Corrosion-resistant, reactive adsorbent for treatment of contaminated water, methods for producing same and use thereof

Abstract

The invention relates to a corrosion-resistant, reactive adsorbent which is made up of element iron on a carbon carrier plus sulfur and additional phosphorus as well as a method for producing this reactive adsorbent and use thereof for removal of reductively degradable pollutants in contaminated groundwater and wastewater.

Claims

1. A corrosion-resistant, reactive adsorbent containing nanoparticles of zero-valent iron on a carbon carrier, and a coating of sulfur and phosphorus on surfaces of the nanoparticles of zero-valent iron.

2. The reactive adsorbent according to claim 1, characterized in that the reactive adsorbent contains 10 to 40 wt % zero-valent iron, 40 to 70 wt % carbon, 0.01 to 5 wt % P and 0.01 to 5 wt % S.

3. The reactive adsorbent according to claim 2, characterized in that the reactive adsorbent contains 0.1 to 2 wt % P and 0.1 to 2 wt % S.

4. A method for producing a reactive adsorbent according to claim 1, characterized in that a basic composite material of zero-valent iron in nanoparticulate form is created on a carbon carrier and is converted to a form that is reactive and corrosion-resistant at the same time by treatment with phosphorus and sulfur compounds in aqueous suspension.

5. The method according to claim 4, characterized in that to create the basic composite material, the carbon carrier is loaded with an iron compound by wet impregnation, and then the iron is reduced to zero-valent iron in a nanoparticulate form by thermal treatment after drying.

6. The method according to claim 5, characterized in that a water-soluble iron(III) compound is used as the iron compound for impregnation.

7. The method according to claim 4, characterized in that the basic composite material is transferred to an oxygen-free aqueous suspension, and the suspension is then treated with dissolved phosphorus compounds and dissolved sulfur compounds.

8. The method according to claim 4, characterized in that water-soluble orthophosphates, polyphosphates, pyrophosphates or metaphosphates are used as the phosphorus compounds.

9. The method according to claim 4, characterized in that water-soluble sulfides, dithionites or dithionates are used as the sulfur compounds.

10. The method according to claim 4, characterized in that activated carbon is used as the carbon carrier.

11. The method according to claim 10, characterized in that powdered activated carbon (PAC) with a particle size of 0.5 to 50 μm is used as the carbon carrier.

12. A process of treating water contaminated with hydrophobic pollutants, the process comprising contacting a corrosion-resistant, reactive adsorbent according to claim 1 with the water contaminated with hydrophobic pollutants, in particular for to achieve dechlorination of chlorohydrocarbon compounds in the water contaminated with hydrophobic pollutants.

Description

DETAILED DESCRIPTION

Exemplary Embodiments

Example 1

(1) Reduction of a Basic Composite Material

(2) a) A commercial powdered activated carbon (PAC, particle size: 0.5 to 50 μm) is loaded with iron(III) nitrate in a PAC-to-Fe weight ratio of 3:1 by wet impregnation from an aqueous solution. Loaded and predried PAC is then first reduced by stepwise heating in a stream of nitrogen, then reduced in a stream of hydrogen at temperatures up to 550° C., while the following chemical reactions take place:

(3) ##STR00001##

(4) The result is a reactive adsorbent, comprised of 25 wt % Fe.sup.0 and 68 wt % C with 7 wt % residues (consisting primarily of iron oxides).

(5) The resulting pyrophoric powder containing iron particles in finely divided form in and on the carbon carrier, is transferred to water bubbled with nitrogen and treated further in various ways. b) By analogy with “a) reduction in a stream of hydrogen,” reduction in a stream of nitrogen was carried out at temperatures up to 750° C., with the carbon of the carrier material PAC acting as the reducing agent in this variant.

(6) The result is a reactive adsorbent of 28 wt % Fe.sup.0, 62 wt % C and 10 wt % residues (consisting primarily of iron oxides). c) Stability testing of the basic composite material

(7) ZVI-C basic composite material (prepared according to Example 1a) was stored at an approximately neutral pH in tapwater (10 g/L) bubbled with nitrogen while agitating gently. The ZVI corrosion was tracked continuously by measuring the hydrogen formed. After 10 days, the supernatant water was decanted, and the moist residue was mixed with concentrated hydrochloric acid. In doing so, all the iron compounds dissolved. Hydrogen was formed from metallic iron according to the equation Fe.sup.0+2 HCl.fwdarw.H.sub.2+FeCl.sub.2. The volume of the hydrogen was determined and used as a measure of the ZVI still present at the respective point in time. After storing for two weeks in an aqueous suspension, approx. 75% of the original ZVI content of 25 wt % could still be detected. After another 4 weeks, 50% of the original content was still detectable, and 20% after two months. The corrosion rate of the ZVI can be described with its half-life (t.sub.50%_ZVI) as a stability parameter. The stability of this batch with respect to anaerobic corrosion at t.sub.50%_ZVI of approx. 6 weeks is still not sufficient for the desired active period of a few months to years.

