Evaluation of the Applicability of Concrete Sludge for the Removal of Cu, Pb, and Zn from Contaminated Aqueous Solutions

metals

Article Evaluation of the Applicability of Concrete Sludge for the Removal of Cu, Pb, and Zn from Contaminated Aqueous Solutions

Jongchan Yoo, Heeyoung Shin * and Sangwoo Ji Center for Carbon Mineralization, Climate Change Mitigation and Sustainability Division, Korea Institute of Geoscience and Mineral Resources (KIGAM), 124 Gwahak-ro, Yuseong-gu, Daejeon 34132, Korea; [email protected] (J.Y.); [email protected] (S.J.) * Correspondence: [email protected]; Tel.: +82-42-868-3574; Fax: +82-42-868-3429

Received: 9 July 2018; Accepted: 24 August 2018; Published: 25 August 2018

Abstract: In this study, the possibility of using concrete-sludge recycling as an immobilizer, including dried sludge (DS), precipitated calcium carbonate (PCC), and calcite-coated particles (CCP), was evaluated for the remediation of Cu-, Pb-, and Zn-contaminated aqueous solutions. Experimental variables characterizing immobilization, including reaction time, initial solution pH, and immobilizer dosage, were selected. After DS was applied, 98.3% of Cu, 99.9% of Pb, and 95.2% of Zn were removed via metal-hydroxide coprecipitation onto the surface of DS, which increased in pH within the shortened contact time. On the contrary, PCC and CCP removed metals (above 99.9% removal) via the formation of metal carbonates, which was highly dependent on both the pH and the carbonates released from the immobilizers. The acidic solution pH contributed to the dissolution of calcite (leading to an increase in carbonates in solution), thus enhancing the removal of metals. An increase in PCC and CCP dosage (liquid to solid ratio of 1000 to 100) was effective in removing Cu with an increasing ﬁnal pH and number of carbonates in solution. Our results show that concrete sludge can be recycled to reduce environmental loads, including alkaline wastewater discharge, waste disposal, CO2 emissions, and metal-contaminated aqueous solutions.

Keywords: heavy metals; concrete sludge; immobilization; carbon mineralization; environmental load; calcium carbonate

1. Introduction The metallic contamination of groundwater by industrial developments and human activities (e.g., mining, smelting, and refining) have serious, harmful effects on human health and the ecosystem. Several remediation techniques, including immobilization (e.g., sorption, solidification, and stabilization), permeable reactive barriers, coprecipitation, coagulation, membrane filtering, and ion exchange, were developed in an effort to remediate metal(loid)-contaminated groundwater over the past few decades [1,2]. Among them, the immobilization method using Fe/Mn/Al oxides, clay minerals, or calcium (Ca)-bearing amendments is an attractive technique for the inhibition of the mobility and toxicity of metallic contaminants in a groundwater environment [3,4]. Ca-bearing materials/waste/minerals, such as gypsum (CaSO4·2H2O), quicklime (CaO), slaked lime (Ca(OH)2), fly ashes, hydroxyapatites (Ca5(PO4)3(OH)), and limestone (CaCO3), are attractive alternative immobilizers that are easily accessible and cost-effective for reducing contamination [5–10]. In Korea, vast amounts of waste concrete are annually generated by reconstruction projects for deteriorated and aged buildings. Screening and crushing processes are essential for the recycling of waste concrete as aggregate materials, which are classified as dry- and wet-based processes. Although it is simpler, cheaper, and easier to maintain the system using dry-based processes,

