Supplementary Data (SD)
Journal of Hazardous Materials
Soil retention of hexavalent chromium released from construction and demolition waste in a road-base-application scenario
Stefania Buteraa*, Stefan Trappa, Thomas F. Astrupa, Thomas H. Christensena
a Technical University of Denmark, Department of Environmental Engineering, Building 115, 2800 Lyngby, Denmark
* Corresponding author: Stefania Butera,
Technical University of Denmark, Department of Environmental Engineering, Building 115, 2800 Lyngby, Denmark
,
tlf: +45 72201951, fax: +45 4593 2850.
This SD document includes text, tables, and figures with details of materials and experimental methods, measured pH, concentrations and relative standard deviation of every sampling, results of all blank tests, detailed description and results of models and sensitivity analysis.
1. Experimental
Three subsoils, retrieved from a soil archive in Denmark, were used for the test; the soils were characterized by moderate acidity, and low soil organic matter (SOM) content (0.4-0.7%) as expected in the subsoil under a road base. The main features of the soils (S1, S2, S3) are presented in Table S1. Soil S1 had the lowest pH, S2 had the highest organic matter content, while S3 was characterized by the highest cation exchange capacity (CEC), clay content, Fe ad Mn concentrations. Cr background concentration in soils was in the range 4-20 mg∙kg-1TS.
Table S1. Main properties of the three analyzed soils. (n.d.= not determined). TS: total solid.
S1 / S2 / S3Depth / 50-100 / 55-100 / 65-100
Horizon / C-horizon / C-horizon / C-horizon
Texture / Fine sand - mixed with clay / Coarse sand / Clay mixed with sand
Clay (<0.002 mm) / % / 8.1 / 2.4 / 14
Silt (0.002-0.02 mm) / % / 8 / 0.8 / 11
Fine sand (0.02-0.2 mm) / % / 49 / 16 / 49
Coarse sand (0.2-2 mm) / % / 35 / 80 / 26
Soil Organic Matter (SOM) / % / 0.3 / 0.4 / 0.3
Cation exchange capacity (CEC) / meq∙kg-1TS / 47 / 26 / 80
pH (H2O) / - / 5.3 / 6.1 / 7.3
pH (CaCl2) / - / 4.4 / 5.3 / 6.4
Total N / % / 0.011 / 0.008 / 0.014
Extractable B / mgB∙kg-1TS / 0.43 / 0.08 / 0.51
Ca / gCa∙kg-1TS / 1.3 / 0.32 / 2.4
Fe / gFe∙kg-1TS / 9.4 / 3.8 / 17
Dithionite-citrate Fe / gFe∙kg-1TS / 2.8 / n.d. / 5.6
K / gK∙kg-1TS / 1.2 / 0.49 / 2.4
Mg / gMg∙kg-1TS / 1.9 / 0.57 / 3.3
Na / mgNa∙kg-1TS / 110 / 70 / 180
Cd / mgCd∙kg-1TS / 0.08 / 0.04 / 0.04
Co / mgCo∙kg-1TS / 3.1 / 1.2 / 5.0
Cr / mgCr∙kg-1TS / 12 / 4.1 / 20
Cu / mgCu∙kg-1TS / 1.9 / 0.35 / 3.8
Mn / mgMn∙kg-1TS / 240 / 91 / 270
Dithionite-citrate Mn / mgMn∙kg-1TS / 82 / n.d. / 55
Ni / mgNi∙kg-1TS / 7.2 / 3.0 / 10
Pb / mgPb∙kg-1TS / 5.1 / 2.2 / 5.1
Zn / mgZn∙kg-1TS / 19 / 9.5 / 28
Two C&DW leachates obtained from batch tests EN 12457 at L/S 2 and 10 L·kg–1TS were used [1,2]. The choice of having two different liquid-to-solid ratios (L/S) was made in order to assess the effect of different background ion concentrations; the C&DW used to produce the leachate originated from previous investigations and sampling details can be found elsewhere [3]. Leachate obtained at L/S 10 had very low Cr concentrations (20 mg Cr·L–1) which were not considered relevant for this study, and therefore it was spiked with 1 mg·L–1 K2CrO4 solution to approximately 100 mg Cr·L–1 (i.e. the Cr concentration in the L/S 2 leachate). Additionally, aliquots of both leachates were spiked to approximately 500 and 200 mg·L–1 based on previous column studies on C&DW [4] in order to represent typical maximum (initial), and medium chromium levels from road base scenarios, respectively. The leachates were measured after spiking, and their composition is given in Table S2, together with the soil leachate composition as measured after 48h contact time with each of the three soils, as a reference. It should be noted that soil leachate composition was highly variable depending on contact time, especially for Ca, Fe, K, Na, P, Si (major elements) suggesting that equilibrium was likely not reached between the solid and the liquid phase. Therefore such values should be taken with caution and are only provided as a reference for comparison with the C&DW leachates.
