Soil and sediment remediation

Abstract

A batch process of remediation of soil and sediment contaminated with toxic metals includes the steps of treating contaminated soil and sediment with a solution containing aminopolycarboxylic chelating agent, rinsing the soil/sediment solid phase to remove residues of mobilized toxic metals, treating the used process waters to recycle chelating agent and clean process solutions and placing the remediated soil/sediment on a permeable horizontal reactive barrier to prevent emission of contaminants. In the batch process, toxic metals are removed from process solutions by alkaline adsorption of polysaccharide adsorbents. By applying alkaline adsorption the efficiency of toxic metal removal from process solutions and alkaline and acidic recycling of chelating agent is significantly improved.

Claims

1. A batch process for remediation of soils/sediments contaminated with toxic metals selected from the group consisting of Pb, Zn, Cd, Cu, Hg, Ni, Mo, Tl, Cr, U, Cs and Sr, in a series of batch processes, said batch process comprising: (a) washing or leaching a solid phase of contaminated soil/sediment with washing solution, said washing solution comprising EDTA or other chelating agent from the group of aminopolycarboxylic acids and their salts and mixtures of thereof; (b) separating the washing solution from the solid phase to obtain a washed solid phase and used washing solution; (c) rinsing of the washed solid phase with one or several rinsing solutions; (d) subsequent separation of said rinsing solutions to obtain a rinsed solid phase and one or several used rinsing solutions; (e) alkalinisation of the used washing solution and used rinsing solutions obtained in steps (b) and (d), respectively, to value between pH 11.5 and pH 13.5 using calcium containing base, thereby precipitating mixture comprising used Ca(OH).sub.2 and toxic metal hydroxides; (f) subsequent supplementation of polysaccharide material to the used washing solution and used rinsing solutions treated in step (e) for alkaline adsorption of toxic metals; (g) subsequent separation of (i) the precipitated mixture of used Ca(OH).sub.2 and metal hydroxides, and (ii) polysaccharide material with adsorbed toxic metals from the used washing and rinsing solutions; and (h) re-using of the rinsing solution separated in step (g) for rinsing the solid phase in the subsequent batch process in said series of batch processes.

2. The batch process of claim 1, further comprising the steps: (i) acidification of the separated used rinsing solutions of step (g) containing chelating agent to the value between pH 3.0 and pH 1.5 using H.sub.2SO.sub.4, thereby precipitating the chelating agent; and (j) subsequent separation of the precipitated chelating agent from rinsing solutions of step (i).

3. The batch process of claim 1, further comprising the steps: (k) addition of the chelating agent separated in step (j) and of fresh chelating agent to the used washing solution of the step (g), thereby recycling the washing solution with chelating agent concentration between 10 and 250 mol per ton of soil/sediment processed and with pH value between 3.0 and 9.0 to be used in step (a) of the subsequent batch process in said series of batch processes; and (l) formulation of the rinsed solid phase of the step (c) by at least one additive selected from the group consisting of soil fertilizers, soil conditioners, toxic metals immobilizators and microbial inoculants to obtain the remediated soil/sediment.

4. The batch process of claim 1, wherein the volume of fresh water for final rinsing of the washed solid phase in step (c) is defined by the difference in water content in the rinsed solid phase and in the contaminated soil/sediment and by other water losses during the process.

5. The batch process of claim 1, wherein the used rinsing solution which is last separated from the solid phase in step (d) can be directly without further treatment used as a first rinsing solution in step (c) of the succeeding batch in a series of batch processes.

6. The batch process of claim 1, wherein of the group of used rinsing solutions treated in steps (e) and (f), the used rinsing solution last separated from the solid phase in step (d) is alkalinised with 0.1 to 30 kg m.sup.3 of fresh Ca containing base and treated with 0.2 to 10 kg m.sup.3 of fresh polysaccharide material for alkaline adsorption of toxic metals.

7. The batch process of claim 1, wherein the mixture of used Ca(OH).sub.2 and toxic metal hydroxides and the used polysaccharide material with adsorbed toxic metals separated from the given process solution in step (g) are re-used in alkalinisation step (e) and alkaline adsorption step (f) of the process solution which is series of batch processes in step (d) separated from the solid phase before the given process solution.

8. The batch process of claim 1, wherein the mixture of said used Ca(OH).sub.2 and toxic metal hydroxides precipitated in step (e) can be supplemented with fresh calcium, sodium or potassium containing base and with used Ca(OH).sub.2 and toxic metal hydroxides from previous batches.

9. The batch process of claim 1, wherein the mixture of used Ca(OH).sub.2 and toxic metal hydroxides and the used polysaccharide material with adsorbed toxic metals which are separated from the used washing solution in step (g) are in whole or in part removed from the process to ensure balance of inputs and outputs of calcium containing base and polysaccharide adsorbent throughout the series of batches.

