JP3443573B2 - Arsenic-removing filter medium and method for purifying water containing arsenic - Google Patents

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a filter medium for removing arsenic from groundwater. In particular, the present invention relates to arsenic removal filter media that are renewable and can be efficiently used to filter water at normal drinking water pH.

[0002]

Arsenic is a naturally occurring element found in ground and surface waters and is present in high concentrations in many parts of the world. Also, there are numerous areas where groundwater is contaminated with arsenic from industrial activities such as mining operations, waste pile runoff and pesticide manufacturing.

It has long been known that arsenic is a highly toxic and suspected carcinogen and can be deadly in its pure form. The effects of long-term consumption of naturally occurring water containing high concentrations of arsenic have been the subject of numerous studies. It is known that so-called chronic arsenic poisoning results in thickening and discoloration of the skin, cancer of the liver, kidneys and skin and loss of blood circulation in the limbs resulting in a gangrenous condition known as blackfoot disease.

Since arsenic is toxic and carcinogenic, the maximum allowable concentration of arsenic in drinking water has been determined by government agencies. In Canada, the maximum allowable concentration in drinking water is 25 μg / L by Health and Welfare Canada. The World Health Organization stipulates a maximum allowable concentration of 10 μg / L. Currently, the USA limit is 50 μg / L,
The U.S. Environmental Protection Agency limits this limit to perhaps 10 μg /
We plan to reduce it to a low standard value of L. It is known that many countries have already reduced the limit value to 10 μg / L.

Numerous methods have been used in the past to remove arsenic from water. Arsenic removal capacity as a secondary effect has been demonstrated in surface water treatment systems using conventional treatment sequences such as lime softening and coagulation / coagulation / filtration. Advanced technologies such as ion exchange treatment, activated alumina treatment and membrane treatment processes such as reverse osmosis have shown very good results under certain conditions, although very few tests have been performed. However, previous studies have shown that the underlying water matrix significantly affects arsenic removal. For example, basicity affects the coagulation process; sulfate affects ion exchange and membrane treatment processes; the performance of activated alumina decreases as pH and fluoride concentration increase; and coprecipitation with iron increases Suppressed by chloride.

In addition to the possible presence of inhibitors in water, other difficulties associated with removing arsenic from groundwater include cost, complexity and method of use. For example, lime water softener or coagulator /
Coagulation / filtration equipment is expensive and requires relatively high degree of care in its operation. It is also known that these devices produce large amounts of residue, which presents a disposal problem. Activated alumina and ion exchange media are expensive to manufacture. It costs money to build a membrane filtration device,
It is technically difficult to operate, and often more than 50% of the feed water is wasted. It is known that in some cases, less than 50 liters of high-purity water is often produced per 100 liters of water entering the system, with the balance being discarded.

In recent years, studies have been conducted on the use of sand coated with iron oxide as a filter medium. It has been shown that iron oxide coated sand effectively removes arsenic from groundwater. However, this filter medium is mainly
It has not been developed on a large scale because it requires complicated and costly preparatory operations involving the firing of iron oxide coatings on the surface of sand. To date, the use of this manufacturing method has been limited to producing small quantities for laboratory experiments.

Similar work has been done on the use of iron oxide impregnated porous support materials as filter media. For example, Canadian Patent 1,067,627 describes a method and apparatus for removing arsenic from water by passing water containing arsenic through a ferric hydroxide impregnated porous support material. ing. The Canadian patent above shows two examples of filter media for removing arsenic from groundwater. In the first example, the filter medium is impregnated with 4.4% ferric ion as Fe (OH) ₃ , and in the second example, Fe (O
H) ₃ is impregnated with 0.97% ferric ion. The test was carried out in the first case with water of pH 4.7,
In the case of pH 3.9 water is used. The Canadian patent further states that arsenic removal is preferably performed with water having a maximum pH of 4.4.

The Canadian patent further describes that the filter media may be regenerated in certain applications.
However, the Canadian patent does not describe how to regenerate the filter media or whether re-impregnation with Fe (OH) ₃ is performed to effect the regeneration.

From the above, there is a demand for a filter medium that can be used for well water when it flows out from the ground and that does not require pH adjustment before and after the filtration operation. Will be understood. There is also a need for a filter medium that can desorb and wash away arsenic and that can restore its adsorption capacity to an effective level without re-impregnating ferric ions.

