Solidification mechanism of heavy metal of red mud-based engineered cementitious composites

doi:10.21203/rs.3.rs-3131092/v1

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Solidification mechanism of heavy metal of red mud-based engineered cementitious composites

https://doi.org/10.21203/rs.3.rs-3131092/v1

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In this paper, the occurrence form and leaching characteristics of heavy metals in red mud were clarified, the solidification mechanism of heavy metals by the hydration products and solidification agents was explored. The results show that the contents of heavy metals in red mud from high to low are Cr, Pb, Ni, Cu, As and Cd, which are 754 mg/kg, 78.2 mg/kg, 41.8 mg/kg, 33.2 mg/kg, 15.0 mg/kg and 0.77 mg/kg respectively. The potential ecological hazard level is moderate. The occurrence form of heavy metals in red mud-based cementitious materials gradually changes from effective state to stable state with the increase of curing age. When the curing age is 120 days, the curing rate tends to be stable, and the curing rates of As, Cu, Ni, Cd, Cr, Pb reach 99.91%, 97.62%, 97.02%, 96.83%, 95.91%, 91.11% respectively. The lifting effect of three heavy metal curing agents is: 5A zeolite < fly ash < attapulgite. The optimum heavy metal curing agent is attapulgite with a content of 6%. When the curing period is 28 days, the curing rate of heavy metal element Pb reaches 99.74%, and the curing rate of Cr reaches 99.40%, increased by 8.86% and 4.28% respectively. The results can realize the green and safe application of red mud based cementitious materials.

red mud-based cementitious material

heavy metals

heavy metal risk assessment

solidification agent

solidification mechanism

Bayer red mud is a by-product of extracting Al₂O₃ from bauxite (Wang et al. 2008). China is one of the world’s largest aluminum producer, producing about 120 million tons of red mud annually (Liu et al. 2009 and Wang et al. 2021), but the utilization rate of red mud is less than 5%. It has become a social problem to be solved urgently.

The components of red mud is mainly Fe₂O₃, Al₂O₃, SiO₂ and CaO (He et al. 2013 and Kardelen Kaya et al. 2016), which has the potential to prepare cementitious materials (Hu et al. 2018, Geng et al. 2017, Pan et al. 2003). Utilizing high-activity chemical components (Gao et al. 2020) such as Si, Al and Ca in red mud, a new type of cementitious material which classified in alkali activated materials (Ding et al. 2020) can be prepared to replace the Ordinary Portland cement, which can not only meet the engineering requirements of infrastructure construction, but also solve the problem of land pollution caused by red mud stockpiling.

However, the heavy metals in red mud may cause nearby soil salinization and water pollution (Liu et al. 2021d Khairul et al. 2019). The content of heavy metals, especially Cr (Sarath Chandra K. et al. 2022 and Luo et al. 2022), in red mud reaches 754 mg/kg, which is far beyond the acceptable range of the environment according to “Soil environmental quality Risk control standard for soil contamination of agricultural land (GB 15618 − 2018)” and “Soil environmental quality Risk control standard for soil contamination of development land (GB36600-2018)”. Ren et al. found that the hardness of groundwater increased and the pollution of heavy metals such as As and Cr intensified during the application of red mud. The pollution disposal of red mud has become the general concerned question, but there are still few researches on heavy metals leaching, solidification mechanism and ecological risk assessment of red mud. Ding et al. used red mud and slag to prepare red mud-slag based geopolymer, and systematically studied the effects of red mud content, activator modulus and alkali equivalent on heavy metals solidification performance. Zhou et al. found that alkali-activated red mud would participate in geopolymerization reaction, which increased the solidification rate of heavy metals. Cheng et al. prepared slag-red mud cementitious material, which has the advantages of environmental friendliness and can effectively inhibit the leaching of heavy metals. Jiannan Pei et al. prepared ultra-lightweight ceramsite and the alkali and heavy metals in red mud were immobilized during sintering.

