The Effects of Portland and Sulphoaluminate Cements Solidification/Stabilization on Semi-Dynamic Leaching of Heavy Metal from Contaminated Sediment

Abstract

Cement-based solidification/stabilization technology is widely applied in the treatment of heavy metals in river sediment because it is an effective treatment, with the advantages of saving time and being economically and environmentally friendly. In this study, the heavy metal polluted sediment in Shanghai Fuxing Island Canal was used as the raw material, the cement solidified form was prepared by adding 10% Portland cement or sulphoaluminate cement, and semi-dynamic leaching tests were carried out on the solidified forms. In this study, we compare two types of cements as metal conditioners and curing agents aiming to determine the more economical and effective way to utilize river sediments. The results showed that the compressive strength of Portland cement solidified form (PSF) increased with an increase in curing time, which could reach 0.75 MPa after 28 days and met the requirements of general subgrade engineering. The compressive strength of sulphoaluminate cement solidified form (SSF) reached 0.35 MPa after curing for 1 day, however, it decreased later. The semi-dynamic leaching test results showed that the mobility of Cu and Cd in the cement solidified form was low, and the migration mechanism of heavy metals was mainly diffusion. The mobility of heavy metals in the PSF was lower than in the SSF, thus, the PSF had a better solidification effect and was more suitable for treating heavy metal-contaminated sediment.

1. Introduction

With the rapid development of urbanization and industrialization, heavy metals in wastewater have been discharged to natural rivers through municipal pipe networks and surface runoff and have accumulated in river sediments through migration and transformation, finally exceeding the self-purification capacity of rivers [1]. When river environments change significantly, the original adsorption/desorption balance is broken and heavy metals in the sediments are released, which can result in secondary pollution [2]. Heavy metal pollution in river sediment has become a global environmental problem, and therefore is a popular research topic. In recent years, heavy metal-contaminated sediment as a type of waste from river purification and management has become a concern, especially in China. The potential ecological risks of Cu and Cd are prominent, which is consistent with the studies on heavy metal pollution in river sediment [3]. It has been reported [4] that Cu has adverse effects on human health and long-term exposure to Cd could cause lung cancer, lung adenocarcinoma, renal function injury, and fracture.

Solidification/stabilization technology is one of the effective methods to treat heavy metal-contaminated sediment worldwide, it is economical, feasible, and highly efficient [5,6]. Because the metals are undegradable, solidification/stabilization is an efficient way to treat contaminated sediment. Cement [7] is a traditional hydraulic inorganic cementitious material. According to mineral composition, cement can be divided into three types: Portland cement, sulphoaluminate cement [8], and aluminate cement. Cement solidification is widely used in practice because of its good treatment effect, short treatment period, and economical efficiency [9]. The mechanical properties of aluminate cement are unstable during the later stage of the solidification reaction, and therefore, it is less applied. It has been reported [10] that the existence of heavy metals was unfavorable to the compressive strength of Portland cement, and that the cement composition affected its compressive strength and permeability during the stabilization process. In this research, Portland cement and sulphoaluminate cement are selected to study their properties in sediment solidification, as well as to study the migration behaviors of two heavy metals, i.e., Cu and Cd in PSF and SSF.

A semi-dynamic leaching test [11,12] can simulate the diffusion process and dissolution mechanism of heavy metals in solidified and stabilized sediment, and quantitatively describe the mobility of heavy metals by calculating the diffusion coefficient. The mobility of various heavy metals can be compared through the diffusion coefficient when the total amount of heavy metals in sediment is different, and therefore, can quantitatively evaluate the environmental safety of solidified and stabilized heavy metal sediment. Semi-dynamic leaching test methods include the USEPA method 1315 [13], ASTM C 1308-08 [14], and ANS16.1 [15]. Moon et al. [16,17] found that the dissolution mechanism of heavy metals in most cement stabilized heavy metal-contaminated soils was diffusion, however, there were very few discussions about the influence of the leaching solution pH on the heavy metal diffusion coefficient. Some other authors [18] have compared different organic acids used to leach heavy metals from polluted river sediment and found that the best leaching solution was tartaric acid at a concentration of 0.4 mol/L for 2 h. Gui et al. [19] analyzed the differences between leaching rates and leaching concentrations of heavy metals by using different leaching measures. They found that the leaching rates were mainly affected by occurring forms, ion exchange of H⁺, and acid etching to solid matrix; acid etching to solid matrix played the major role. However, there are limited comparisons between Portland cement and sulphoaluminate cement for heavy metal solidification/stabilization through a semi-dynamic leaching test.