Furthermore, there is a need for a filter medium that is relatively inexpensive, effective for a wide range of water qualities, and that can be used efficiently in residential wells as well as in industrial, commercial and urban installations. Has been done.

[0012]

The present invention has a large arsenic adsorption capacity, is particularly efficient at removing arsenic from groundwater at drinking water pH (6.5-8.0), and is easy and inexpensive to mass produce. A filter material for removing arsenic is provided. More importantly, the filter media of the present invention can be saturated with arsenic and then regenerated several times for reuse.

[0013]

According to a broad definition, in a first aspect of the invention, for the removal of arsenic essentially consisting of calcined diatomaceous earth particles and ferric ions bound to the calcined diatomaceous earth particles. A filter medium, a mixture of calcined diatomaceous earth particles and ferric chloride is prepared, and this mixture is allowed to stand for 16 hours, whereby ferric chloride is completely impregnated in the diatomaceous earth particles,
The mixture thus obtained has a pH of at least 9.0.
A filter element for arsenic removal is provided, which is produced by a method of gradually adding ferric chloride to ferric chloride to completely convert it into ferric hydroxide by gradually adding sodium hydroxide until the value of is reached.

The filter medium thus obtained has a strong and durable bond between ferric ions and diatomaceous earth particles. The filter medium according to the first aspect of the present invention can be regenerated several times, and the reduction of the arsenic adsorption capacity of the filter medium at that time is minimal. The test results show that the reduction of the arsenic adsorption capacity of the filter medium is 10% or less in each time the filter medium is regenerated.

Another feature of the filter media according to the invention is that the arsenic is positively bound to the filter media and leaching of arsenic is prevented downstream of the filter containing the arsenic-saturated filter media.

It is believed that the above-mentioned advantages are mainly obtained by the strong and durable bond between the ferric ion and the diatomaceous earth particles formed during the preparation of the filter medium.

Further, according to the above-mentioned method, ferric iron ions are loaded on the diatomaceous earth support at a particularly high rate, which is considered to be another factor contributing to the performance of the filter medium to some extent.

According to another aspect of the present invention, the step of forming a mixture of diatomaceous earth particles and ferric chloride in the above method comprises the steps of: It consisted of adding 4 ml of a 2.1 M FeCl ₃ .6H ₂ O solution, then the sodium hydroxide used in the step of gradually adding sodium hydroxide until the mixture had a concentration of 10N. These additional limitations to the method of making the filter media of the present invention are advantageous in providing filter media having ferric hydroxide bound to diatomaceous earth particles in a large proportion of 1.36 g / g diatomaceous earth particles. That is, this method is advantageous in providing a filter medium containing ferric ions in a large proportion of 30% by weight.

The filter medium according to this aspect of the invention has a thickness of about 1200 μm.
It is particularly advantageous in that it has an arsenic adsorption capacity of g / g, can efficiently utilize water at the drinking water pH and can repeat regeneration several times. It has also been found that the filter media of the present invention is effective in removing other metals present with arsenic in the water to be filtered. In the test, with the filter medium of the present invention, 99% or more of copper, 98% of lead,
Over 98% of uranium was removed. It has often been found that the filter media of the present invention also has the ability to remove iron from water. Furthermore, the filter media according to this aspect of the invention showed no reduction in arsenic filtration efficiency for water containing up to 250 mg / L chloride or sulphate.

Such high loading of ferric ions by the diatomaceous earth material is preferred in most arsenic filtration systems,
Nevertheless, lower loadings of ferric ion also show advantageous results and may be preferred for use in less damaging applications, for example. Therefore, according to still another embodiment of the present invention, a calcined diatomaceous earth particle having a particle size of 30 mesh to 60 mesh and a bonded 5
There is provided an arsenic removal filter material consisting essentially of -30 wt% ferric ion. The arsenic removal filter material according to this aspect of the invention can be used to filter water at drinking water pH and its regeneration can be repeated several times.

According to another aspect of the present invention, there is provided a method of regenerating an arsenic-removing filter medium containing calcined diatomaceous earth particles and ferric ion bound to the calcined diatomaceous earth particles after being saturated with arsenic. To be done. The regeneration method consists of slowly passing sodium hydroxide at a concentration of 0.5 N to 2.0 N downward through the arsenic removal filter medium. This reduces wear on the arsenic removal filter media and substantially retains its performance. It was found that this method was effective in desorbing at least about 82% arsenic from the arsenic-removing filter medium, and still retained about 90% of the arsenic-removing filter medium's arsenic adsorption capacity. It has been found that this method of regenerating the filter medium has the advantage that it does not require chemical regeneration of the filter medium, for example by desorbing arsenic from the filter medium and then reimpregnating the support material with ferric ions.