Heavy metals in red mud based engineered cementitious composites (RECC) will cause serious pollution to groundwater and soil if not disposed properly. The potential ecological pollution risk of red mud-based cementitious material severely restricts its application in infrastructure construction projects (Y. Pontikes and G.N. Angelopoulos 2013, M. Andac et al. 1998). It is necessary to explore the occurrence forms and leaching characteristics of heavy metals in RECC, clarify the solidification mechanism of heavy metals. Furthermore, the synergistic effect of solidification agent on heavy metals solidification in RECC was investigated, the results are helpful for realizing the high value-added mass utilization of red mud in infrastructure construction.

2.1 Raw Materials

Bayer red mud was collected from Wei Qiao Pioneering Group, which is dried, ground and sieved by 100 mesh sieve. Sodium silicate was bought from Shandong Yousuo Chemical Industry Technology Co. Ltd. The heavy metal solidification agent is selected from fly ash, 5A zeolite and attapulgite. Fly ash, 5A zeolite and attapulgite were bought from Changsheng Environmental Protection Material Co. Ltd, Jiangxi Xintao Technology Co. Ltd, Changzhou Dingbang Minerals Technology Co. Ltd, respectively. PbCrO₄ with analytically pure was purchased from Shandong Chengchuan Life Technology Co., Ltd. Figure 1 and Table 1 showed particle size distribution of red mud, fly ash, 5A zeolite and attapulgite.

Table 1

Physical parameters of raw materials
	Red mud	5A zeolite	Attapulgite	Fly ash
D50 (µm)	16.188	3.815	8.447	8.918

2.2 Heavy metal risk assessment method

The heavy metal contents were tested according to Soil and Sediment- Determination of Aqua Regia Extracts of 12 Metal Elements-Inductively Coupled Plasma Mass Spectrometry (HJ 803–2016), and the Cr(Ⅵ) content was tested according to Soil and Sediment—Determination of Cr(VI)—Alkaline Digestion/Flame Atomic Absorption Spectrometry (HJ1082-2019). The occurrence forms of heavy metal were tested by Tessier sequential extraction method.

The potential risk of heavy metals in red mud were evaluated by single factor index method, Nemerow comprehensive index method and potential ecological risk index method. The toxicity coefficients of Cu, Pb, Cd, Ni, Cr and As are 5, 5, 30, 5, 2 and 10, respectively, according to the toxicity coefficient values calculated by Hakanson (L. Hakanson 1980, Teklit Zerizghi et al. 2021). The corresponding relationship between single factor index (P), Nemerow comprehensive index (P_N) and pollution level is shown in Table 2. Besides, the corresponding relationship between potential ecological risk index (RI) and pollution degree is shown in Table 3.

Table 2

Corresponding relationship between P, P_N and pollution level
Level	P/P_N	Pollution level
1	<0.7	No risk
2	0.7 ~ 1	Warning line
3	1 ~ 2	Low pollution
4	2 ~ 3	Medium pollution
5	>3	Heavy pollution

Table 3

Corresponding relationship between RI and pollution degree
Degree	RI	Pollution degree
1	<150	Low risk
2	150–300	Medium risk
3	300–600	High risk
4	≥ 600	Very high risk
The calculation formulas of P, P_N and RI are shown as follows (L. Hakanson 1980, Teklit Zerizghi et al. 2021):

$$P=C_{r}^{i}=\frac{{C_{m}^{i}}}{{C_{n}^{i}}}$$

$${P_N}=\sqrt {\frac{{(\frac{1}{n}\sum\nolimits_{{i=1}}^{n} {{P_i}{)^2}+{P_{\hbox{max} }}^{{^{2}}}} }}{2}}$$

$$RI=\sum\nolimits_{{i=1}}^{n} {E_{r}^{i}} =\sum\nolimits_{{i=1}}^{n} {T_{r}^{i}} \times C_{r}^{i}$$

where $C_{m}^{i}$ is the measured data of heavy metal, $C_{n}^{i}$ is the evaluation standard of heavy metal, $E_{r}^{i}$ is potential ecological hazard coefficient of for a given heavy metal, $T_{r}^{i}$ is toxicity coefficients for a given heavy metal.