Metals 2018, 8, 666; doi:10.3390/met8090666 www.mdpi.com/journal/metals Metals 2018, 8, 666 2 of 17 the wet-based crushing process is commonly applied to remove impurities and to wash the concrete surface [11] and reduce air pollution. During this process, a significant amount of concrete sludge with a high moisture content (approximately 92.5 wt%), containing fine-grained particles from the mortar, is unfortunately generated. The liquid phase, which involves Ca-rich and severely alkaline wastewater, is separated from the sludge through a flocculation–precipitation process, before being recycled during the wet-based crushing process without any treatment, such as pH neutralization. The carbon mineralization process, which is a part of the carbon capture utilization and sequestration (CCUS) technology, is widely applied for reducing atmospheric carbon-dioxide (CO2) concentrations. During this process, carbonate-based materials, including CaCO3, magnesite (MgCO3), and dolomite (CaMg(CO3)2), are produced as commercially valuable materials [12,13]. In our previous study, a carbon mineralization process was performed in an effort to neutralize Ca-supersaturated alkaline wastewater from concrete sludge [14]. The process produced precipitated calcium carbonate (PCC; rhombic calcite, CaCO3) with a purity of 99.2%, and sequestered CO2 through a wastewater neutralization process. The solid phase (waste), which is emitted at a rate of 10 tons/day in waste concrete recycling plants, is useless in any industry; therefore, we should consider the concept of reuse with respect to waste recycling. Some studies investigated whether the application of concrete sludge as an (ab)sorbent could remove harmful elements/metalloids, including fluoride (F), boron (B), and arsenic (As) [15–17]. Concrete sludge contains a considerable number of Ca ions due to the dissolution of calcium hydroxide (Ca(OH)2), and is strongly alkaline. Therefore, these properties could be applied to the removal of divalent heavy metals in aqueous solutions [8,18,19]. In this study, the applicability of materials originating from concrete sludge as a metal-immobilizer was evaluated for the reduction of the mobility of metals, including copper (Cu), lead (Pb), and zinc (Zn), in aqueous solutions. Three materials from concrete sludge used in this study were prepared as follows: (1) dried sludge (DS; solid phase); (2) PCC produced via the carbon mineralization process using wastewater (liquid phase); and (3) calcite-coated particles (CCP) produced via the carbon mineralization process using dried sludge and wastewater (solid–liquid phase). The presented study focused on reducing environmental loads with respect to the neutralization of alkaline wastewater, CO2 sequestration, the treatment of metal-contaminated aqueous solutions, and recycling of concrete sludge waste (Figure1). Metals 2018, 8, x FOR PEER REVIEW 2 of 16 wet-based crushing process is commonly applied to remove impurities and to wash the concrete surface [11] and reduce air pollution. During this process, a significant amount of concrete sludge with a high moisture content (approximately 92.5 wt%), containing fine-grained particles from the mortar, is unfortunately generated. The liquid phase, which involves Ca-rich and severely alkaline wastewater, is separated from the sludge through a flocculation–precipitation process, before being recycled during the wet-based crushing process without any treatment, such as pH neutralization. The carbon mineralization process, which is a part of the carbon capture utilization and sequestration (CCUS) technology, is widely applied for reducing atmospheric carbon-dioxide (CO2) concentrations. During this process, carbonate-based materials, including CaCO3, magnesite (MgCO3), and dolomite (CaMg(CO3)2), are produced as commercially valuable materials [12,13]. In our previous study, a carbon mineralization process was performed in an effort to neutralize Ca-supersaturated alkaline wastewater from concrete sludge [14]. The process produced precipitated calcium carbonate (PCC; rhombic calcite, CaCO3) with a purity of 99.2%, and sequestered CO2 through a wastewater neutralization process. The solid phase (waste), which is emitted at a rate of 10 tons/day in waste concrete recycling plants, is useless in any industry; therefore, we should consider the concept of reuse with respect to waste recycling. Some studies investigated whether the application of concrete sludge as an (ab)sorbent could remove harmful elements/metalloids, including fluoride (F), boron (B), and arsenic (As) [15–17]. Concrete sludge contains a considerable number of Ca ions due to the dissolution of calciumMetals 2018 hydroxide, 8, 666 (Ca(OH)2), and is strongly alkaline. Therefore,3 of 17 these properties could be applied to the removal of divalent heavy metals in aqueous solutions [8,18,19].