The further utilization of sediments as a resource is actually highly relevant to sustainability. For instance, the stabilized and solidified bricks prepared from sediments and cement can substantially reduce mining productions. The solidified sediments can even be used as slope protection materials. Based on this, the raw material for this research is the heavy metal-contaminated sediment in the Shanghai Fuxing Island Canal. Two different types of cement are added to prepare the cement solidified form, and the mechanical properties of the solidified form are measured after curing for a certain period of time. Through a semi-dynamic leaching test, the change of pH during the leaching process is studied, the influences of two different types of cement on the mobility of heavy metals are discussed, and the diffusion coefficients of heavy metals under different leaching solution pH are analyzed, in order to provide a reference for the application of cement solidification/stabilization technology. Metal ion leaching tests were conducted corresponding to the properties of the two cements. The mechanism was also discussed according to the testing results. There are few studies concerning the relations among cement categories and their properties on the stabilization and mechanical strength.

2. Materials and Methods

2.1. Materials

The test sediment was taken from the Fuxing Island Canal in Shanghai (under the HaiAn road bridge). The surface sediment (0–10 cm) was collected using a stainless-steel grab sediment sampler (XDB0201, Xindibiao, Beijing, China) [20]. The water content was measured immediately with a halogen moisture meter (Mettler Toledo HB43-S, Mettler Toledo Instrument Co., Ltd. Shanghai, China). The main physical and chemical properties of the sediment are shown in

Table 1. The main physical and chemical properties of the sediment.

Properties	Value	Method
Natural moisture content (%)	38	Test Methods of Soils for Highway Engineering (JTG 3430-2020) [21]
Liquid limit (%)	36.8
Plastic limit (%)	24.3
Plasticity index (%)	12.7
pH value (L:S = 1:1)	7.56	ASTM D4972-13 [22]
Redox potential (mv)	−69.5	Redox potential tester
Organic matter content (%)	2.2	Loss-on-ignition method
Particle size distribution (μm)	5–75	Laser particle size distribution instrument
Chemical composition (%)	-	Analysis method for clay minerals and ordinary non-clay minerals in sedimentary rocks by X-ray diffraction (SYT 5163-2010) [23]
Fe2O3	5.72
Na2O	1.12
SiO2	63.75
CaO	5.33
MgO	3.27
K2O	3.11
Al2O3	15.98
Soil classification	Sandy clay

. The cements tested are PI42.5 Portland cement and sulphoaluminate cement produced by Conch Cement (Shanghai, China) Co., Ltd.

The heavy metals added to the sediments were in the form of Cu(NO₃)₂·3H₂O and Cd(NO₃)₄·4H₂O and were incubated for 15 days. The sediments before/after incubation were digested with mixed acids following a method detailed in previous studies [24,25]. Then, the metal concentrations in the digests were analyzed by inductively coupled plasma mass spectrometry (ICP-MS) (PerkinElmer, NexIon 300X, Shelton, CT, USA) The acid used in the sample analysis was Guaranteed Reagent to avoid contamination. The quality assurance and quality control (QA/QC) procedures for total concentration analysis were performed by analyzing three reagent blanks and triplicates for each sample (RSD ≤ 10.8%). The recoveries of a standard reference material sediment (SRM 1646a, National Institute of Standards and Technology) were in the range of 80~120%. The contents of heavy metals are listed in

Table 2. Contents of heavy metals in the sediment (mg/kg).