Another feature of the present invention is that the manufacturing costs associated with the materials, equipment and operations used are low, and therefore the selling price to the industry is low, which makes such filter media economically available to the public. It is to make it possible.

[0023] Very importantly, the filter media of the present invention is intended for use in drinking water applications for use by the National Sanitation Foundation (NSF Internatio).
nal) by Drinking Water System Components-H
Certified under ANSI / NSF Standard 61 entitled ealth Effects.

Other advantages and novel features of the invention will be apparent from the detailed description below.

[0025]

BEST MODE FOR CARRYING OUT THE INVENTION Preferred embodiments of the present invention will be described below with reference to the drawings.

The inventor has made and tested a number of filter media, including some conventional filter media. However, in the present specification, for the sake of clarity, only the test results of the filter media prepared according to the preferred embodiment are shown. This test result was compared with the test result using activated alumina, which is considered to be the most common arsenic adsorption medium currently used. Also, for clarity, the filter media made according to the preferred embodiment is referred to as filter media G2 or filter media G2 ^™ .

The term filter media G2 refers to filter media made according to the preferred embodiment and filter media made with modifications to the contents and structure of the filter media that are within the scope of the invention.

Preparation of Activated Alumina For reference, the activated alumina filter media used in the tests described below were prepared from standard 14 mesh material and washed with tap water.

Preparation of Filter Media G2 A preferred support material for the preparation of filter media G2 is calcined diatomaceous earth consisting of particles with a particle size of about 30 mesh (0.85 mm) to about 60 mesh (0.42 mm). This support material is available under the name MP79 from Eagle-Picher Minerals (Reno, NV, USA).

To the calcined diatomaceous earth, 4 mL of 2.1 per 1 g of the diatomaceous earth was added.
It was coated with M FeCl ₃ · 6H ₂ O solution. The solution was incorporated into diatomaceous earth using an inversion type agitator operating at 60 rpm.
The resulting slurry was then left for 16 hours to allow ferric chloride to penetrate the diatomaceous earth particles. After infiltration, excess solution was drained from the filter media and then sufficient tap water was added to submerge the entire filter media. Then 10 N NaOH was added slowly over 10-15 minutes to slowly raise the pH of the slurry to a final value of about 1 to about 9. Finally, the filter medium was washed with tap water to remove excess ferric hydroxide that was not bound to the filter medium particles. When the Fe concentration in the wash water flowing out became 0.1 mg / L or less, the wash was stopped.

Filter media G2 was prepared as a batch with each filter media containing about 75 g of calcined diatomaceous earth. Therefore, the above mixture,
The standing and reaction times can be adjusted according to other batches.

It has been found that this method is particularly effective for loading the filter medium with as much ferric ion as possible. The obtained filter medium G2 contains ferric hydroxide adsorbed on the calcined diatomaceous earth and bound by ionic bonds in a large amount of 1.36 g per 1 g of calcined diatomaceous earth. In other words, the filter medium G2 contains a large amount of ferric ion of 0.71 g, that is, 30% by weight of ferric ion per 1 g of calcined diatomaceous earth.

The particularly high loading of ferric ions by the calcined diatomaceous earth is considered to be an important contributor to the results and advantageous characteristics of the filter media G2 as described above. This particularly high loading of ferric ions by the filter media particles is believed to be a contributing factor in obtaining an arsenic removal filter media with great adsorption capacity that is effective and can be regenerated several times at drinking water pH (6.5-8.0). However, it is believed that such high ferric ion loadings are preferred, but not absolutely necessary, to some extent to achieve at least some of the characteristics as described above.

According to the test method described below,
As a result of the test, during the preparation of the filter medium G2, a strong and durable bond was formed between the ferric ion and the diatomaceous earth, and the regenerating characteristics accompanying this were as low as 5% by weight of the ferric ion. It was also found to be found in some G2 containing Effective use at drinking water pH was also found in certain G2's containing about 5% by weight ferric ion. Therefore, the nature of the bond formed between the ferric ion and the diatomaceous earth support during the preparation of filter medium G2 according to the method described above yields a filter medium for arsenic removal that is reproducible several times and is effective at drinking water pH. However, it is believed to be as important as the concentration of ferric ion in the diatomaceous earth support.