Waste concrete

Wet-based Crushing/Screening

Concrete sludge

• Liquid-solid separation

Liquid phase Solid phase

• Oven-drying at 105°C

Carbon Carbon mineralization mineralization Process (Liquid) Process (slurry)

• CO2 sequestration • Alkaline wastewater neutralization

Precipitated- Neutralized- Neutralized- Calcite-coated calcium Dried sludge aqueous aqueous Particles carbonate (DS) solution solution (PCC) (CCP)

• Wastes recycling • Remediation of metal-contaminated aqueous solution

Immobilizer for removing metals Water (Cu, Pb, and Zn) recycling in aqueous solution Figure 1. A schematic ﬂow diagram of waste/wastewater recycling and the application of carbon Figure 1. A schematicmineralization flow diagramprocesses using of waste waste/wastewater concrete sludge. recycling and the application of carbon mineralization processes2. Materials andusing Methods waste concrete sludge.

2.1. Preparation of Immobilizers In this study, the applicability of materials originating from concrete sludge as a metal- immobilizer was evaluated2.1.1. Dried Sludge for (DS)the reduction of the mobility of metals, including copper (Cu), lead The concrete sludge sample used in this study was collected from a resource recycling plant (Pb), and zinc (Zn),of in construction aqueous waste solutions. in Incheon, Korea.Three The materials concentration from of the concre componentste insludge the sludge used in this study

Metals 2018, 8, 666 4 of 17

solution (liquid phase) diluted and acidified by 5 vol% nitric acid (HNO3) for dissolution of hydroxide-precipitates was analyzed using inductively coupled plasma optical emission spectroscopy (ICP-OES; 720-ES, Agilent, Santa Clara, CA, USA). The moisture content of the concrete sludge was determined based on oven-drying at 105 ◦C until there was no change in weight. The sludge was separated into liquid and solid phases via vacuum filtration using a membrane filter (Advantech, Tokyo, Japan) with a 0.45 µm pore size. The filtrate sample was completely sealed to prevent a decrease in pH and Ca concentration in solution via natural CaCO3 precipitation between Ca and atmospheric CO2, and was kept in an amber bottle at room temperature. The particle size distribution of the sludge was determined via the wet-sieving method using 20-, 100-, 325-, and 500-mesh sieves. In order to achieve a DS sample (<25 µm), the slurry containing fine particles was sieved through a 25 µm sieve, and was filtered with a 0.45 µm pore-sized membrane filter. After oven-drying the residual solids at 105 ◦C for 24 h, they were used as a metal-immobilizer sample.

2.1.2. Precipitated Calcium Carbonate (PCC) and Calcite-Coated Particles (CCP) Laboratory-scale carbon mineralization experiments, with a neutralization of the alkaline wastewater in concrete sludge, previously described in Reference [14], were carried out to produce the PCC and the CCP. The experiment was conducted according to the following procedure: 2.5 L of sludge solution, with solids removed, was carbonated in a reactor at ambient temperature and pressure, with a CO2 gas flow rate of 8 mL CO2/min/L solution (99.9% CO2 purity) until the solution pH dropped to 8.5 (reaction time of approximately 65 min). The CO2 gas flow rate was chosen to achieve the optimal CO2 sequestration (100.0%), removal of Ca in solution (99.2%), and purity of PCC (99.2%) when the solution pH was neutralized (target solution pH of 8.5). The process using 2.5 L of sludge solution produced approximately 2.7 g of PCC, which was highly dependent on the solution pH and Ca concentration. In order to coat completely the surfaces of the mother particles (DS) with calcite, CCP was produced under the same conditions using a mixture of 2.5 L of sludge solution and 12.5 mg of DS (0.005 w/v). A fixed quantity of DS, which was dispersed with a little sludge solution in order to prevent its aggregation, was added into the sludge solution. After the solution pH reached 8.5 (reaction time of approximately 65 min), the slurry was immediately filtered with a membrane filter (0.45 µm pore size) before the solid residue (CCP) was oven-dried at 105 ◦C.