Heavy Metals	Cd	Cr	Ni	Pb	Zn	Cu
Sediment from Fuxing Island Canal	0.35	82.7	26.5	68.5	96.4	45.2
Background value [26,27]	0.13	75.0	31.9	25.5	86.1	28.6
Prepared sediment	0.98	68.0	26.9	158	92.3	68.5

.

2.2. Experimental Design

The unconfined compressive strength of the solidified form was tested according to the standard for geotechnical testing method (GB/T 50123-1999) [28]. The effective diffusion coefficient De was calculated according to ASTM C1308-08 [14] as follows:

First, the cumulative leaching rate (CLR) of heavy metal X under different leaching times was calculated according to Equation (1):

$$\mathrm{CLR}=\frac{{\displaystyle \sum {\mathrm{M}}_{\mathrm{n}}}}{{\mathrm{M}}_0}=\frac{{\displaystyle \sum _{\mathrm{i}=1}^{\mathrm{n}}{\mathrm{c}}_{\mathrm{x},\mathrm{i}}{\mathrm{V}}_{\mathrm{x},\mathrm{i}}}}{{\mathrm{M}}_0}=\frac{{\mathrm{M}}_{\mathrm{t}}}{{\mathrm{M}}_0}$$

(1)

where M_n is the mass of heavy metal X leaches at the Nth leaching (mg), M₀ is the total amount of heavy metal X in the solidified form (mg), C_x,i is the heavy metal concentration in the ith leachate (mg/L), V_x,i is the volume of the ith leachate (cm³), M_t is the accumulated amount of heavy metal in the leachate duration time t (mg).

De is obtained by fitting the slope of CLR-t^1/2:

$${\mathrm{D}}_{\mathrm{e}}=\frac{\mathsf{\pi }}{4}{\left(\frac{\mathrm{CLR}}{{\mathrm{t}}^{1/2}}\cdot \frac{\mathrm{V}}{\mathrm{S}}\right)}^2$$

(2)

where V is volume of the solidified form (cm³) and S is surface area of the solidified form (cm²).

The design of the semi-dynamic leaching test refers to Liu et al. [29], and the selected pH value refers to the average acid rain pH of 4.38 in Pudong, Shanghai [30]. The leachate pH values were set at 4.00 and 7.00. After 28 days of solidified form curing, leaching experiments were conducted on the solidified form at different initial pH values. Among them, the leachate in 4 groups of experiments was replaced regularly, which was marked as the RL group. The remaining 4 groups of leachate were kept unreplaced and were marked as the UL group. The leaching test lasted for 43 d (1032 h), and the procedure is shown in Figure 1. The metal concentration in leachate was measured by ICP-MS after 0.43 μm membrane filtration.

3. Results

3.1. Unconfined Compressive Strength Test

Figure 2 shows the unconfined compressive strength test results for PSF and SSF that was cured at 20 °C and 90% humidity for 1, 3, 7, 14, and 28 days.

It can be seen that with an increase in curing time, the compressive strength of PSF was obviously increased. The unconfined compressive strength of PSF cured for 28 days reached 0.75 MPa, which met the requirements of general subgrade engineering materials. The compressive strength of SSF on the 1st curing day was 0.35 Mpa, which was almost equivalent to the compressive strength of PSF on the 7th day, then, it decreased slightly, and increased again after the 14th day. The reason might be that the addition of sulphoaluminate cement increased the hydration reaction rate of calcium ions and reduced the free water content of the system, which significantly enhanced the strength of the solidified form in the early stage. In the later stage, with more products and erosion ions generated during the reaction, the porosity of the solidified form increased and the strength decreased accordingly [31]. The relationship model of the water/cement ratio vs. compressive strength of cement proposed by Singh et al. [32] also confirmed this result.