10. The batch process of claim 1, further comprising the step: (m) deposition of the washed solid phase as remediated soil/sediment on a permeable horizontal reactive barrier.

11. The batch process of claim 10, wherein in the remediated soil/sediment deposition step (m) the permeable horizontal reactive barrier contains calcium-containing base as a reactive material and adsorbents selected from polysaccharides, aluminium and iron oxides and anion exchange resins as auxiliary reactive materials.

12. The batch process of claim 5, wherein the mixture of said used Ca(OH).sub.2 and toxic metal hydroxides precipitated in step (e) can be supplemented with fresh calcium, sodium or potassium containing base and with used Ca(OH).sub.2 and toxic metal hydroxides from previous batches.

Description

DESCRIPTION OF DRAWINGS

(1) FIG. 1. Flowchart of the preferred embodiment of the process according to the invention Soil and sediment remediation.

(2) FIG. 2. Alkalinisation of solutions with different concentration of EDTA with Ca(OH).sub.2 to pH 12.5. Increasing amounts of lime are needed due to pH buffering capacity of chelating agent.

(3) FIG. 3. Removal of Pb from the used process solution by alkaline precipitation and by alkaline adsorption on polysaccharide material at different pH. Used process solution contained 12 mM EDTA and 233 mg L.sup.1 Pb. The pH of the process solution was adjusted by addition of Ca(OH).sub.2 from 9.5 to 12.8 and afterwards by addition of NaOH. As a polysaccharide absorbent 4 g L.sup.1 of waste paper was used.

(4) FIG. 4. Removal of Pb from the used process solution by alkaline adsorption on different polysaccharide materials at pH 12.5. After initial substitution of Pb in the EDTA chelate with Ca from Ca containing base and removal of precipitated Pb the process solution contained 12 mM EDTA and 40 mg of Pb.

(5) FIG. 5. The dynamic of alkaline (pH 12.3) adsorption of Pb on recycled paper after alkaline substitution of Pb in the EDTA chelate with Ca from Ca containing base.

(6) FIG. 6. Removal of Pb from the used third (uRS3) and second rinsing (uRS2) and from used washing solution (uWS) by (A) single process of simultaneous substitution and adsorption and (B) by two separate processes of first alkaline substitution and subsequent alkaline adsorption. 3 kg m.sup.3 of fresh Ca(OH).sub.2 was supplied to uRS3. Used Ca(OH).sub.2 from uRS3 was amended with 2 kg of fresh Ca(OH).sub.2 and supplied to uRS2. Used Ca(OH).sub.2 from uRS2 was amended with additional 2 kg of fresh Ca(OH).sub.2 and supplied to uWS. 5 kg m.sup.3 of paper was supplied to uRS3 and re-used for alkaline adsorption in uRS2 and than in uWS.

(7) FIG. 7. EDTA balance (percentage of the total EDTA input in the process): chelant recycled from the used washing (A) and second rinsing solution (B), chelant lost from the process due to the soil binding (C), chelant lost from the process with wastes (D), fresh EDTA supplied to the process (E). Average EDTA concentrations from 8 remediation batches using contaminated soil from Mezica, Slovenia, were used in EDTA balance calculation.

(8) FIG. 8. Conductivity of the used washing (uWS), first (uRS1), second (uRS2) and third rinsing solution (uRS3) during the course of 24 consecutive remediation batches using soils from former Pb smelter sites in Mezica, Litija, Domzale in Slovenia, Arnoldstein in Austria and Pribram in Czech Republic.

(9) FIGS. 9A and 9B. Removal of Pb from the used washing (uWS) solution (FIG. 9B) and second rinsing (uRS2) solution (FIG. 9A) after alkalinisation with Ca(OH).sub.2 mixtures and with fresh Ca(OH).sub.2. Used process waters and Ca(OH).sub.2 mixtures with different Pb concentration were tested.

(10) FIG. 10. Deposition of remediated soil/sediment on permeable horizontal reactive to prevent toxic metals leaching and emissions.

(11) FIG. 11. Leaching of Pb from remediated soils from former Pb smelter sites in Mezica, Litija, Domzale in Slovenia, Arnoldstein in Austria and Pribram in Czech Republic after simulated rainfall. Remediated soils were placed into soil columns with and without installed permeable horizontal reactive barriers. Barriers were constructed in the bottom of soil columns by mixing fresh Ca(OH).sub.2 with remediated soil (weight ratio 1:10). The amounts of Ca(OH).sub.2 tested were 0.2, 0.5, 1.0 and 2.0% of the weight of remediated soil in the column.