Removing arsenic from water with acid-activated clay

Abstract

The description relates to a composition and a method for reducing the concentration of arsenic in water. Contaminated water is contacted with acid-activated clay characterized by a removal efficiency for arsenic of at least 95 wt %. Following sufficient contact, the water is separated from the acid-activated clay. In preferred form, the acid activated clay is characterized by a BET surface area of at least about 200 m.sup.2/gram.

Claims

1. A method for removing arsenic from contaminated water which comprises contacting the contaminated water with: a. an acid-activated clay selected from the group consisting of bentonite, montmorillonite, hectorite, talc, vermiculite, saponite, nontronite, kaolinite, halloysite, illite, and chlorite and mixtures thereof, wherein the acid-activated clay dosage to the volume of the contaminated water is in the range from about 1 to about 25 g/liter of contaminated water; and b. a water-soluble oxidizer that can oxidize arsenite to arsenate, wherein the oxidizer dosage to the volume of the contaminated water is in the range from about 0.2 to about 5 mg/liter of contaminated water.

2. The method of claim 1, wherein the acid-activated clay is bentonite.

3. The method of claim 1, wherein the acid-activated clay is generated by spraying the clay with an acid or acid solution or by incorporating the acid by kneading, at a temperature between 20° C. and 60° C. for at least 1 hour.

4. The method of claim 1, wherein the acid activated clay has specific surface area of at least 100 m.sup.2/g, preferably has specific surface area of least 200 m.sup.2/g.

5. The method of claim 1, wherein the oxidizer comprises chlorine.

6. The method of claim 1, wherein the oxidizer is calcium hypochlorite.

7. The method of claim 1 used for the batch wise arsenic removal from a volume of contaminated water.

8. A method according to claim 1, wherein the acid-activated clay dosage to the volume of the contaminated water is in the range from about 2 to about 15 g/liter of contaminated water.

9. A method according to claim 1, wherein the oxidizer dosage to the volume of the contaminated water is in the range from about 0.4 to about 2.5 mg/liter of contaminated water.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention will be better understood and its advantages will become more apparent when the following detailed description is read in conjunction with the accompanying drawings, in which:

(2) The FIGURE is a is a schematic diagram of one embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

(3) Reference will first be made to the FIGURE, which is a schematic diagram of one embodiment of the invention. The invention can be carried out in a manner illustrated in the FIGURE, wherein an inlet 10 for water to be treated, and an inlet 11 for sorbent replenishment. The inlet 11 can also be used for the introduction of a suitable oxidizing agent, which will be added in an amount sufficient to oxidize substantially all of the arsenic present to the highest valence form. The water to be treated and filled into reaction tank 12, will contain water-soluble arsenic (As) compounds including, but not limited to, arsenite [AsO.sub.3.sup.2−] and arsenate [AsO.sub.4.sup.2−].

(4) Arsenic in wastewater mainly includes dissolved As(III) and As(V) chemical compounds. The distribution between dissolved As(III) and As(V) is dependent on the redox potential of the wastewater. Under oxidizing conditions, the predominant species is As(V), which exists as arsenate. Depending on the pH of the water, arsenate includes a mixture of deprotonated oxyanions of arsenic acid (H.sub.2AsO.sub.4.sup.−, HAsO.sub.4.sup.2−, and AsO.sub.4.sup.3−). Under mildly reducing conditions, As(III) is also stable and exists as arsenite. Depending on the pH of the water, arsenate includes a mixture of arsenious acid (H.sub.3AsO.sub.3, H.sub.2AsO.sub.3.sup.−, HAsO.sub.3.sup.2−).

(5) The acid-activated bentonite clays used in accord with the invention can remove >95 wt % of arsenate in wastewater but only <25 wt % of arsenite in wastewater. Few solid sorbents can adsorb and remove arsenite effectively in water. Since wastewater usually contains both arsenite and arsenate (the distribution will depend on the original source of arsenic and redox potential), it is most effective to use an oxidant to oxidize arsenite to arsenate, then to adsorb arsenate. See, for example, Table 1 in Int. J. Environ. Res. Public Health 2016, 13, 62; doi:10.3390/ijerph13010062, which is incorporated herein by reference. Among the suitable oxidants are: oxygen, ozone, chlorine, chlorine dioxide, hypochlorite and salts, hydrogen peroxide, potassium permanganate, and biological oxidation. The preferred oxidants are chlorine, hypochlorite and salts, hydrogen peroxide, and potassium permanganate. More preferred oxidants are chlorine and hypochlorite and salts. More preferred oxidant is calcium hypochlorite salt [Ca(ClO).sub.2] that is a water-soluble salt. When calcium hypochlorite dissolves in water, it generates both hypochlorite and chlorine. Once oxidized, the arsenic is very efficiently removed according to the invention.