3.2. pH Fluctuation of the Leachate during Leaching

Figure 3 shows the leachate pH fluctuations during leaching. As the leaching time proceeded, the leachate of PSF and SSF had similar pH change trends of becoming alkaline, which indicated continuous leaching of alkaline substances. Under the same initial conditions, the leachate pH of the UL group was higher than that of the RL group. When the initial pH of the leaching solution was 4.00, the leachate pH had a sharp increase between the leaching times of 96 and 192 h, which might have been due to the continuous erosion of H⁺ and SO₄²⁻ to the solidified form which led to the formation of microcracks, thus, resulting in the release of alkaline substances from the the inner channel [33].

During the first 24 h, a large amount of Ca(OH)₂ generated from cement hydration was rapidly dissolved into the leachate. The pH of the RL group leachate kept increasing even if the leaching solution was replaced at regular intervals and the SSF leachate pH was slightly lower than that of PSF. Between 24 and 144 h, the leachate pH was still increasing, however, the pH difference between RL and UL groups began to occur. From 144 to 192 h, the pH increasing trend began to flatten, which indicated that the dissolution of Ca(OH)₂ was close to saturation. The change trend of leachate pH was similar to the research conducted by Xue et al. on cement stabilized lead-contaminated soil leachate [34].

3.3. The Analysis on Cu Leaching

From Figure 4 it can be observed that the pH of leaching solution has an obvious effect on the leaching of Cu. When the pH was 4.0, both the Cu concentration in the leachate and the cumulative leaching content of Cu were higher than under the condition of pH 7. The cumulative leaching content of Cu in the RL group was 0.201 μg/cm² for PSF and 0.917 μg/cm² for SSF at pH 4. As the leaching solution pH became 7, the cumulative leaching content of Cu in the RL group decreased to 0.098 μg/cm² for PSF and 0.390 μg/cm² for SSF. This indicated that the acidic condition promoted the release of Cu from the solidified sediment.

During 0–96 h, the leaching concentration of Cu fluctuated significantly, for that the hydration reaction was not complete in the early stage and Cu was not completely stabilized. As the leachate pH increased, hydration products such as C-S-H were negatively charged on the surface, which led to the enhanced adsorption of Cu²⁺. When the pH continued to increase, Cu(OH)_n⁽ⁿ⁻²⁾ began to form and the adsorption was inhibited.

The leaching concentration of Cu in SSF leachate was significantly higher than that in the PSF group, which indicated that Cu was more easily dissolved and diffused from the SSF system. The drop in the data might be related to the adsorption effect of the solidified sediments [35]. When the leaching solution pH was 4.0, the cumulative leaching content of Cu in the SSF-RL group was 4.5 times that in the PSF-RL group. These results proved that Cu was more prone to be neutralized, precipitated, and adsorbed in the hydrated calcium silicate system than in the calcium sulphoaluminate system, therefore, Portland cement was more applicable for the solidification of Cu. Remond [36] found that the solubility order of cement hydration products was Ca(OH)₂ > monosulfide calcium sulphoaluminate (AFM) > trisulfide calcium sulphoaluminate (AFT) > hydrated calcium silicate C-S-H. The main hydration products of the sulphoaluminate cement system were AFT and AFM; therefore, the sulphoaluminate cement coating with hydration products to fix heavy metals was not as stable as the Portland cement system.

From Figure 5, it can be observed that the CLR-t^1/2 slope of the PSF group is lower than that of the SSF group, which means Cu in the PSF group has lower mobility than the SSF group. According to Rachana et al., De < 3 × 10⁻⁹ cm²/s indicated that the low mobility of heavy metals, 3 × 10⁻⁹ cm²/s < De < 1 × 10⁻⁷ cm²/s indicated middle mobility, and De > 1 × 10⁻⁷ cm²/s indicated high mobility of heavy metals [37].

Table 3. Mobility of Cu during leaching.