(8) In parallel with the stability test, a reactivity test was also carried out with this batch (prepared according to Example 1a). The ZVI-C basic composite material in a concentration of 1 g/L in aqueous suspension (deionized water) was mixed with 10 mg/L tetrachloroethene (PCE) for this purpose and agitated gently but continuously in a sealed reaction vessel. The concentrations of PCE and chloride ions in solution were determined by analysis by gas chromatography and ion chromatography. PCE was removed from the aqueous solution quickly and almost completely by adsorption on the composite material. Then a dechlorination reaction took place, and was tracked on the basis of the increase in the chloride concentration and the occurrence of gaseous chlorine-free hydrocarbons, in particular ethene and ethane, as products of complete dechlorination. Dechlorination approximately conforms to first-order kinetics and will be described on the basis of the half-life parameter (t.sub.50%_PCE, reaction time for a 50% conversion of PCE). The half-life of the PCE under the conditions described was t.sub.50%_PCE approx. 12 days.

Example 2

(9) Preparing a Basic Composite Material Treated with Sulfur (Comparative Example)

(10) Basic composite material ZVI-C (prepared according to Example 1a) was transferred to deionized water bubbled with nitrogen (10 g/L) and agitated moderately for 24 hours after adding 100 mg/L sodium sulfide (Na.sub.2S nonahydrate). Next, the solids were removed by decanting and were freed of the remaining dissolved sulfide by washing several times with deionized, degassed water.

(11) The result was a reactive adsorbent of 24 wt % Fe.sup.0, 68 wt % C, approx. 0.15 wt % S and the remainder iron oxides.

(12) The material treated with sulfur was suspended in tapwater and tested for corrosion resistance and dechlorination reactivity, as described in Example 1c. Initially only a slight formation of hydrogen was measured due to iron corrosion (4% of the maximum value after two weeks). However, then the iron corrosion accelerated progressively, reaching approx. 90% of the maximum value after a total of 10 weeks. For the period of accelerated ZVI corrosion, the half-life t.sub.50%_ZVI amounted to only approx. 1 week.

(13) In contrast with anaerobic corrosion, the sulfur treatment did not have a significant effect on the initial dechlorination activity of the composite material. PCE was dechlorinated from the beginning with a half-life t.sub.50%_PCE of approx. 14 days. However, its degradation was further accelerated in the accelerated ZVI corrosion phase (t.sub.50%_PCE approx. 5 days).

(14) As indicated above, a plain sulfur treatment causes only temporary stabilization of the basic composite material. Then there is accelerated ZVI corrosion, leading to an unsatisfactory overall lifetime of the material.

Example 3

(15) Preparing a Basic Composite Material Treated with Phosphorus (Comparative Example)

(16) Basic composite material ZVI-C (prepared according to Example 1a) was transferred to deionized water bubbled with nitrogen as described in Example 2 (10 g/L) and then agitated moderately for 24 hours after adding 200 mg/L dibasic sodium phosphate. The pH was then shifted slightly to the basic range (to pH 8.5). Next, the solids were removed by decanting and the mixture was freed of the remaining dissolved phosphate by washing several times with deionized degassed water.

(17) The result was a reactive adsorbent of 25 wt % Fe.sup.0, 68 wt % C, approx. 0.2 wt % P and the remainder iron oxides.

(18) The basic composite material ZVI-C treated with phosphorus was suspended in tapwater and tested for corrosion resistance and dechlorination reactivity as described in Examples 1c and 2.

(19) The ZVI corrosion was not reduced significantly by the phosphate treatment. A half-life t.sub.50%_ZVI of approx. 5 to 6 weeks was measured. The dechlorination activity was reduced slightly to approx. 70% in comparison with the untreated material (t.sub.50%_PCE=17 days).

(20) As indicated above, a simple phosphate treatment does not have a positive effect on the stability and reactivity of the ZVI-C composite material.