[0035]

EXAMPLES Example 1: Bench scale column test (colu
mn test) Each column test was carried out using a filter consisting of two plastic columns with a diameter of 15 mm and a length of 150 mm connected in series. A nominal volume of 25 ml (20 g) of filter medium G2 was charged to each column; therefore, the total amount of filter medium per filter is 50 ml. Tap water containing 200 μg / L arsenic was added to each column by 5 mL using a lightweight pump.
Supplied at a flow rate of / min. Therefore, the empty floor contact time (empty b
The ed contact time) was 5 minutes required for passage through the first column and 10 minutes required for passage through two columns. Samples were collected at regular time intervals from the sample collection points provided behind the first and second columns.

The results of these tests are shown in FIGS. As can be seen from these figures, before the outlet concentration exceeded 25 μg / L, 500
By treating over a bed volume of 0, filter media G2 provides outstanding results. Filter material G2
Up to a bed volume of 4000 still yields an arsenic content of 2 μg / L. This performance is similar to that of activated alumina, both of which provide an arsenic adsorption capacity of over 1200 μg / g.

Samples of treated water from the column test described above were subjected to comprehensive metal scan and general chemical analysis. Typical results are shown in Tables 1 and 2 below. As can be seen from these tables, the filter medium G2 did not change the water quality to be poor. In addition to removing arsenic, the filter medium G2 provides 80
~ 90% copper and 98-99% lead are also removed. In comparison, activated alumina removes 62% copper and 70% lead. Other studies described below confirmed the ability of filter media G2 to remove copper and lead.

[0038]

[0039]

Example 2 Pilot Scale Test A pilot filter was charged with a filter medium G2 having a nominal capacity of 18.2 liters (10 kg), and this filter was washed with water having a pH of 1.
It was operated by continuous feeding at a flow rate of 82 L / min. The results with this filter are shown in FIG. A breakthrough occurred after a bed volume of about 1500, as shown by the curve labeled Run 1. Leakage was 25 μg / L in arsenic concentration at the filter outlet
Is defined as the point when The results of this test are shown in Example 1.
It is understood that it is worse than the result obtained in the bench scale test described in. This difference is believed to be due to the pH of the water being 5.8 in the bench scale test and 7.0 in the pilot scale test.

After the leakage occurred, the filter medium was regenerated by passing a small amount of 2N NaOH through the filter medium G2, as will be described later. If you put this filter back in the operating line,
Water with an arsenic concentration of 2 μg / L or less was immediately produced. Interestingly, as shown in Run 2 of Figure 3, the performance of the filter media G2 after regeneration is better than the performance in the first run performed with water at the same pH of 7.0. This increase in performance is believed to be due to the effective oxidation of some ferric ions on the filter media that were not oxidized during the initial filter media preparation process by the sodium hydroxide used in the regeneration process. During this second test run 2, more than 2800 bed volumes of water passed through the filter, but the effluent arsenic concentration in the outlet water remained at 2 μg / L.

In addition to the daily test for arsenic, a complete metal analysis and general chemical analysis were performed on the water at the inlet and outlet of the pilot filter. The results are shown in Tables 3 and 4. The results shown in these tables show that, as in the bench scale column test filters, the water quality is not adversely affected by the filter media G2 and that large amounts of 94% and 84% copper and lead respectively are removed. Is recognized.

[0043]

[0044]

Example 3: Testing on a commercial apparatus Using a filter containing filter media G2, Rosehill Center, Holly, Michigan, USA.
Arsenic was removed from the feed water to commercial equipment at Hill Centre). The arsenic concentration at the filter inlet and at the filter outlet can be measured by the Environmental Protection Association.
on Agency (EPA) and University of Michigan (UM) personnel. Filter size is 1200μ with expected operating period of 2 years at a continuous water flow rate of 60 gallons / min.
It was determined according to the arsenic adsorption capacity of g / g. The test results were available at set flow rates for almost 6 months; meanwhile, the filter medium G2 showed no signs of saturation or the need for regeneration. The test results on this commercial device are shown in FIG.

Example 4: Effect of pH Filter media G2 and activated alumina were tested to determine the arsenic adsorption capacity for water at pH's in the range 6.0 to 8.0. The results are shown in Table 5 below.