2.3 Preparation of RECC

The design mix ratio of red mud to slag is specified as 6:4, and alkaline activator is 1.8 M sodium silicate solution with the dosage of 8 wt.% (Gao et al. 2022). The RECC are evenly mixed according to the design mix ratio, mechanically stirred for 3 min, and poured into 40 mm×40 mm×40 mm PVC molds. The samples are cured for 24 h at the environment of 20 ± 1℃ and 90% humidity, and then demoulded.

3.1 Heavy metal risk assessment results of red mud

3.1.1 Potential pollution elements

The contents of heavy metals in red mud are shown in Table 4, in which Cr, Pb, Ni, Cu, As and Cd, are 754 mg/kg, 78.2 mg/kg, 41.8 mg/kg, 33.2 mg/kg, 15.0 mg/kg and 0.77 mg/kg, respectively. It can be seen from Table 4 that the contents of Cr and Cr(Ⅵ) are far exceeding the risk screening value, becoming the main pollution element in red mud.

Table 4

Analysis of heavy metal elements in red mud
Heavy metal	Test results (mg/kg)	GB 15618 − 2018	GB36600-2018
Cu	33.2	100	2000
Pb	78.2	170	400
Cd	0.77	0.8	20
Ni	41.8	190	150
Cr	754	250	-
As	15.0	25	25
Cr(Ⅵ)	7.67	-	3.0

Tessier sequential extraction method was used to test the occurrence forms of heavy metals in red mud, including Cu, Pb, Cd, Ni, Cr and As. The results are shown in Table 5. Residual fraction is the main form of heavy metals in red mud, but the Exchangeable fraction of Cu, Pb, Cd, Ni, Cr and As reaches 7%, 6%, 7%, 4%, 6% and 12%, and pollution risk still exists.

Table 5

Occurrence forms of heavy metal elements in red mud
Heavy metal Occurrence forms	Cu	Pb	Cd	Cr	As	Ni
Heavy metal Occurrence forms	mg/kg
Exchangeable fraction	0.40	4.92	0.02	250.27	1.82	0.27
Carbonate-bound fraction	1.66	3.14	0.09	65.01	2.70	1.49
Fe-Mn oxide-bound fraction	1.47	1.33	0.05	26.48	1.36	1.32
Organio-bound fraction	1.74	1.99	0.10	43.22	0.79	1.51
Residual fraction	24.01	35.28	1.19	560.67	30.93	37.74

3.1.2 Heavy metal risk assessment

Nemerow comprehensive index method takes the risk value corresponding to Soil environmental quality—Risk control standard for soil contamination of agricultural land (GB 15618 − 2018) as the red mud reference value, and the results are shown in Table 6. The single factor index of six heavy metals in red mud is Cr (3.016) > Cd (0.9625) > As (0.6) > Pb (0.46) > Cu (0.332) > Ni (0.22). That is, Cr showed heavy pollution, followed by Cd pollution reaching the warning line, and the contents of the other four heavy metals, including Pb, Cu, As and Ni, did not exceed the standard. The Nemerow comprehensive index of red mud is 2.23, reaching the medium pollution level, and Cr is the primary pollution factor of red mud.

Potential ecological risk index method comprehensively considers the toxicity of heavy metals. Taking the soil background values in Shandong Province as the reference standard (Pang et al. 2018), the potential ecological risk of red mud was evaluated by the method of potential ecological risk index. The results correspond to Table 6, showing that the potential ecological index (RI) of red mud is calculated to be 248.39. It is concluded that the ecological risk of red mud is medium risk.