(6) The sorbent discovered according to the invention is acid-activated clay, which is commonly used as an adsorbent to remove colored pigments (carotenoids, chlorphyll) and colorless pigments (phospholipids) from edible and in-edible oils..sup.5 It can be used in a natural clay material, such as calcium bentonite (natural clay material) form or as the purified montmorillonite form, once acid activated by thoroughly wetting with a concentrated (e.g., at least 10% by weight) strong mineral acid, such as hydrochloric, sulfuric or nitric, heating and then drying. The acid-activated clay of the invention has been found effective without the use of iron or other added promoters. The wetting can be accomplished easily in a stirred reactor, with reaction times of from several minutes to six or more hours. .sup.5. Adeyemo, A. A., Adeoye, I. O., Bello, O. S.; “Adsorption of dyes using different types of clay: a review”; Appl Water Sci (2017) vol. 7 PP. 543-568.

(7) The acid-activated clay is typically prepared by mixing the clay with an acid or acid solution or by incorporating the acid by kneading at a temperature between room temperature (20° C.) and 60° C. for at least 1 hour. Preferably, the acid activated clay has specific surface area of at least 100 m.sup.2/g, preferably has specific surface area at least about 150 m.sup.2/gram, and most preferably of least 200 m.sup.2/g, as measured by a standard BET specific surface area measurement (Micromeritics Tristar II, N.sub.2, 3-point analysis).

(8) According to a preferred form of the process, the oxidizer dosage to the volume of the contaminated water is in the range from about 0.2 to about 5, preferably 0.4 to about 2.5, more preferably from about 0.8 to about 1.5 mg/liter of contaminated water.

(9) The reaction tank 12 is preferably stirred, such as by rotary rake 14 for a residence time in the tank of from about 1 to about 24 hours. The residence time will be selected by a determination of the degree of heavy metal reduction desired and the relative freshness of the acid treated bentonite sorbent. Again, the useful life will be determined by the degree of heavy metal reduction desired as well as the concentration of the heavy metal. Following a sufficient reaction time in reaction tank 12, the mixture of water and sorbent is drained via line 16 to a settling tank 20, from which purified water is extracted via line 22 and sorbent is removed via line 18 for treatment or disposal.

(10) In a broad sense, the compositions of the invention will comprise: a water-soluble oxidizer that can oxidize arsenite to arsenate at the ambient temperature and pressure in water, and an acid-activated bentonite clay. Some clays are mixed, and all are impure as mined, so the invention extends to all of those equivalent acid-activated clays including those selected from the group consisting of bentonite clay, montmorillonite, hectorite, talc, vermiculite, saponite, nontronite, kaolinite, halloysite, illite, and chlorite and mixtures thereof.

(11) Also, in a broad sense, the process will comprise: contacting a contaminated water with a water-soluble oxidizer that can oxidize arsenite to arsenate at the ambient temperature and pressure and an acid-activated clay as defined above. For treating a volume of contaminated water, the acid-activated clay dosage to the volume of the contaminated water is typically in the range from about 1 to about 25, preferably 2 to about 15, more preferably from about 4 to about 10 g/liter of contaminated water.

(12) The following examples are presented to further explain and illustrate the invention and are not to be taken as limiting in any regard. Unless otherwise indicated, all parts and percentages are by weight.

Example 1

(13) This example compares three acid-activated bentonites, three sodium bentonites, a calcium bentonite, a hectorite, a kaolinite, an activated carbon, a zeolite, and a diatomaceous earth for the removal of arsenic (As) from water. All of these solid adsorbents are in fine powder forms with mean particle sizes less than 50 μm.

(14) An As(V) stock solution (1,000 mg/L) was prepared by dissolving 0.425 g of sodium arsenate (Na.sub.2HAsO.sub.4.7H.sub.2O, Sigma-Aldrich, 98.0%) in distilled water in a 100 mL volumetric flask. The actual As(V) solution used for the adsorption tests was 1 mg/L (it was actually 0.97 mg/L based on MWL's result, see below) that was made by adding 1 ml of the 1,000 mg/L stock As(V) solution into a 1 L volumetric flask and add distilled water to the mark.

(15) The arsenic removal wt % was calculated by [(C.sub.0−C.sub.f)/C.sub.0]×100%, where C.sub.0 is the original concentration of arsenic, C.sub.f is the final concentration of arsenic.

(16) For each adsorption run, 50 mL of the pre-made 1 mg/L As(V) solution was added into a 125 mL Erlenmeyer flask. A measured amount (0.5 g) of solid adsorbent was added into the solution above. The adsorbent and the As solution were agitated on an orbital shaker (Benchmark Orbi-Shaker, Sigma-Aldrich) at 200 rpm for 30 min. The liquids after the agitation were centrifuged at 5,000 rpm for 30 min (Sorvall Legend X1 Centrifuge, Thermo Scientific). After the centrifuge, small amounts of the clear top liquid were sent to an analytical lab for total As analysis.

(17) All the As levels were analyzed by Midwest Laboratories, Inc. (MWL, Omaha, Nebr.) based on EPA 200.8 method via Inductively Coupled Plasma Mass Spectroscopy (ICP-MS). The limit of detection for ICP-MS was 0.1 μg/L.