[0047]

As can be understood from the results shown in Table 5, the performances of both filter media deteriorate remarkably as the pH increases.
However, the performance of filter medium G2 is more stable over the entire pH range. The arsenic adsorption capacity of filter medium G2 depends on the pH of water.
When increasing from 6.0 to 8.0, only 34% decrease, while the arsenic adsorption capacity of activated alumina decreases to 58% in the same pH range.

Example 5: Influence of Sulfate and Chloride Containing 200 μg / L of arsenic, and further containing the following elements in the following ratios: (a) 500 mg / L sulfate, (b) 500 mg / L L containing chloride, (c) 250 mg / L chloride, or (d) 250 mg / L sulfate, using feed water containing 430 bed volumes for 3 days, 2 pieces containing filter medium G2 Activated the filter. The exit arsenic concentration at the end of this period is shown in Table 6.

[0050]

At the concentration of 500 mg / L, both sulfate and chloride show remarkable effects, but the outlet arsenic concentration is 430
Even after a period corresponding to the bed volume, 10-11
Only μg / L. At a concentration of 250 mg / L, neither chloride nor sulphate showed any effect. Then 250m
The test at g / L was conducted for a further 6 days, ie the total bed volume was 115
It continued for a period of 0, but it had no effect.

Example 6 Removal of Copper A filter medium G2 was tested with a feed water pH of 5.6 and an inlet copper concentration of 3580 μg / L. The removal rate of copper was 80% one day after the operation, but rapidly decreased to 50% or less. Increase the pH at the filter inlet to 8.0 and increase the inlet copper concentration to 5520 μg
When set to / L, the outlet concentration did not exceed 10 μg / L over the operation period of 600 bed volumes. This has a removal efficiency of 99.
It shows that it is 8%. The result is shown in FIG.

Example 7 Removal of Lead A lead removal test was conducted with the feed water pH set to 5.6 and the lead concentration at the inlet set to 100 μg / L. The removal efficiency is as shown in FIG.
The concentration at the filter outlet did not exceed 3 μg / L. The removal efficiency obviously decreased at pH 8.0 and was as low as 50% after feeding 750 bed volumes of water.

Therefore, if filter medium G2 is used to remove copper, the pH of the water at the filter inlet should be adjusted to 8.0 and the pH of the water containing lead should be adjusted to 5.6. . The preferred method when both metals are present in the water to be treated is to pass water of pH 5.6 over the filter medium G2 to remove lead first and then adjust the pH to the level of drinking water, then , A second pass of water over the second bed of filter media G2, either the same filter media as the first or in series with the first, to remove the copper.

Example 8: Uranium Removal Further tests were conducted to remove uranium from water. The feed water pH was adjusted to 6.5, and the uranium filter inlet concentration was 120.
It was set to μg / L. As a result of the removal test, the outlet concentration did not exceed 4 μg / L.

Restoration of Filter Media G2 As used herein, the term renewable, renewable or restoration refers to the desorption and washout of arsenic from the filter media and the filter media reaching a new state. It is used to describe the restoration or recovery of the filtration efficiency of filter media to a level.

The regeneration of the filter medium G2 is preferably carried out in situ by slowly passing NaOH downward through the filter medium. In-situ regeneration is preferred because it avoids the safety considerations and measures associated with handling arsenic-saturated filter media and is more appropriate to implement automatic desorption and cleaning systems. . 0.5N, 1.0N and 2.
Regeneration of the arsenic-saturated filter medium G2 was carried out using 0N NaOH and in each case the% of arsenic recovered was determined. The results are shown in Table 7.

[0058]

From the results shown in Table 7, it is understood that all three NaOH concentrations are likewise very favorable for the desorption of arsenic from filter media samples.

The filter media G2 was tested for regeneration several times. Table 8 shows the results of a test in which the filter medium G2 was regenerated several times and its use in the pilot filter. In this table, the restoration of filter medium G2 and the restoration of activated alumina are compared.

It was observed that during the first and fifth regeneration cycles, the adsorption capacity of the filter medium G2 was reduced by 33%, but the performance of the activated alumina was very significantly reduced, ie to 54%. Was given. This means that the reduction rate of the performance is 10% or less per cycle for the filter medium G2, whereas it is 18% per cycle for the activated alumina.
It means%.

