NL2031251B1

NL2031251B1 - Method for cleaning a contaminated sand containing composition

Info

Publication number: NL2031251B1
Application number: NL2031251A
Authority: NL
Inventors: Wilhelmus Van Dam Hendrikus; Lugthart Mark
Original assignee: Boskalis Bv Baggermaatschappij
Priority date: 2022-03-11
Filing date: 2022-03-11
Publication date: 2023-09-20

Abstract

The present invention relates to a method for cleaning a contaminated sand containing composition. In accordance with the present invention charcoal particles are added to a sand containing composition before or after making an aqueous slurry. The method of the invention results both in cleaner washing water, as well as a cleaner sand product.

Description

METHOD FOR CLEANING A CONTAMINATED SAND CONTAINING COMPO-

SITION

The present invention relates to a method for cleaning a contaminated sand containing composition.

Introduction

Soil pollution is a serious problem, especially in case of degradation resistant contaminants, which include hydro- carbons such as perfluoroalkyl and polyfluoroalkyl sub- stances (PFAS). PFAS have gained particular attention be- cause these compounds are mobile in soil subsurfaces and highly resistant to natural degradation processes. This may render soil source areas potential sources of groundwater contamination. Moreover, government regulations allow much lower concentrations of PFAS than many other contaminants.

It is therefore of high importance to remove PFAS from soil and to prevent entry of PFAS in groundwater.

Soil washing with water has been applied to contami- nants such as PFAS, for instance as described in WO 2018/232461 Al. Conventional approaches such as the approach described in WO 2018/232461 Al require cleaning of the liq- uid rinse water waste streams produced during the cleaning process, which normally rely on conventional water treatment technologies such as granular activated carbon (GAC) or ion exchange. This involves dedicated equipment and requires time and resources. The inventors therefore consider that there is room to improve efficiency of soil cleaning, in particular with regard to cleaning of sand containing com- position, in particular the remediation of sandy soils.

Description of the invention

The present invention relates to a method for cleaning a contaminated sand containing composition, comprising: i) obtaining an aqueous slurry containing sand particles,

contaminants and charcoal particles; ii) maintaining said aqueous slurry for a sufficient time to allow binding of sald contaminants to said charcoal particles; iii) separat- ing said charcoal particles from the sand particles to ob- tain a first fraction comprising charcoal particles with contaminants bound thereto, and a second fraction comprising sand particles.

In accordance with the present invention charcoal par- ticles are added to a sand containing composition before or after making an aqueous slurry. The slurry is maintained for a sufficient time to allow adsorption of said contaminants to said charcoal particles. After that the charcoal parti- cles can be separated from the sand particles and be disposed of or recycled for another cycle of adsorption. The rinsing water used in the slurry can also be separated from the sand and/or the charcoal particles by conventional means for de- watering solids, including without limitation screening, centrifugation, hydrocyclone or upwash classification or the like.

The inventor has found that the method of the invention results both in sand that is sufficiently depleted from contaminants as well as rinsing water that is sufficiently depleted from contaminants. Both products are clean to such an extent that they can be released in the environment with- out further treatment.

It is in particular noted in this respect that sand under most circumstances carries some associated water ad- hering to the sand particles. This water may contain con- taminants. Conventional cleaning methods result in a cleaned sand with a substantial amount of adhering water. This sand particle associated water still contains substantial amounts of contaminants, which may leach into the groundwater in due course if the sand is used in a soil area. The inventors have observed that with the cleaning method according to the invention, associated water of cleaned sand contains sub- stantially decreased amounts of contaminants compared to cleaned sand obtained by conventional water based sand cleaning methods. In other words, the method of the in- vention results both in cleaner washing water, as well as a cleaner sand product, with minimal leaching of contaminants in the groundwater when the sand is used in soil areas.

As a further advantageous effect, the present invention does not require additional water treatment steps in order to remove contaminants from rinse water. Instead, the pro- cess of depletion of contaminants takes place in the sand slurry by adsorbing any contaminants to the charcoal parti- cles that are present therein. In other words contaminants can be neutralized in a single process unit, for instance a separation unit, such as a hydrocyclone. Conventional soil processing units can be used in this regard. Accordingly, the present method can be implemented in an existing con- ventional process line for remediation of sandy soils. This makes the method according to the invention efficient with regard to the required equipment, resources and time.