2.1.3. Characterization of Immobilizers The pH of the immobilizers was determined using a mixture with a particles-to-deionized (DI) water ratio of 1:5 (w/v) according to the Korean Standard Test Method (KSTM-ES 07032) before the pH of the mixture was measured using a pH meter (ORION VSTAR-PH, Thermo Fisher Scientific Inc., Waltham, MA, USA). The surface morphology and elemental mapping, as well as the specific surface area of the immobilizers, were analyzed using scanning electron microscopy with energy-dispersive spectroscopy (SEM-EDS; 6380LA, JEOL Ltd., Tokyo, Japan), and an N2-based Brunauer–Emmett–Teller (BET) surface-area analyzer (BET analyzer; Tristar 3000, Micromeritics, Norcross, GA, USA), respectively. The mineralogy of the immobilizers was analyzed by X-ray diffraction (XRD; X0pert MPD, Philips, Amsterdam, The Netherlands) in combination with Reitveld quantification analysis. The XRD analysis was conducted with a θ/2θ goniometer using a Cu-Kα radiation. The patterns were obtained by step scanning from 3◦ to 65◦ with a step size of 0.01◦ 2θ.

2.2. Immobilization Experiments

2.2.1. Reaction Time

Copper nitrate trihydrate (Cu(NO3)2·3H2O; Junsei Chemical Co. Ltd., Tokyo, Japan), lead nitrate (Pb(NO3)2; Junsei Chemical), and zinc nitrate hexahydrate (Zn(NO3)2·6H2O; Junsei Chemical) were dissolved in deionized (DI) water to prepare the artiﬁcial metal-contaminated solutions. Speciﬁcally, Metals 2018, 8, 666 5 of 17

2.5 g of immobilizers (DS, PCC, and CCP) were added into 250 mL of metal solution at a concentration of 100 mg/L in an Erlenmeyer flask, before being mixed using a magnetic stirrer for 1440 min at room temperature. Ten milliliters of sample were collected periodically and filtered through a 0.45 µm syringe filter, before immediately measuring the pH of the filtrate. The concentration of elements in the filtrate was analyzed using ICP-OES. After 1440 minutes, the mixture was filtered via gravity filtration using a 0.45 µm pore-sized filter paper, and the residue was used for the identification of mineralogy using XRD, following oven-drying at 105 ◦C. In addition, some experiments using an increased concentration of 250 mL of metal solution (1000 mg/L) for 1440 min were conducted with 2.5 g of PCC, considering the detection limit of the XRD analysis. The chemical species, according to the pH and Eh in the aqueous phase, were simulated using the geochemical modeling program (Geochemist’s Workbench; GWB, Ver. 12.0.2, Aqueous Solutions LLC, Andover, MA, USA) based on the Lawrence Livermore National Laboratory (LLNL) thermo database (thermo.com.V8.R6+.tdat).

2.2.2. Solution pH and Immobilizer Dosage Batch experiments as a function of solution pH were carried out in order to evaluate the immobilization efficiency of metals. A mixture of 0.3 g of immobilizer (DS, PCC, and CCP) with 30 mL of metal solution with a concentration of 100 mg/L was adjusted to pH values of 1–5 with 1 M HNO3 in a 50 mL centrifuge tube before being mixed using an end-over-end shaker at 30 rpm for 1440 min at room temperature. In addition, the immobilization efficiency of the metals in aqueous solution, according to immobilizer dosage, was evaluated at 1:200, 1:500, and 1:1000 (w/v) with metal solutions of 100 mg/L for 1440 min at room temperature. After mixing, the final mixture was filtered using a 0.45 µm syringe filter before the filtrate was used to analyze the metal concentration using ICP-OES. The filtrate pH was immediately measured using the pH meter to prevent pH changes due to atmospheric CO2. The experimental conditions are summarized in Table1.