Group	Time (h)	CLR-t1/2 Slope	R2	De/m2·s−1	Mobility
PSF, pH = 7.0	0~96	2.82 × 10−7	0.9152	8.69 × 10−18	low
	96~1032	1.43 × 10−7	0.9615	2.24 × 10−18	low
PSF, pH = 4.0	0~96	4.59 × 10−7	0.9633	2.31 × 10−17	low
	96~1032	3.63 × 10−7	0.9713	1.44 × 10−17	low
SSF, pH = 7.0	0~96	1.20 × 10−6	0.9964	1.57 × 10−16	low
	96~1032	6.12 × 10−7	0.9702	4.10 × 10−17	low
SSF, pH = 4.0	0~96	1.52 × 10−6	0.9806	2.53 × 10−16	low
	96~1032	2.02 × 10−6	0.9744	4.47 × 10−16	low

shows that all the De was lower than 3 × 10⁻⁹ cm²/s, which indicated low Cu mobility under the tested conditions. The De value under pH 4.0 was higher than under pH 7.0; therefore, heavy metals are more mobile in a low pH environment. Cu also showed higher mobility during 0~96 h. The Cu in the PSF group had low mobility, and the mechanism was mainly diffusion. When the pH was 4.0, the De of the PSF group was 2.31 × 10⁻¹⁷ m²/s at most, while for the SSF group, this value was 4.47 × 10⁻¹⁶ m²/s, the difference was almost one magnitude order.

3.4. The Analysis on Cd Leaching

From Figure 6 it could be observed that, during 0~96 h, the Cd concentration in the leachate had varied degrees of fluctuations, and then tended to be stable. If the leaching solution was not replaced, the Cd concentration might reach equilibrium at 96 h, and the subsequent concentration in the leachate was almost unchanged. Wang et al. [38] found that Cd²⁺ would form precipitates such as Cd(OH)₂ and CdCO₃ during the solidification in cement, and the equilibrium state was easily affected by the concentration of various ions in the leaching solution. An acidic environment also promoted the release of Cd: when the leaching solution pH was 4.0, the cumulative leaching content of Cd in the RL group was 0.024 μg/cm² for PSF and 0.022 μg/cm² for SSF; when the leaching solution pH was 7.0, the cumulative leaching content of Cd in the RL group was 0.019 μg/cm² for PSF and 0.018 μg/cm² for SSF.

The results listed in Figure 7 and

Table 4. Mobility of Cd during leaching.

Group	Time (h)	CLRt1/2 Slope	R2	De/m2·s−1	Mobility
PSF, pH = 7.00	0~96	5.54 × 10−6	0.9852	3.37 × 10−15	low
	96~1032	2.34 × 10−6	0.9977	6.01 × 10−16	low
PSF, pH = 4.00	0~96	7.09 × 10−6	0.9952	5.50 × 10−15	low
	96~1032	2.54 × 10−6	0.9975	7.09 × 10−16	low
SSF, pH = 7.00	0~96	5.35 × 10−6	0.9907	3.13 × 10−15	low
	96~1032	2.06 × 10−6	0.9973	4.66 × 10−16	low
SSF, pH = 4.00	0~96	6.56 × 10−6	0.9955	4.72 × 10−15	low
	96~1032	2.27 × 10−6	0.9973	5.64 × 10−16	low

indicate the low mobility of Cd during cement solidification. However, the De values of Cd were generally greater than Cu by one or two orders of magnitude. Cd was more mobile at a lower pH and during the early stage of 0~96 h, which was similar to Cu. The correlation coefficient for Cd was higher than Cu, which showed that the precipitate formed from Cd has higher stability, and its migration was mainly through ion dissolution and diffusion.

3.5. The Compressive Strength Change of the Solidified Form

From

Table 5. The compressive strength change of the solidified form after leaching.