The method of the invention is directed to cleaning a sand containing composition. A sand containing composition is meant to be understood as any kind of composition that contains sand, preferably in a high percentage, such as > 50 w/v % of sand particles. Normally the composition is a sandy soil that requires remediation because of contami- nants. In the context of the present invention sand parti- cles are suitably mineral particles with a particle size between 40 um and 6 mm, such as particles between 60 um and 4 mm, depending on the desired sand product for further use.

Sand particle size ranges can vary depending on the desired properties. Sand fractions having a desired size range can be obtained using conventional separation means including without limitation screening, centrifugation, hydrocyclone or upwash classification or the like.

The method according to the invention is in particu- larly directed at cleaning sand containing compositions be- cause the mineral sand particles do not well adsorb to char- coal particles, while hydrocarbon contaminants such as PFAS have high affinity for charcoal particles. This makes sand containing compositions, and in particular sandy soils, highly suitable to be cleaned in accordance with the inven- tion.

Apart from sand particles the composition may also con- tain fine particles and coarse particles. Within the context of the invention fine particles are particles smaller than the sand particles, i.e. between 0 and 40 - 60 pm, depending on the desired specification of the cleaned composition, such as particles smaller than 60 um or smaller than 40 um.

Fine particles are also referred to in the art as “fines” and include solids such as very fine clay particles and silt. Within the context of the invention coarse particles are particles larger than the sand particles, i.e. larger than 4 or 6 mm, depending on the desired sand particle size range.

It is to be understood that within the context of the invention the term “sand containing composition” may encom- pass a soil composition containing coarse particles and/or fine particles, as well as a soil composition that does not contain coarse particles and/or fine particles. In other words, in the context of the invention a sand containing composition may also relate to a soil consisting essentially of sand particles.

If the sand containing composition to be cleaned con- tains coarse particles, it may be desired to separate these from the smaller particles such as sand particles and fine particles, as the contaminants predominantly appear to be associated with the smaller particles. In this case step 1) of the method is preceded with a step of screening a sandy soil to remove coarse particles from it. This way a sand containing composition containing sand particles, contami- nants and optional fine particles is obtained. This separa- tion can be done by sieving at a 20 mm diameter cut off, followed by sieving at a 4 mm diameter cut off.

In the context of the present invention charcoal par- ticles refer suitably to particles of activated carbon (also called activated charcoal}. In the art activated is sometimes replaced by active. In general, activated carbons are made in particulate form as powders or fine granules less than 1.0 mm in size with an average diameter between 0.15 and 0.25 mm. Smaller particles present a large surface 5 to volume ratio which allows maximal adsorption of contam- inants. It is therefore preferred that the charcoal parti- cles are in the form of charcoal powder. Such a powdered charcoal material is preferably made up of crushed or ground activated carbon particles, 95-100% of which will pass through a designated mesh sieve. The ASTM D5158 classifies particles passing through an 80-mesh sieve (0.177 mm} and smaller as powdered activated carbon.

Because said charcoal particles have high affinity for hydrocarbon compounds the present invention can be suitably applied to remove hydrocarbon contaminants. As mentioned above, within this class PFAS, such as PFOS (perfluoroc- tanesulfonic acid) and PFOA (perfluorooctanic acid), are of particular interest. In this regard the present invention may suitably relate to a method for cleaning a sandy soil from per- and polyfluorcalkyl substances (PFAS) contami- nants. Also dioxins or oily substances or other undesired hydrocarbon compounds can be removed with the method of the invention.

The charcoal particles should be maintained in the slurry for a sufficient time to allow binding of said con- taminants to said charcoal particles. Normally this involves agitation of the slurry during mixing. To allow sufficient binding it is in general desired that in step ii} of the method said charcoal particles are incubated with said slurry for at least 5 minutes. Suitably, said charcoal par- ticles are incubated with said slurry for a duration between 5 minutes and 1 hour. Although longer incubations will also result in sufficient adsorption of contaminants the inven- tors have observed that satisfying results are obtained us- ing these relatively short incubation times, in particular when charcoal particles are added in an amount of 0,01 to 1 wt.% on the basis of the dry weight of the sand containing composition. It is therefore preferred that said charcoal particles are added in an amount of 0,01 to 1 wt.% on the basis of the dry weight of the sand containing composition, preferably approximately 0,5 wt.%.