Table 1. The conditions for the immobilization experiments.

Time Conc. of Metals Initial Metal Immobilizer L/S Ratio 1 Temp. (◦C) (min) (mg/L) Solution pH Effect of reaction time DS 100 PCC Nonadjusted 1440 100 (250 mL : 2.5 g) 25 CCP Effect of initial solution pH 1.0 DS 2.0 100 PCC 1440 100 25 3.0 (30 mL : 0.3 g) CCP 4.0 5.0 Effect of immobilizer dosage DS 100–1000 PCC Nonadjusted 1440 100 (30 mL : 0.03–0.3 g) 25 CCP 1 Liquid-to-solid ratio. DS—dried sludge; PCC—precipitated calcium carbonate; CCP—calcite-coated particles.

3. Results and Discussion

3.1. Properties of Immobilizers Table2 shows the physico-chemical properties of the concrete sludge and the immobilizers used in this study. The pH values of the sludge solution, DS, PCC, and CCP were 12.4, 8.7, 11.0, and 10.5, respectively. Sludge solution pH varied between 12.2 and 12.7, which was highly dependent on the Metals 2018, 8, 666 6 of 17 supply of tap water during the wet-based crushing process. The pH of DS was quite a bit lower (pH of 8.7) because DS contains a considerable amount of quartz, muscovite, and feldspar, which does not affect the pH increase signiﬁcantly, derived from cement/mortar in the wet sieving process of waste concrete. In addition, more than 53% of sludge particles were separated from the sludge with a 500-mesh sieve (<25 µm) using the wet-sieving procedure.

Table 2. Physico-chemical properties of the concrete sludge and the immobilizers used in this study.

Concrete Sludge pH 12.4 ± 0.3 Moisture content (%) 92.5 ± 2.5 Ca2+ (mg/L) (liquid phase) 375.0 ± 63.0 Na+ (mg/L) (liquid phase) 426.5 ± 1.9 K+ (mg/L) (liquid phase) 307.2 ± 1.3 >850 µm 1.0 ± 0.1 850–150 µm 4.2 ± 0.2 Particle size distribution (wt%) 150–45 µm 26.2 ± 1.0 45–25 µm 15.2 ± 3.6 <25 µm 53.4 ± 3.0 Immobilizer DS (<25 µm) 8.7 ± 0.2 pH PCC 11.0 ± 0.0 CCP 10.5 ± 0.1 DS (<25 µm) 23.1 Brunauer–Emmett–Teller (BET) PCC 2.3 surface area (m2/g) CCP 4.7 Quartz (19.3 wt%), Calcite (24.2 wt%), Muscovite (30.9 wt%), Albite (11.3 wt%), DS (<25 µm) Vaterite (6.7 wt%), Orthoclase (5.5 wt%), Mineralogy Chlorite (2.0 wt%) PCC Calcite (100 wt%) CCP Calcite (100 wt%)

H2O 2+ − Ca(OH)2(s) → Ca + 2(OH) (1)

The mineralogy and morphology of the immobilizers are shown in Figures2a–c and3, respectively. Quartz (SiO2), muscovite (KAl2(Si3AlO10)(OH)2), and albite (NaAlSi3O8), derived from cement/mortar, were identiﬁed using XRD analysis to be the dominant minerals in DS. In addition, calcite (CaCO3) was partially observed, formed via the natural carbonation of lime (CaO) with atmospheric CO2 [20]. Metals 2018, 8, x FOR PEER REVIEW 6 of 16

The concentration of Ca in the sludge was in the range of 310–430 mg/L following the dissolution of Ca(OH)2 into the cement phases of concrete, which contributed to the changes in sludge pH according to the following Equation [14]: 퐻 푂 2 2+ − Ca(OH)2(푠) → Ca + 2(OH) (1) The mineralogy and morphology of the immobilizers are shown in Figures 2a–c and Figure 3, respectively. Quartz (SiO2), muscovite (KAl2(Si3AlO10)(OH)2), and albite (NaAlSi3O8), derived from cement/mortar, were identified using XRD analysis to be the dominant minerals in DS. In addition, calcite (CaCO3) was partially observed, formed via the natural carbonation of lime (CaO) with Metals 2018atmospheric, 8, 666 CO2 [20]. 7 of 17