(18) Table 1 shows the results for arsenic removal wt % of these solid adsorbents tested under the same experimental conditions. The results indicate that the acid-activated bentonite adsorbents have very high arsenic removal (>95 wt %) compared to the other adsorbents except for the much more expensive activated carbon sample. These results have been repeated and confirmed.

(19) TABLE-US-00001 TABLE 1 Residual Arsenic Removed Arsenic Absorbent Content (μg/g) wt % Baseline - No Adsorbent 1.060 N/A Acid-activated Bentonite A 0.011 99.01 Acid-activated Bentonite B 0.012 98.91 Acid-activated Bentonite C 0.046 95.64 Sodium Bentonite A 1.015 −4.68 Sodium Bentonite B 0.790 25.47 Sodium Bentonite C 1.040 1.89 Calcium Bentonite 0.839 20.89 Hectorite 0.997 5.98 Kaolinite 0.154 85.49 Mix of Bentonite/Attapulgite 0.485 54.28 Zeolite 1.099 −3.69 Activated Carbon 0.006 99.47 Diatomateous Earth 1.085 −2.31

Example 2

(20) This example shows the relationship between the arsenic removal wt % and specific surface area of different acid-activated bentonite adsorbents. The surface areas of the adsorbents were measured using a standard BET specific surface area measurement (Micromeritics Tristar II, N.sub.2, 3-point analysis) and are presented in Table 2 below.

(21) Table 2 results indicate that a higher the specific surface area of the acid-activated bentonite results in better arsenic removal efficiencies. Specifically, acid-activated bentonites with surface areas above 200 m.sup.2/g have arsenic removal efficiencies above 95 wt %. Other surface properties such as pore volume and mean pore size were also measured but the results show no clear correlations to their arsenic removal results.

(22) TABLE-US-00002 TABLE 2 Removed Arsenic Specific Surface Area Absorbent wt % (m.sup.2/g) Acid-activated Bentonite A 99.01 294.3 Acid-activated Bentonite B 98.91 225.4 Acid-activated Bentonite C 95.64 289.6 Acid-activated Bentonite W 36.85 139.2 Acid-activated Bentonite X 27.85 149.6 Acid-activated Bentonite Y 20.98 67.8 Acid-activated Bentonite Z 19.02 91.0

Example 3

(23) This example shows the dosage effect of the acid-activated bentonite adsorbents for arsenic removal from water.

(24) For each adsorption run, 50 mL of the pre-made 1.06 mg/L As(V) solution was added into a 125 mL Erlenmeyer flask. The amount of solid adsorbent added into the solution above was modified based on the tests. The adsorbent and the As solution were agitated on an orbital shaker (Benchmark Orbi-Shaker, Sigma-Aldrich) at 200 rpm for 30 min. The liquids after the agitation were centrifuged at 5,000 rpm for 30 min (Sorvall Legend X1 Centrifuge, Thermo Scientific). After the centrifuge, small amounts of the clear top liquid were sent to an analytical lab for total As analysis.

(25) All the As levels were analyzed by Midwest Laboratories, Inc. (MWL, Omaha, Nebr.) based on EPA 200.8 method via Inductively Coupled Plasma Mass Spectroscopy (ICP-MS). The limit of detection for ICP-MS was 0.1 μg/L.

(26) Table 3A, Table 3B, and Table 3C data show that when the dosages of acid-activated bentonite adsorbents are above certain critical numbers, their As adsorption efficiencies are very high (>95%). When the dosage level is lower than that number, the As adsorption efficiency starts to decrease. For different acid-activated bentonite adsorbents, their critical dosage levels are not necessarily the same, which might be because each adsorbent has its unique surface properties such as surface acidity and surface area.

(27) TABLE-US-00003 TABLE 3A Acid-activated Bentonite A/ Residual Arsenic Removed Arsenic Water Volume (g/L) Content (μg/g) wt % Baseline - No Adsorbent 1.060 N/A 10 0.011 99.01 5 0.0332 97.04 2 0.1349 87.95

(28) TABLE-US-00004 TABLE 3B Acid-activated Bentonite B/ Residual Arsenic Removed Arsenic Water Volume (g/L) Content (μg/g) wt % Baseline - No Adsorbent 1.060 N/A 10 0.012 98.91 5 0.0200 98.21 2 0.1591 85.79

(29) TABLE-US-00005 TABLE 3C Acid-activated Bentonite C/ Residual Arsenic Removed Arsenic Water Volume (g/L) Content (μg/g) wt % Baseline - No Adsorbent 1.060 N/A 10 0.046 95.64 5 0.1877 83.24 2 0.4709 57.95

(30) The above description is for the purpose of teaching the person of ordinary skill in the art how to practice the invention. It is not intended to detail all of those obvious modifications and variations, which will become apparent to the skilled worker upon reading the description. It is intended, however, that all such obvious modifications and variations be included within the scope of the invention which is defined by the following claims. The claims are meant to cover the claimed components and steps in any sequence that is effective to meet the objectives there intended, unless the context specifically indicates the contrary.