In one embodiment, obtaining an aqueous slurry contain- ing sand particles, contaminants and charcoal particles com- prises mixing a sand containing composition containing sand particles and contaminants with charcoal particles to obtain a particle mixture, followed by mixing said particle mixture with water to obtain said aqueous slurry. In this case a dry or semi-dry, non-slurry sand containing composition is mixed with dry charcoal particles first, followed by suspending the mixture in water to obtain an aqueous slurry. Mixing the charcoal particles with the dry or semi-dry sand containing composition may occur before or after screening out coarse particles, if there would be a need for this.

It is however preferred to generate an initial slurry or pre-slurry in which a sand containing composition is suspended first and then mix the charcoal particles into that initial slurry to obtain a second aqueous slurry con- taining the sand containing composition with contaminants and said charcoal particles. This second slurry can then be used as the aqueous slurry mentioned in step i) as defined above. Therefore, in another embodiment, obtaining an aque- ous slurry containing sand particles, contaminants and char- coal particles comprises mixing a sand containing composi- tion containing sand particles and contaminants with water to obtain a first slurry containing contaminants, followed by adding charcoal particles to said first slurry to obtain said aqueous slurry of step i). This is in particular pre- ferred because adsorption of contaminants to the charcoal will predominantly take place in the liquid phase, i.e. in the agueous suspension. Optimal mixing and adsorption be- tween charcoal and contaminants is also ensured in this case because a risk of charcoal particles aggregating with soil components is minimized. Also for practical purposes this embodiment is preferred, for instance in case of land

: reclamation and dredging which in general involves collect- ing soil in the form of an aqueous slurry, after which the cleaning process can start directly by adding charcoal par- ticles.

The charcoal particles may be added in the form of a powder or granulate but may also be added in the form of an aqueous charcoal slurry. Adding the particles in the form of a slurry is in particular advantageous when the charcoal is to be added to an initial slurry of a sand containing composition, such as the above mentioned first slurry. This prevents floating or floating layers of charcoal, and also has advantages with respect to dosing. A charcoal slurry may for instance be introduced by injection in a transfer line of the slurry of said sand containing composition.

In general, a sandy soil when reclaimed or dredged from an area contains fine particles, with the characteristics as described above. In general it is desired to separate the fine particles from the sandy particles, because fine par- ticles may have detrimental effects on the quality and fur- ther use of the cleaned sand. Importantly, the fine parti- cles normally also contain organic material with affinity for hydrocarbon contaminants. Removal of the fine particles will therefore often be necessary in order to obtain a cleaned sandy soil that meets environmental requirements.

Further, high levels of fine particles may increase water demand to achieve adequate workability and may also decrease compressive strength of concretes made from the cleaned sand. Therefore, in case the sand containing composition to be cleaned comprises fine particles it is preferred that the method of the invention further comprises a step of sepa- rating said fine particles from the sand particles prior or after adding said charcoal particles.

In the context of the invention separating the fine particles from the sand particles can be done by any suitable separating method, for instance by screening, centrifuga- tion, hydrocyclone or upwash classification or any combina- tion of these, wherein hydrocyclone or upwash classification or a combination thereof are preferred for practical pur- poses in view of the small size of the particles.

It is possible to separate said fine particles from the sand particles before adding charcoal to the sand particles.

Although this would require multiple separation steps, i.e. removing the fine particles and removing the charcoal par- ticles in two separate steps, this allows reuse of the char- coal particles, especially if these are not saturated with contaminants.

It is however preferred that said fine particles are separated from the sand particles after adding charcoal to the sand particles. This allows to separate the charcoal particles simultaneously with the fine particles from the sand particles. This is in particular advantageous when pow- dered charcoal is used. In this respect it is preferred that in case of the presence of fine particles, step iii) of the method of the invention comprises separating said charcoal particles and said fine particles from the sand particles to obtain a first fraction comprising charcoal particles with contaminants bound thereto and fine particles, and a second fraction comprising sand particles. The first frac- tion can be further processed to separate water from the solids to obtain clean water and a solid product of fine particles and charcoal bound contaminants.