Q (a) DS (<25 μ m) C V

Q A Q M Q

(b) PCC

C C CC C C C

(d) PCC-Cu

Intensity (cps) M1 M1 M1 M1

(e) H C1 PCC-Pb C1 C1 H H H

(f) PCC-Zn

Z S

10 20 30 40 50 60 2 theta

FigureFigure 2. Qualitative 2. Qualitative mineralogical mineralogical analysis analysis results results before (a (a–c)–c) and and after after (d– (d–f)f) immobilization immobilization experimentsexperiments for for 1000 1000 mg/L mg/L of of metal metal concentration concentration;; Qquartz Q—quartz (SiO2); C (SiOcalcite2); (CaCO C—calcite3); Aalbite (CaCO 3); A—albite(NaAlSi (NaAlSi3O8); 3O M8); M—muscovitemuscovite ((KAl ((KAl2(AlSi2(AlSi3O10)(F,OH)3O10)(F,OH)2); V2);vaterite V—vaterite (CaCO (CaCO3); M13); M1—malachitemalachite (Cu2(OH)(Cu2(OH)2CO23CO);3 C1—cerussite); C1cerussite (PbCO (PbCO3);3); H—hydrocerussite Hhydrocerussite (Pb (Pb3(CO(CO3)32(OH))2(OH)2); 2); S S—smithsonitesmithsonite (ZnCO3); Zhydrozincite (Zn5(CO3)2(OH)6). (ZnCOMetals3); Z—hydrozincite2018, 8, x FOR PEER REVIEW (Zn5(CO 3)2(OH)6). 7 of 16

3.0 μm 3.0 μm 3.0 μm 10.0 μm Ca Ca Ca Ca

O O O O

Si C C Si

Al Al

(a) DS (b) PCC (c) CCP (0.005 w/v) (d) CCP (0.2 w/v)

Figure 3. FigureThe results3. The results of scanning of scanning electron electron microscopy microscopy (SEM) (SEM) elemental elemental dot-mapping dot-mapping analysis of the analysis of initial immobilizers; (a) dried sludge (DS); (b) precipitated calcium carbonate (PCC); (c and d) calcite- the initialcoated immobilizers; particles (CCP). (a) dried sludge (DS); (b) precipitated calcium carbonate (PCC); (c and d) calcite-coated particles (CCP). For PCC and CCP, calcite was formed as the predominant mineral via the carbon mineralization process. Calcite in the supersaturated solution was formed through the stages of spontaneous nucleation and crystal growth [21]. The morphology of PCC exhibited a step-like growth of rhombic or rhombohedral calcite (Figure 3b). On the other hand, rhombohedral calcite was spherically agglomerated with spontaneous crystal growth on the rough surfaces of DS (Figure 3c), which led to the production of calcite-coated particles. Here, the rhombohedral calcite grown on the surfaces of the DS might provide sufficient reaction sites to accelerate the reactions with heavy metals in the aqueous solution during the immobilization processes. Unfortunately, an increase in pulp density of up to 0.2 (w/v) (Figure 3d) when producing CCP led to an inhibition of calcite-coating onto the surfaces of sludge particles, and to a decrease in calcite content (75.9%). The BET surface areas of the immobilizers, in increasing order, were DS (23.1) >> CCP (4.7) > PCC (2.3). These values might be increased due to the heterogenetic and rough fine-particles contained in the DS.