Table 6

Red mud pollution index
	Single factor index	Nemerow comprehensive index	Potential ecological risk index
Cu	0.332	2.23	248.39
Pb	0.46
Cr³⁺	0.9625
Ni	0.22
Cr⁶⁺	3.016
As	0.6

3.2 Effect of self-cementation of hydration products on heavy metal solidification

3.2.1 Occurrence forms of heavy metal in RECC

Tessier sequential extraction method was used to analyze the occurrence forms changes and the solidification effect of heavy metals in RECC at different ages was studied. Exchangeable fraction and carbonate-bound fraction are sensitive to environmental changes, which is considered as available state. However, Fe-Mn oxide-bound fraction, organic-bound fraction and residual fraction tend to be stable and difficult to leach, which is considered to be stable. Therefore, available state is the main form of heavy metal pollution to the surrounding environment. The formula for calculating the solidification rate of heavy metal is as follows:

$${S_{hm}}=(1 - \frac{{{c_{ex}}+{c_{cb}}}}{{{c_{hm}}}}) \times 100\%$$

where ${S_{hm}}$ is solidification rate of heavy metals, ${c_{ex}}$ is the exchangeable fraction content, ${c_{cb}}$ is the carbonate-bound fraction content, ${c_{hm}}$ is total content of heavy metal.

The occurrence forms of heavy metals in RECC at different curing ages are shown in Fig. 2. The solidification rates of Cr, Pb, Cu, Ni, Cd and As in the red mud are 66.66%, 82.73%, 92.96%, 95.84%, 92.41% and 87.98% respectively. Among them, the solidification rate of Cr is the lowest, followed by Pb. When the curing age increases from 60 d to 120 d, the solidification rate of heavy metals tends to be stable, among which the solidification rate of As is higher than 99.90%, that of Cu, Ni, Cd, Cr, Pb is 97.62%, 97.02%, 96.83%, 95.91% and 91.11%, respectively.

3.2.2 Solidification mechanism analysis of self-cementation of hydration products

The content of available state of Cr exceeds the limit of Identification standards for hazardous wastes—Identification for extraction toxicity (GB 5085.3—2007). Although Pb does not exceed the limit, its solidification rate is low, and the content of available state exceeds 1.00 mg/kg. In practical engineering, the extensive application of RECC produces cumulative occurrence effect, which leads to the decrease of safety, and may cause the pollution of groundwater and soil under extreme conditions. Other heavy metals are far lower than the limit. Therefore, considering the content of available state and solidification rate of heavy metals in RECC comprehensively, analyzing the occurrence forms and solidification rate of Cr and Pb in RECC is necessary.

The occurrence forms of heavy metals in red mud and RECC at different curing ages are shown in Fig. 3. Compared with the red mud, the solidification rate of Cr and Pb in the RECC is significantly improved. With the increase of curing age, the heavy metals in RECC gradually change from available state to stable state.

In order to analyze the solidification mode mechanism of heavy metals by hydration products of RECC, 0.6 wt.% PbCrO₄ was added into RECC, and the solidification mode of Pb and Cr was analyzed by means of XRD, XPS and MAS NMR.

The binding energy of Pb and Cr was analyzed by XPS, and the results are shown in Fig. 4. According to the XPS spectrum of PbCrO₄ (Liu et al. 2020), the Cr 2p spectrum is at 588.1 eV and 578.9 eV, and the Pb 4f spectrum is at 142.9 eV and 138.2 eV, respectively. As can be seen from Fig. 4, the binding energy peaks of Pb and Cr shift to the direction of high binding energy peaks in the hydration process, indicating that Pb and Cr participated in the hydration reaction to form stable geopolymers in the RECC, changing their valence state and coordination environment (Ye et al. 2017, Zhang et al. 2018).

The change of coordination environment of Si and Al in RECC was further analyzed by XPS spectrum, and the results are shown in Fig. 5. Due to the existence of Pb and Cr, the binding energy of Si and Al has changed, among which the binding energy of Al is highly sensitive to coordination environment. The peak of Al 2p has shifted from 74.02 eV to 73.76 eV after the addition of lead chromate, which indicates that the coordination number of Al has changed from octahedron to tetrahedron during geopolymerization, resulting in the lower binding energy of Al. It shows that the addition of Pb and Cr changes the coordination environment of elements, besides Pb and Cr participate in the hydration process of RECC. It also proves that heavy metals are solidified in the hydration products of RECC by chemical bonding and physical adsorption.