Solidified Form	Compressive Strength (MPa)	Loss Rate of Compressive Strength (%)
PSF, initial	0.75	-
SSF, initial	0.27	-
PSF, leaching solution pH = 7.0	0.72	4.0
PSF, leaching solution pH = 4.0	0.71	5.3
SSF, leaching solution pH = 7.0	0.23	14.8
SSF, leaching solution pH = 4.0	0.23	14.8

it can be observed that PSF was more stable and the compressive strength of PSF reached 0.71–0.75 MPa, while for SSP, the compressive strength was only 0.23–0.27 MPa. After leaching, the compressive strength of SSF decreased 14.8%, much higher than that of PSF which was only 4.0–5.3%. This might be due to the different structural characteristics of the products formed in hydration of the two cements: Portland cement tended to form rod-shaped garnet during hydration, which had a compact structure and high strength; sulphoaluminate tended to form a circular compound AFT under different Ca/S ratios. The needle form compound interwoven together had certain compressive strength in the early age, but its resistance to dissolution was weaker than that of garnet. The pH of the leaching solution had little effect on the loss of compressive strength. The solidified sediment products were generally used for subgrade, building materials, landfill, etc., which would inevitably have contact with water from precipitation. Therefore, Portland cement was more suitable for treating heavy metal polluted sediment.

3.6. XRD and SEM

Figure 8a shows the change of sediment microstructure before and after solidification. The main components of the sediment were quartz and albite, and the characteristic diffraction peaks were 20.8°, 26.6°, 22.0°, and 27.9°, respectively. The secondary components were potassium mica and clinopyrochlore. The characteristic diffraction peaks of hydrated calcium silicate (29.4°) and ettringite (9.0°, 15.7°, and 22.9°) were also found in the cement hydration products. They were important sources of mechanical strength which played a key role in cement solidification/stabilization. Gypsum was observed after solidifying with Portland cement. When using sulphoaluminate cement, obvious characteristic diffraction peaks of ettringite appeared, which was due to different compositions and quantities of hydration products formed from cement with calcium sulphoaluminate as the main ingredient.

Figure 8b–d shows the surface morphology of the original sediment and the solidified sediment. The original sediment particles were stacked to form many macropores. After solidification, many cement hydration products, such as plate Ca(OH)₂, flocculent C-S-H, and acicular AFT (AFM) were generated, which constantly filled the pores among sediment particles and formed the sediment particles–hydration products junctional complex. This cementation state significantly improved the mechanical properties of solidified sediment, and heavy metals could be sealed in this stable structure. Comparing the SEM of PSF and SSF, the hydration products of Portland cement were mainly Ca(OH)₂ and C-S-H, while more acicular AFT and AFM were observed in sulphoaluminate cement hydration products, which was consistent with the results of XRD. Therefore, as compared with SSF, PSF had higher mechanical strength and optimal mechanical property.

3.7. Implication

With the increased urgent demand for river water and sediment treatment and recovery in cities and rural places, the disposal of exposed sediments has become a critical issue for water quality maintenance. Cement solidification has significant advantages in solidification/stabilization of heavy metals. In this study, we compared two normal cements on the physical strength and leaching behavior of Cu and Cd and found that Portland cement was superior to sulphoaluminous cement with respect to cost, curing time, and mutual functions between chemicals and metal ions. For the most part in city areas and rural places, as a usual and economic building material, using Portland cement as a solidification reagent is still the most effective approach considering the subsequent properties and usefulness, for example, bricks, river slope protection, and construction materials.

4. Conclusions

In this study, we used Portland cement and sulphoaluminate cement as the cementatory solidifying agent to stabilize heavy metal in sediment from Shanghai Fuxing Island Canal. The cement adding amount was 10%, and the solidified forms were cured under 20 °C and humidity of 90%. The main findings include:

(1)

The compressive strength of SSF reached 0.35 MPa after curing for one day, and then decreased. The compressive strength of PSF increased with the curing time, and reached 0.75 MPa after 28 days.

(2)

In general, the solidification effect of Portland cement on Cu was better than that on Cd, with significantly lower mobility even in acidic conditions. The adsorption and chemical bonding force of hydrated silicate to Cu were much higher than that of calcium sulphoalmuninate which resulted in the lower release of Cu from PSF. The difference in stabilization effect of Cd using two cements was less significant than that of Cu.

(3)

Portland cement was more suitable for treating heavy metal-contaminated sediments due to the higher stability of PSF. When the pH of leaching solution was 4.0, the compressive strength decline of PSF and SSF after leaching was 5.3% and 14.8% respectively.