3.2. Metal Immobilization in Aqueous Solution

3.2.1. Effect of Reaction Time The removal of metals from the solution and the change in solution pH with reaction time are shown in Figure 4, with the efficiencies (R) calculated according to the following equation:

퐶 푅 (%) = (1 − 푡) × 100, (2) 퐶0

Metals 2018, 8, 666 8 of 17

For PCC and CCP, calcite was formed as the predominant mineral via the carbon mineralization process. Calcite in the supersaturated solution was formed through the stages of spontaneous nucleation and crystal growth [21]. The morphology of PCC exhibited a step-like growth of rhombic or rhombohedral calcite (Figure3b). On the other hand, rhombohedral calcite was spherically agglomerated with spontaneous crystal growth on the rough surfaces of DS (Figure3c), which led to the production of calcite-coated particles. Here, the rhombohedral calcite grown on the surfaces of the DS might provide sufﬁcient reaction sites to accelerate the reactions with heavy metals in the aqueous solution during the immobilization processes. Unfortunately, an increase in pulp density of up to 0.2 (w/v) (Figure3d) when producing CCP led to an inhibition of calcite-coating onto the surfaces of sludge particles, and to a decrease in calcite content (75.9%). The BET surface areas of the immobilizers, in increasing order, were DS (23.1) >> CCP (4.7) > PCC (2.3). These values might be increased due to the heterogenetic and rough ﬁne-particles contained in the DS.

3.2. Metal Immobilization in Aqueous Solution

3.2.1. Effect of Reaction Time The removal of metals from the solution and the change in solution pH with reaction time are shown in Figure4, with the efﬁciencies (R) calculated according to the following equation:

C R (%) = 1 − t × 100, (2) C0 where Ct and C0 are the concentrations of the metals remaining in solution after a specified reaction time and before the reaction, respectively. The immobilizers increased the removal of Cu, Pb, and Zn from solution with increasing reaction time, with a removal efficiency of 98.3–100.0% for Cu, 99.9–100.0% for Pb, and 95.2–100.0% for Zn for DS; 19.6–99.9% for Cu, 99.9–100.0% for Pb, and 30.1–99.9% for Zn for PCC; and 9.2–100.0% for Cu, 99.9–100.0% for Pb, and 19.6–100.0% for Zn for CCP during 1440 min (Figure4a-1,b-1,c-1). Interestingly, the removal trends of the metals were significantly influenced by the changes in solution pH during the reaction, regardless of immobilizer type (Figure4a-2,b-2,c-2). The behavior of the metals in aqueous solution in the presence of the immobilizers was highly influenced by the equilibrium pH, reaction time, and immobilizer dosage. In particular, divalent metals, including Cu, Pb, and Zn, formed various complexes via a combination with anionic ligands in aqueous solution, depending on the equilibrium pH [22]. The removal mechanisms of the metals by calcite-bearing immobilizers, including PCC and CCP, can be associated with the interactions between metals and the carbonates released from calcite, as well as the equilibrium pH. However, in the case of DS, the metals might be removed through the coprecipitation of metal hydroxides (Me–OH) onto the surface of DS under high-pH conditions, rather than through metal-carbonate precipitation, due to an insignificant amount of calcite contained in DS [5,23]. Metals 2018, 8, x FOR PEER REVIEW 8 of 16 where Ct and C0 are the concentrations of the metals remaining in solution after a specified reaction time and before the reaction, respectively. The immobilizers increased the removal of Cu, Pb, and Zn from solution with increasing reaction time, with a removal efficiency of 98.3–100.0% for Cu, 99.9– 100.0% for Pb, and 95.2–100.0% for Zn for DS; 19.6–99.9% for Cu, 99.9–100.0% for Pb, and 30.1–99.9% for Zn for PCC; and 9.2–100.0% for Cu, 99.9–100.0% for Pb, and 19.6–100.0% for Zn for CCP during 1440 min (Figures 4a-1,b-1,c-1).