SEM-EDS was used to analyze the micro-morphology and the distribution law of Pb and Cr of RECC with PbCrO₄, and the results are shown in Fig. 6. According to the energy spectrum analysis, both Pb and Cr participate in the geopolymerization reaction of RECC, and N(C)-S-A-H amorphous gel are generated, which can effectively solidify Pb and Cr. This is because heavy metals have a charge balance reaction with Al(OH)₄⁻, and can replace metal cations such as Al³⁺ and Ca²⁺ to participate in the hydration reaction, thus being fixed in the stable hydration products.

The mineral phases of Pb and Cr in the RECC were analyzed by XRD, and the results are shown in Fig. 7. Compared with the XRD spectrum of the control group, the diffraction peak of hydration products of RECC with PbCrO₄ shifted and produced a new diffraction peak, which changed the type of hydration products of RECC. Pb participated in the hydration reaction and reacted with Si, Al and Fe in the system to produce PbSiO₃, PbFe₁₂O₁₉ and Pb₂Al₃(SiO₄)(Si₂O₇)(O, OH)₂, realizing the transformation from available state to stable state. Cr reacts with elements such as Na, Si, Al and Fe to produce NaCr + 3Si₂O₆ and Fe(Cr, Al)₂O₄, which realizes the transformation of Cr(Ⅵ) to low toxicity Cr(Ⅲ), and reduces the leaching and migration of heavy metals in RECC.

According to the above analysis, compared with the red mud, the reason for the change of occurrence forms of heavy metals in RECC is that the hydration products of RECC solidify heavy metals by self-cementation. Al³⁺ combines with four oxygen to form [Al(OH)₄]⁻, which can react with heavy metals in RECC in charge balance (Li et al. 2019). The available state of heavy metals combine with Si (Al) oxygen tetrahedron or iron oxygen polyhedron to form stable three-dimensional network-like compact structure, which fixes the heavy metals in stable hydration products, realizes the transformation of heavy metals from available state to stable state.

3.3 Synergistic effect of solidification agent on heavy metals solidification in RECC

3.3.1 Effect of solidification agent on solidification rate of heavy metals

In order to further improve the solidification rate of heavy metals in RECC, the effects of solidification agents on the solidification rate and occurrence forms of heavy metals was studied. Attapulgite, fly ash and 5A zeolite were added to improve the solidification rate in RECC, and the solidification mechanism was analyzed by mechanical properties, FT-IR and XRD.

The solidification rate of Cr and Pb in RECC with solidification agent is shown in Fig. 8. It is clear that fly ash, attapulgite and 5A zeolite can all improve the solidification rate of heavy metals in RECC, and the order of solidification effect is 5A zeolite < fly ash < attapulgite. The best solidification agent for heavy metals is attapulgite with 6 wt.%. When curing time is 28 d, the solidification rates of Pb and Cr are increased by 8.86% and 4.28% respectively. The solidification rate of Cr and Pb in the RECC increased significantly, and the heavy metals tended to be stable, which weakened the leaching and migration of heavy metals in the RECC.

3.3.2 Compressive strength

The 28 d compressive strength of RECC with 2%, 4% and 6% solidification agent was tested, and the result was shown in Fig. 9. As can be seen, with the increase of the dosage of fly ash, the compressive strength first increases and then decreases. When the dosage of fly ash is 2 wt.%, the optimum compressive strength reaches 19.62 MPa. With the increase of the dosage of attapulgite, the compressive strength gradually increases. When the dosage of attapulgite increases to 6%, the maximum compressive strength reaches 24.15 MPa. With the increase of the dosage of 5A zeolite, the compressive strength first increases and then decreases. When the dosage of 5A zeolite increases to 4%, the compressive strength reaches the optimal value of 18.30 MPa.

3.3.3 Analysis of microstructure

The FT-IR analysis of hydration products of RECC with solidification agent is shown in Fig. 10. The absorption peak of CO₃²⁻ is at 1380 cm^− 1, and the unreacted Na-O and Ca-O in the stone react with CO₂ to form carbonate. The absorption peak of Si-O-T(T = Si, Al, Fe, Ca, etc.) is at 1070 cm^− 1 to 988 cm^− 1, and the Na-O bond is at 770 cm^− 1. With the increase of dosage of solidification agent, the absorption peaks of Si-O-T and Na-O increase, which indicates that the solidification agent promotes the geopolymerization of RECC.