In this respect the method of the invention according may suitably comprise after step iii) a further step of: iv) mechanically dewatering said first fraction to obtain fil- trate water and a sludge cake comprising said fine particles and charcoal particles with contaminants bound thereto. This can be done by means of gravity dewatering, and dewatering equipment including a filter press, belt filter press or decanter centrifuge. During such a process usually floccu- lants are used to optimize separation of solids from water.

The sludge cake can be disposed of via conventional way of disposing chemical waste.

After disposal of contaminated charcoal any rinsing water used in the cleaning process is sufficiently clean to be discharged in the environment. It is also possible to recycle any clean process water into the suspension of step i) of the method of the invention. In that respect it is preferred that any filtrate water obtained from separating the contaminated charcoal particles and optional fine par- ticles is recycled for use in step i) in another cycle of cleaning a contaminated sand containing composition in ac- cordance with steps i) to iii} of the method of the inven- tion.

Examples

The following tests represent exemplary examples of the cleaning process of the invention. The examples are not intended to limit the scope of the claims.

Comparative example

In a first series of tests a sandy soil was cleaned as follows. A sandy soil was first screened to separate coarse particles from it. This separation was performed by sieving at a 20 mm diameter cut off, followed by sieving at a 4 mm diameter cut off. A slurry containing sand particles and contaminants was obtained with a liguid/solid (L/S) ratio of 4. Such a slurry meets the requirements for separation in conventional separation installations. After the sieving steps an upwash classification step was performed to sepa- rate fine particles from the sand. A standard Boskalis sep- aration installation was used for this purpose.

The amount of PFOS in the water adhered to sand parti- cles in semi-dry state before making a suspension and after dewatering was determined by analyzing water leaching from the sand. Also the total amount of PFOS in the suspension was determined before separation and classification to re- move fine particles. The amount of PFOS in the washing water after separation and classification was also determined and represented as ug per litre water.

Three samples with different amounts of PFOS based on the weight of dry solids (ds) in the suspension were used:

Sample 1, Sample 2 and Sample 3. The initial total amounts in the suspension ranged from 11 to 1000 ug per kilogram dry solids. The results of the PFOS measurements are shown in table 1 below.

TT

Soil sample 2

PFOS in slurry before separa- | 1000 55 11 tion and classification {pg/kg ds)

PFOS in sand particles product | 52 6.4 after separation and classifi- cation (pg/kg ds)

Removal efficiency 91.8%

PFOS in adherent water of | Not 59.3 10.9 sandy soil before suspension | tested (ng/kg ds) (particle associ- ated water)

PFOS in adherent water of sand | Not 5.2 0.7 particles after separation and | tested classification (ng/kg ds) (particle associated water) associated water tested ter {pg/ltr)

Table 1: cleaning sand without active carbon powder

Table 1 shows that PFOS are removed from the sand with an efficiency between 88.4% and 94.5%. The amount of PFOS in sand particle associated water has been removed with an efficiency between 91.2% and 93.6%. There is however a sub- stantial amount of PFOS in the washing water: between 2.1 and 120 ug/ltr depending on the initial level of contamina- tion. Therefore, in order to be released in the environment subsequent steps of removal of the PFOS from the water would be required.

Test example

In a second series of tests a sandy soil was cleaned as described for the comparative example above, with the exception that active carbon powder was added to the sandy soil suspension in an amount of 0.5 wt? on the basis of dry solids of the soil sample. Four samples were tested: samples 4, 5, 6 and 7 with incubation times of said carbon powder in the slurry of 5, 15, 30 and 60 minutes, respectively.

Fa

Soil sample 4 5 5 |] powder in slurry

PFOS in slurry before separa- | 1300 1300 1300 1300 tion and classification (ug/kg ds)

PFOS in sand particles product | 31 12 39 27 after separation and classifi- cation (pg/kg ds) 97.6% 99.1% 97.0% 97.9%

PFOS in adherent water of | 880 880 880 880 sandy soil before suspension (ng/kg ds) (particle associ- ated water)

PFOS in adherent water of sand | 0.2 0.1 0.2 0.1 particles after separation and classification (ng/kg ds) (particle associated water) associated water ter (ug/ltr)

Table 2: cleaning sand with active carbon powder

Table 2 shows that PFOS are removed from the sand with an efficiency between 97.0% and 99.1%. The amount of PFOS in sand particle associated water has been removed with an efficiency of 100%. The amount of PFOS in the washing water is negligible: between 0.006 and 0.063 ng/ltr depending on residence time of the active carbon in the slurry. Thus, by mixing active charccal powder in the sandy soil slurry the cleaned sand is cleaner, not in the least because the sand particles associated water was unexpectedly fully depleted from contaminants. The cleaned sand can be processed for further use without the need for further cleaning and with- out the risk of leaching contaminants in the groundwater.