Batch Cr retention tests were carried out in PE bottles by mixing, in full factorial design, each soil type with each leachate type at an L/S ratio of 10 L·kg–1TS. All tests were carried out in duplicate. Additionally, 6 blanks were run (i.e. leachates without soil) to take into account possible processes taking place in the leachate itself over time. Three more blanks consisted of soil mixed with distillated water, to account for possible releases from the soil itself. The bottles were rotated during the whole duration of the test, i.e. approximately 6 months, during which six samples were collected from each bottle at specific times (2 d, 10 d, 30 d, 80 d, 130 d, 180 d); additionally, a first sample was taken from each bottle right before the start of the test. As a result of the samplings, the L/S ratio varied by less than 15% in total. Table S3 provides an overview of the test design.
Table S2. Composition of the three soil leachates after 48h contact time in batch test, and composition of the C&DW leachates at L/S 2 and 10 (produced according to batch standard test methods EN 12457-1 and EN 12457-2 respectively [1,2] (n.d.= not determined).
Soil leachate S1 / Soil leachate S2 / Soil leachate S3 / Leachate L/S 2 / Leachate L/S 10pH [-] / 5.37 / 6.01 / 7.26 / 11.4 / 11.4
Conductivity [mS/cm] / n.d. / n.d. / n.d. / 1.2 / 0.7
Ionic strength [M] / n.d. / n.d. / n.d. / 0.017 / 0.010
Solution concentrations [mg∙L-1]
Al / 210 / 160 / 10,000 / 1,100 / 1,600
As / 16 / 31 / 53 / 15 / 12
Ba / <7 / 16 / 72 / 160 / 140
Be / <0.5 / <0.5 / <0.5 / <0.5 / <0.5
Ca / 650 / 1,100 / 5,200 / 120,000 / 80,000
Cd / 1 / <0.5 / <0.5 / <0.5 / 1.6
Co / <1.5 / <1.5 / <1.5 / 10 / 4.8
Cr / 14 / 15 / 23 / 110/240/490 / 100/230/520
Cu / 15 / <7 / 12 / 93 / 67
Fe / 95 / 41 / 7,500 / 76 / <34
K / 3,700 / 1,400 / 2,700 / 150,000 / 24,000
Li / 5 / <1.5 / 7 / 35 / 13
Mg / 200 / 230 / 1,700 / 220 / 78
Mn / 22 / 40 / 65 / 2.2 / 6.7
Mo / 19 / <3.5 / 7 / 17 / 7.2
Na / 5,600 / 4,800 / 11,000 / 91,000 / 20,000
Ni / 12 / 14 / 24 / 29 / 7.9
P / 140 / 2,000 / 370 / 130 / 22
Pb / 11 / 18 / 44 / 19 / 19
S / 1,500 / 580 / 1,500 / 64,000 / 24,000
Sb / <7 / 17 / 64 / 33 / 27
Se / 79 / 34 / 29 / 10 / 76
Si / 1,500 / 640 / 19,000 / 14,000 / 11,000
Sn / 11 / 34 / 34 / 10 / 13
Sr / <3.5 / 5 / 24 / 1,100 / 530
V / <7 / 8 / 27 / 33 / 23
Zn / 7 / 22 / 32 / 10 / 9.0
Table S3. Overview of the test design. All tests were run in duplicate. Seven samples were taken during 6 month time (including initial sample before mixing of solid and liquid phase).
L/S 2 / L/S 10 / BlankCr conc [mg∙kg-1TS] / 100 / 200 / 500 / 100 / 200 / 500 / No leachate (DW)
Blank / No soil / 100b-LS2 / 200b-LS2 / 500b-LS2 / 100b-LS10 / 200b-LS10 / 500b-LS10 / -
Test with soil / Soil S1 / S1-100-LS2 / S1-200-LS2 / S1-500-LS2 / S1-100-LS10 / S1-200-LS10 / S1-500-LS10 / S1-b
Soil S2 / S2-100-LS2 / S2-200-LS2 / S2-500-LS2 / S2-100-LS10 / S2-200-LS10 / S2-500-LS10 / S2-b
Soil S3 / S3-100-LS2 / S3-200-LS2 / S3-500-LS2 / S3-100-LS10 / S3-200-LS10 / S3-500-LS10 / S3-b
At each sampling, 20mL of eluate were extracted after decantation, and filtrated (0.45mm PTFE filters) after pH measurement. In some cases additional filtration was necessary due to the presence of small particles, using first vacuum filtration (1.5 mm glass microfiber filters) and then 0.22 mm nylon filters. Prior tests indicated that the additional filtration steps did not have any significant influence on the measured concentrations. During the samplings, care was used to minimize the exposure of the leachate-soil mixtures to atmospheric oxygen. However this could not be fully avoided, and therefore the measurements of reducing capacity of soils should be interpreted as a conservative estimation because experiments have not been carried out in anaerobic conditions.