The XRD analysis of hydration products of RECC with solidification agent is shown in Fig. 11. Compared with the control group, with the addition of attapulgite, 5A zeolite and fly ash, there is no new diffraction peak in the XRD spectrum, which indicates that there is no new type of hydration product. However, the diffraction peaks of the hydration products including ettringite, anorthite and calcite are enhanced, and the camel peak strength of N(C)-S-A-H amorphous gels is enhanced, which indicates that more gels are produced by the addition of solidification agent. This is because the addition of solidification agent increases the alkalinity, enhances the dissolution of silicon and aluminum components and promotes geopolymerization, thus improving the compressive strength and heavy metal solidification rate of RECC.

(1) With the increase of curing age, the occurrence form of heavy metals in RECC gradually changes from the effective state to the stable state, and the solidification rate of heavy metals gradually increases and tends to be stable. When the curing age is 120 days, the solidification rates of As, Cu, Ni, Cd, Cr and Pb reach 99.91%, 97.62%, 97.02%, 96.83%, 95.91% and 91.11% respectively.

(2) Heavy metal ions can replace Al³⁺ in the hydration reaction, and then the effective heavy metal elements can form a stable three-dimensional network, thus realizing the transformation of heavy metal elements from effective to stable.

(3) Fly ash, palygorskite and 5A zeolite can improve the solidification rate of heavy metals in red mud based cementitious materials. With the increase of the content, the leaching amount of heavy metals in red mud based cementitious materials decreases gradually. The improvement effect of curing rate is as follows: 5A zeolite ༜ fly ash ༜ attapulgite, the best heavy metal curing agent is 6% attapulgite, the curing rate of heavy metal element Pb reaches 99.74% and the curing rate of Cr reaches 99.40% when the curing period is 28 days, increased by 8.86% and 4.28% respectively.

Author contribution:

Zhaofeng Li: conceptualization, methodology, review and editing, Funding acquisition.

Chen Zhang: methodology, review and editing.

Yifan Gao: conceptualization, reviewing and editing.

Jian Zhang: methodology, original draft preparation, Funding acquisition.

Chuan Wang: original draft preparation.

Funding:

This study was financially supported by the Shandong Province Key R&D Program (Major Technological Innovation) (Project No. 2020CXGC011405), Natural Science Foundation of Shandong Province for Youths (Project No. ZR2022QE119), Natural Science Foundation of Shandong Province (Project No. 2020KE006) and Shandong Province Key R&D Program (Major Scientific and Technological Innovation Project) (Project No. 2021CXGC010301)

Statements and Declarations:

We confirm that there are no known conflicts of interest associated with this publication and there has been no significant financial support for this work that could have influenced its outcome.

Data Availability:

The datasets used and/or analyzed during the current study are available from the corresponding author on reasonable request.

Ethics approval: Not applicable.

Consent to participate: Not applicable.

Consent for publication: Not applicable.

Competing interest: The authors declare no competing interests.

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Solidification mechanism of heavy metal of red mud-based engineered cementitious composites

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Abstract

Figures

1. Introduction

2. Materials and methods

2.1 Raw Materials

2.2 Heavy metal risk assessment method

2.3 Preparation of RECC

3. Results and discussion

3.1 Heavy metal risk assessment results of red mud

3.1.1 Potential pollution elements

3.1.2 Heavy metal risk assessment

3.2 Effect of self-cementation of hydration products on heavy metal solidification

3.2.1 Occurrence forms of heavy metal in RECC

3.2.2 Solidification mechanism analysis of self-cementation of hydration products

3.3 Synergistic effect of solidification agent on heavy metals solidification in RECC

3.3.1 Effect of solidification agent on solidification rate of heavy metals

3.3.2 Compressive strength

3.3.3 Analysis of microstructure

4. Conclusion

Declarations

References

Status:

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