Also the washing water is clean at levels that permit release in the environment.

Claims

CONCLUSIONS

A method for cleaning a contaminated sand-containing composition, comprising: i) obtaining an aqueous slurry comprising sand particles, contaminants and carbon particles; ii) maintaining the aqueous slurry for a sufficient time to allow binding of the contaminants to the coal particles; iii) separating the coal particles from the sand particles to obtain a first fraction comprising coal particles with associated contaminants and a second fraction comprising sand particles.

Method according to claim 1, wherein the method takes place in a single process unit, which is a separation unit.

A method according to claim 1 or 2, wherein step i) comprises: mixing a sand-containing composition comprising sand particles and contaminants with coal particles to obtain a particle mixture; and mixing the particle mixture with water to obtain the aqueous slurry.

A method according to claim 1 or 2, wherein step i) comprises: mixing a sand-containing composition comprising sand particles and contaminants with water to obtain a first slurry; and adding coal particles to the first slurry to obtain the aqueous slurry.

A method according to claim 4, wherein the coal particles are added to the first slurry in the form of an aqueous coal slurry.

A method according to any one of the preceding claims, wherein the sand-containing composition comprises fine particles and wherein the method further comprises a step of separating fine particles from the sand particles before or after adding the coal particles.

A method according to claim 6, wherein fine particles are separated from the sand particles before adding coal.

A method according to claim 6, wherein fine particles are separated from the sand particles after adding coal.

A method according to claim 8, wherein step iii) comprises separating the coal particles and the fine particles from the sand particles to form a first fraction comprising coal particles with associated contaminants and fine particles and a second fraction comprising sand particles. to obtain a vessel.

A method according to claim 9, comprising the further step of: iv) mechanically dewatering the first fraction to obtain filtrate water and a sludge cake comprising the fine particles and coal particles with associated contaminants.

A method according to any one of the preceding claims, wherein water originating from the aqueous slurry obtained after step iii) is recycled for use in step 1) in a further cycle of the cleaning

applying a contaminant sand-containing composition according to steps 1) to iii) of claim 1.

Method according to any of the preceding claims, wherein step 1) is preceded by a step of sieving a sand-containing composition to remove coarse particles.

A method according to any one of the preceding claims, wherein the contaminants are hydrocarbon compounds.

The method of claim 13, wherein the hydrocarbon compounds include per- and polyfluoroalkyl compounds (PFAS).

A method according to any one of the preceding claims, wherein the separation in step iii) is carried out by means of sieving, centrifugation, hydrocyclone or upstream classification or any combination thereof.

16. Method according to any of the preceding claims, wherein in step ii) the coal particles are incubated with the aqueous slurry for at least 5 minutes

ry.

A method according to claim 16, wherein in step ii) the coal particles are incubated with the aqueous slurry for a period of time between 5 minutes and 1 hour.

A method according to any one of the preceding claims, wherein the coal particles are added in an amount of 0.01 to 1% by weight based on the dry weight of the sand-containing composition, preferably about 0.5% by weight. %.

A method according to any one of the preceding claims, wherein the carbon particles are in the form of activated carbon powder.

A method according to claim 19, wherein the carbon powder is a powdered activated carbon according to ASTM D5158.

A method according to claim 19, wherein the coal powder is in particulate form with a powder particle or granule size of less than 1.0 mm in size with an average diameter between 0.15 and 0.25 mm.

22. Method according to any of the preceding claims, wherein the sand-containing composition contains sand particles in a percentage of > 50 w/v%.

23. Method according to any of the preceding claims, wherein the sand particles in the sand-containing composition contain mineral particles with a particle size between µm and 6 mm, such as particles between 60 µm and 4 mm.

at.