Cr was determined by ICP-OES; limit of detection (LOD) was 1.5 mg∙L-1. Additionally, cationic Cr was separated by cation exchange cartridge (Maxi-Clean 1.5 ml IC-H Alltech) as described by Ball and McCleskey [5]; at existing pH conditions, all Cr(VI) was present in anionic form (chromate/dichromate), and therefore determination of Cr(VI) by subsequent ICP-OES was possible.
2. Modeling
2.1. Cr reduction model
Measured Cr(VI) concentrations in the C&DW leachate and subsoil mixture were fitted according to a second order kinetic model:
(S1)
where CCr(VI)(t) (meq∙L-1) is the dissolved Cr(VI) concentration at time t (days), k (L∙ meq-1∙d-1) is a second order reduction rate constant, and R(t) (meq∙L-1) is the reduction capacity of the subsoil. Measured concentrations (mg∙L-1) were converted to meq∙L-1 by assuming 3 eq∙mol-1 Cr(VI) (three electrons are needed for the reduction of Cr(VI) to Cr(III)). The equation was solved numerically in Microsoft Excel™:
(S2)
(S3)
where pw is the pore water fraction (assumed 0.3), which takes into account that in the subgrade only 30% of the volume is filled with water, meaning that only 30% of the reduction capacity is used. Parameters k and R0 (i.e. R(t=0)) were fitted by minimizing the sum of absolute residuals between experimental data and model-predicted concentrations (i.e. modulus of the difference between measured and calculated value). Outliers, defined as those measured Cr(VI) concentrations larger than CrTOTAL concentration, were excluded from the fit. Goodness of fit was evaluated correlating measured and fitted data using the R2 criterion.
2.2. Cr subsoil migration model
To provide a range of realistic results, the calculation of vertical transport and reduction of Cr(VI) was carried out according to a best and a worst case scenario, by choosing:
- A best and a worst-case source term, i.e. release functions from the C&DW layer in the road base. Two C&DW samples from previous lysimeter experiments [4] were selected, giving respectively the lowest and highest Cr(VI) release from C&DW layer in the road base; and subsequently
- Two sets of k and R0 from the second order reduction model described above (Eq. S1), to represent the least and the most reducing soil/leachate combination (corresponding to experiments S3 L/S 10 and S1 L/S 2 respectively).
First, a release function of Cr(VI) from the C&DW layer above the soil was modelled based on measured concentrations from six C&DW lysimeters [4] and calculated as:
(S4)
where CCr(VI)(t) (meq∙L-1) is the dissolved Cr(VI) concentration at time t (years), mCr(VI)(t) (meq∙kg-1) is the amount of Cr(VI) available for dissolution in the C&DW, and Kd C&DW (L∙kg-1) is a distribution or partition coefficient for Cr(VI) in the C&DW between the C&DW itself and the liquid phase in contact with the C&DW. Based on data reported by Butera et al., 2015 [4], Cr(VI) is released from C&DW lysimeters in high initial concentrations with a steep decrease towards an almost constant concentration after L/S ≈ 1 L∙kg-1. The Kd C&DW is therefore not constant. Since most of the total release is expected to take place in the initial period (approximately L/S ≤ 1 L∙kg-1), it was considered more important to approximate the first part of the curve closer. Therefore, two Kd C&DW were estimated as the minimum and maximum (among the six C&DW samples investigated by Butera et al. [4]) cumulative release until L/S = 1 L∙kg-1 (approximately 20 mg·kg–1and 150 mg·kg–1) divided by the minimum and maximum initial concentration respectively (approximately 40 mg·L–1and 450 mg·L–1); minimum and maximum values, chosen among the six C&DW samples of Butera et al., [4] corresponded to samples named CDW1 and CDW3. From L/S 1 onwards (until approximately 100 years), the release from C&DW was approximated by a discontinuous function constituted by the measured concentrations from the lysimeters at each sampling.
Figure S1. Comparison between measured (Butera et al., 2015 [4]) and modelled Cr(VI) concentrations: “Calculated Cr(VI) min” represents the best case scenario, i.e. lowest release (corresponding to C&DW sample 1), while “Calculated Cr(VI) max” represents the worst case scenario, i.e. highest release (corresponding to C&DW sample 3).
Then, the Cr(VI) released from the C&DW was estimated at each time t according to Eq. S3. Time can be equivalently expressed as L/S value, through the formula [6]:
(S5)