[0062]

Therefore, the restoration of the filtration efficiency of the filter material G2 is
From a low concentration of H of at least 0.5 N to at least 2.0
It is achieved by using a high concentration of N. It was found that when regeneration was carried out in-situ by circulating sodium hydroxide and washing with water and sodium hydroxide was slowly passed downward through the filter medium, the performance gradually decreased.

Industrial Application of Filter Media G2 Filter media G2 is simple to prepare and very effective in removing arsenic from water. In parallel tests under the same conditions, filter media G2 provides arsenic removal capacity similar to activated alumina. Contains 200 μg / L arsenic, total 50
00 bed volumes of water were treated until the outlet concentration of the filter exceeded 25 μg / L. This corresponds to an arsenic adsorption capacity of 1200 μg or more per 1 g of the filter medium. For an empty bed contact time of 10 minutes, this means that washing and regeneration are required after 830 hours or 35 days of continuous operation.

For example, cleaning and regenerating a filter containing about 10 Kg of filter media requires about 2 hours, which has a down time of only 0.2% and a waste liquid generation capacity of treated water. This means about 1.0% of the capacity.

In the above-mentioned test, the raw water having a very high arsenic concentration of 200 μg / L was used, but 95% of the supply water in North America contains arsenic at a concentration of 50 μg / L or less. Are known. Therefore, the operating time between washing and regeneration will theoretically be four times longer than the operating time for such water.

In the case of residential use, it is preferable to install a conventionally known filtration cartridge containing the filter medium G2 in the water supply system and drop the water flow through the filter. The volume of the filtration cartridge is preferably selected so that the residence time through the filter medium G2 is at least about 10 minutes. Desorption, washing and regeneration of the filter medium G2 are preferably carried out in situ, for example using tubing systems, pumps and timers known to those skilled in the field of water softeners.

For large scale applications, it is preferred to mount the filter media G2 on a filter bed or large filter reservoir known to those skilled in the art. Similarly, the amount of filter media G2 and the size of the bed or tank are preferably selected such that the residence time through the filter media G2 is at least about 10 minutes.

Further details regarding the manufacture, installation and use of the filter media of the present invention will be apparent from the foregoing, and therefore further description of the manufacture, use and regeneration of filter media G2 is unnecessary and omitted.

[Brief description of drawings]

FIG. 1 shows the arsenic concentration in the inlet and outlet streams of a bench scale filtration column containing activated alumina.

FIG. 2 shows the arsenic concentration in the inlet and outlet streams of bench scale column filters containing the filter media of the present invention.

FIG. 3 shows the arsenic concentration in the inlet and outlet streams of a pilot filter containing the filter media of the present invention.

FIG. 4 shows the arsenic concentration in the inlet and outlet streams of a commercial filtration device containing the filter media of the present invention.

FIG. 5 shows the removal of copper from water at pH 8.0 when the filter media of the present invention is used in a pilot filter device.

FIG. 6 shows the removal of lead from water at pH 5.6 when the filter media of the present invention is used in a pilot filter device.

Continuation of front page (72) Inventor Eritscuell. Winch Ester Canada E3 Bi6 Wy5, New Brunswick, Fred Rickton, Stney Brook Crescent 97 (72) Inventor Michael Geay. Macmillan Canada E3B1 1C2 2 New Brunswick, New Maryland, Berkeley Drive 183 (72) Inventor Ronald Sii. Berry USA 95616 Cabot Street, Davis, Calif. Ornithia 1135 (56) References JP-A-6-304472 (JP, A) JP-A-7-246390 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) B01J 20/00-20/34 B01D 39/00-39/20 C02F 1/28

Claims (2)

(57) [Claims] 1. A arsenic-removing filter material for removing dissolved arsenic from water, which essentially consists of calcined diatomaceous earth and 5% to 30% by weight of ferric ion bound to the calcined diatomaceous earth. An arsenic-removing filter medium that can be repeatedly regenerated several times after being saturated with arsenic. 2. Passing water containing dissolved arsenic through a filter medium consisting essentially of calcined diatomaceous earth and 5% to 30% by weight ferric ion bound to the calcined diatomaceous earth, A method for purifying water containing dissolved arsenic, wherein the adsorption of arsenic by the filter medium is effectively carried out at the pH of drinking water and the regeneration of the filter medium after being saturated with arsenic can be repeated several times. A method for purifying water containing dissolved arsenic, comprising: