the coagulation process and microfiltration technology are combined, using microfiltration membrane technology to replace the coagulation and precipitation method in the precipitation process or using the coagulation process as pretreatment of microfiltration membrane technology will absorb the advantages of both coagulation and microfiltration technologies to remove arsenic.

The coagulation and microfiltration process firstly transfers the dissolved arsenic in drinking water from the liquid phase to the solid phase then uses a microfiltration membrane to retain the arsenic-containing flocs by its good solid-liquid separation effect. The water will filter the membrane after drinking water achieves the standard. The coagulation microfiltration process has a good effect on arsenic removal because of its low-cost, high-water production rate, and simple operation. Therefore, the process is a better choice to remove arsenic from drinking water. At present, the research on the coagulation microfiltration arsenic removal process is still at the initial stage, but as the price of membrane components continues to fall, the coagulation microfiltration process in drinking water will have more favorable conditions and a better environment.

III. The background of groundwater with radium

Currently, the pollution of surface water sources by industrial and agricultural wastewater, domestic sewage, etc. is becoming increasingly serious. About 1/3 of the world's population draws drinking water from polluted water sources. Of the more than 500 rivers in China, about 400 are polluted to varying degrees, and it is well-known that there is a correlation between water sources and disease. Therefore, the choice and use of unpolluted water sources, for the miasma of people's health are extremely important, groundwater, because it is in the ground, through the physical, chemical, and biological, especially the purification process of the soil, generally not easy to be directly contaminated by the environment, take my hometown China as one example, China's vast territory, with an abundance of groundwater resources, from ancient times, there is a good tradition of taking water, but in the choice of groundwater sources, people just take care of chemical pollution and ignore Radium pollution.

Although the health effects of drinking low-level radioactive contaminated water are not obvious in the short term, the property of radium is very similar to calcium in the body's metabolism and will accumulate in the bones after being ingested by the body. Due to the radioactive decay of radium, there is an increased chance of bone tumors and other cancers. Some people believe that there may be no radiation safety dose for pavement. But in fact, Radium levels in farm wells ranged from 0-6.4 picocurie or pCi/L. MCL is 5 pCi/L. Gross alpha ranges from 1.4-19.4 pCi/L, and MCL is 15 pCi/L.

3. Mechanism of radium removal from groundwater

The pH value of groundwater is generally between 5-8 and soluble radium exists in the form of Ra2*. In the presence of SO4 2-, Ra2+ is adsorbed on the heavy product stone according to the following reaction: BaSO4+Ra2+=Ba(Ra)SO4+Ba2+. Soft manganese reaction with potassium permanganate and in alkaline media, water, and manganese dioxide are produced that let H+ become exchangeable ions so the following reaction can occur with Ra2+.

Its Ra 2+ plate is adsorbed on soft manganese or qualified sawdust.

Zeolite is a three-dimensional shelf-like pin composed of SiO4, or AlO4, due to Al2+, and Si4+ replacement, excess negative charge is generated in the pins structure, which leads to cations such as Na+, K+, Ca2+, etc. entering the pins through the cavities to maintain the charge balance. In solutions, when the cations with balanced charge in the cavities diffuse along the pores, it is possible to exchange with the appropriate cations in water (such as Ra2+), to achieve the purpose of removal from the solution, of course, does not exclude the factor of adsorption on the surface activity of the adsorbent.

4. Some properties when removing radium

4.1

When removing Radium with barite, the concentration of SO4 2- will be influenced a lot. It is generally believed -satisfactory results are obtained when the So4 2- concentration is >500 mg/L.

4.2

Standing stones, especially natural zeolite-like ores, have been widely used in water treatment processes. It has a good effect on Ra removal. However, in the fixed filter column adsorption, it is easy to produce bubbles and form a short circuit, so it has to be flushed frequently to guarantee normal adsorption.

4.3

Soft manganese ore has the characteristics of a wide source, good physical properties, and long service life. Using it as the treatment material, not only removal rate of Ra" is high, but also the purification process is simple, and the treatment cost is low. Due to the contact catalytic oxidation on the surface of soft manganese ore, Fe2* and Mn+ contained in groundwater are oxidized first, and then precipitates such as Fe(OH): and Mn(OH), are formed and retained. Thus, while removing Ra, Fe+ and Mn2+ are also effectively removed. The process of soft manganese ore removal of Ra can be easily integrated with the purification of the existing water supply system. For example, by adding a soft manganese layer to an ordinary sand filter, the purpose can be achieved.

Therefore, the soft manganese ore is the groundwater removal Ra is an extremely ideal adsorption material.

4.4

Potassium permanganate-activated sawdust also has an excellent effect on the adsorption of Ra, which is like soft manganese ore in substance. Because of its higher surface activity, its penetration capacity and penetration volume are larger than that of soft manganese ore. But, due to its high production cost, poor physical properties and easy decay of wood chips, etc., it does not seem to be superior to using soft manganese ore.

5. methods to remove radium

The addition of barium chloride reagent is important for the removal of Ra in water. On one hand, the presence of sulfate in the water can make Bacl2 which is added to form BaSO4 in a short time and BaSO4 has a strong capacity to absorb Ra. In addition, since Ba2+ and Ra2+ have similar ion radii (1 4.3 nm and 1 5.3 nm) so the adsorption of radium by barium sulfate can occur as follows

Ba S04 + Ra2+ = B a (Ra) S 04 + B a2 +Mn 2+ in the alkalinization of water will occur the reaction and the product Mn(OH)2 is easily oxidized by oxygen to form Mn(OH)2

Mn 2 + + 2 OH- = Mn (O H) 2

Mn(O H)2 + l/2 O2= Mn O (O H) 2

Therefore, the air aeration operation can accelerate the Mn2+ precipitation [3]. In addition, in alkaline media, Mn (OH)2 has a positive effect on removing Ra 2+ and purification of Ra2+ in water.

6. Conclusion

With barite, soft manganese ore, potassium permanganate activated sawdust, artificial zeolite, etc. as adsorbent materials, the removal of Ra was out for the groundwater of a factory. Soft manganese ore has the characteristics of good physical properties, wide source, and long service life. Using it as an adsorbent material for Ra, it has a high removal rate, a simple purification process, and low treatment cost, it can effectively remove Fe2+ and Mn+ from groundwater while removing Ra, so it is an extremely ideal adsorption material.