Example 4

(21) Preparing a Reactive Adsorbent According to the Invention by Treating the Basic Composite Material with a Phosphorus Compound and a Sulfur Compound

(22) Basic composite material ZVI-C (prepared according to Example 1a) was transferred to deionized water bubbled with nitrogen (10 g/L), as described in Examples 2 and 3, and then, after adding 200 mg/L dibasic sodium phosphate, and shortly after that, 100 mg/L sodium sulfide, it was agitated moderately for 24 hours. The pH was shifted slightly into the basic range (to pH 9). Then the solids were removed by decanting and freed of the remaining dissolved phosphate and sulfide by washing several times with deionized degassed water. The result was a reactive adsorbent of 24 wt % Fe.sup.0, 68 wt % C, approx. 0.2 wt % P, approx. 0.15 wt % S and the remainder iron oxides.

(23) The basic composite material ZVI-C treated with phosphorus and sulfur was suspended in tapwater and tested for corrosion resistance and dechlorination reactivity again as described in Examples 1c, 2 and 3.

(24) Result

(25) ZVI corrosion was greatly reduced through the combined phosphate-sulfide treatment. After a reaction time of 6 months in suspension in tapwater, the residual ZVI content was still 60% of the initial value. This corresponds to a half-life t.sub.50%_ZVI of approx. 7 months. No accelerated corrosion phase was observed, such as that described for the S-modified composite material in Example 2.

(26) The dechlorination activity of the reactive adsorbent according to the invention was not influenced negatively by the combined S—P pretreatment. PCE was dechlorinated with a half-life t.sub.50%_PCE=10 days. The persistently high dechlorination activity was demonstrated by repeated addition of PCE over the entire observation period of 6 months.

Example 5

(27) Basic composite material ZVIC (prepared according to Example 1 b) was tested by analogy with example 4 both with and without (as in Example 1c) the addition of phosphorus and sulfur but with variations in the order of addition of the phosphorus and sulfur compounds: a) As in Example 1c, 10 g/L basic composite material ZVI-C from Example 1 b was placed in deionized water at an approximately neutral pH, and its corrosion resistance was investigated and the material was subjected to a reaction test with PCE. Much like the basic composite material ZVI-C from Example 1a, a corrosion tendency as with an iron half-life of t.sub.50%_ZVI of approx. 6.5 weeks was found for the basic composite material ZVI-C prepared according to Example 1 b. Dechlorination of the PCE was also accomplished with first-order kinetics and a half-life of t.sub.50%_PCE of 12 days. It was thus found that the two basic composite materials ZVI-C prepared in Example 1 had a comparable initial reactivity. b) As in Example 4, the basic composite material ZVI-C (prepared according to Example 1 b in a stream of nitrogen at 750° C.) was stabilized with phosphorus and sulfur compounds. To do so, the procedure described in example 4 was selected but first the 100 mg/L sodium sulfide was added to the 10 g/L reactive adsorbent and 200 mg/L dibasic sodium phosphate was added after an interval of 10 minutes and then agitated moderately for 24 hours. After decanting the solids and washing with deionized, degassed water several times, the reactive adsorbent was found to contain 26 wt % Fe.sup.0, 63 wt % C, approx. 0.2 wt % P, approx. 0.15 wt % S and residues of iron oxides. The material was subjected to corrosion tests and dechlorination tests, which show as, in Example 4, that the combined use of phosphorus and sulfur additives definitely has a positive effect on the long-term stability of the reactive adsorbent, while maintaining its dechlorination activity. The corrosion was greatly suppressed. The half-life of iron t.sub.50%_ZVI in tapwater was slightly longer than 6 months (residual Fe.sup.0 content still amounting to 53% of the starting value). The dechlorination activity with respect to PCE was even somewhat higher than that found in Example 4. The half-life t.sub.50%_PCE for the first addition of PCE was found to be 8.5 days. The half-life of the pollutant for each of the two additional PCE additions was 9 days.

(28) The results in Examples 4 and 5b can be assessed as similar. In both experiments, the advantage of the combined addition of phosphorus and sulfur to the basic composite material ZVI-C can be recognized. In each case, it was found that the corrosion tendency of the iron metal was greatly reduced, but the rate of dechlorination was at least approximately the same. Thus, the iron component in the composite material can be utilized much more effectively for the target reaction (i.e., dechlorination) and increases the lifetime of the purification material for the water purification.