Interestingly, the removal trends of the metals were significantly influenced by the changes in solution pH during the reaction, regardless of immobilizer type (Figures 4a-2,b-2,c-2). The behavior of the metals in aqueous solution in the presence of the immobilizers was highly influenced by the equilibrium pH, reaction time, and immobilizer dosage. In particular, divalent metals, including Cu, Pb, and Zn, formed various complexes via a combination with anionic ligands in aqueous solution, depending on the equilibrium pH [22]. The removal mechanisms of the metals by calcite-bearing immobilizers, including PCC and CCP, can be associated with the interactions between metals and the carbonates released from calcite, as well as the equilibrium pH. However, in the case of DS, the metals might be removed through the coprecipitation of metal hydroxides (Me–OH) onto the surface of DS under high-pH conditions, rather than through metal-carbonate precipitation, due to an insignificant amount of calcite contained in DS [5,23]. Metals 2018, 8, 666 9 of 17

120 10 (a-1) (a-2) 100 9

80 8

60 7

40 SolutionpH 6

Cu removal, Cu %

20 5

0 4 0 200 400 600 800 1000 1200 1400 1600 0 200 400 600 800 1000 1200 1400 1600 120 10 (b-1) (b-2) 100 9

80 8

60 7

40 SolutionpH 6

Pbremoval, %

20 5

0 4 0 200 400 600 800 1000 1200 1400 1600 0 200 400 600 800 1000 1200 1400 1600

120 10 (c-1) (c-2) 100 9

80 8

60 7

40 SolutionpH 6

Zn removal, Zn % DS DS 20 PCC 5 PCC CCP CCP 0 4 0 200 400 600 800 1000 1200 1400 1600 0 200 400 600 800 1000 1200 1400 1600 Reaction time (min) Reaction time (min)

Figure 4. The removal efﬁciency of metals, and the changes in solution pH over time during the immobilization experiments for Cu (a), Pb (b), and Zn (c).

Calcite might affect the behavior of metals through the interactions occurring via ionic exchange, surface complexation, and precipitation [24]. In the case of precipitation, the stability of the chemical species of metals is heavily inﬂuenced by their solubility under certain pH and Eh conditions [4]. The concentration of Ca dissolved in solution during the reactions is shown in Figure5. In the aqueous 2+ − 2− phase, calcite can be dissolved into Ca , HCO3 , and CO3 according to the following stoichiometric equations [25]:

2+ − − 2+ 2− CaCO3(s) + H2O ↔ Ca + HCO3 + OH ↔ Ca + CO3 + H2O. (3) Metals 2018, 8, x FOR PEER REVIEW 9 of 16

Figure 4. The removal efficiency of metals, and the changes in solution pH over time during the immobilization experiments for Cu (a), Pb (b), and Zn (c).

Calcite might affect the behavior of metals through the interactions occurring via ionic exchange, surface complexation, and precipitation [24]. In the case of precipitation, the stability of the chemical species of metals is heavily influenced by their solubility under certain pH and Eh conditions [4]. The concentration of Ca dissolved in solution during the reactions is shown in Figure 5. In the aqueous phase, calcite can be dissolved into Ca2+, HCO3−, and CO32− according to the following stoichiometric equations [25]:

2+ − − 2+ 2− CaCO3(s) + H2O ↔ Ca + HCO3 + OH ↔ Ca + CO3 + H2O. (3)

The dissolution of calcite is dependent on the solubility product (Ksp of calcite = 10−8.48) [24], which increases with an increasing number of protons (i.e., acidic pH). In addition, the solutions containing each metal, which show different solution pH, might affect the dissolution of calcite because of their different acid dissociation constant (pKa) values of metal complexes in the solution. Although bicarbonate (HCO3−) is the predominant ion in weakly alkaline pH ranges (approximately 8.0–9.0), based on the acid dissociation constant (pKa) for carbon species, metal-carbonate minerals are preferentially precipitated via complexation between Me2+ and CO32− in the presence of metals in solution. Therefore, the CO32− concentration ([CO32−]) consumed by the precipitation of metal carbonates, regardless of atmospheric CO2 dissolution, can be indirectly calculated from [Ca2+] in solution, based on the molar ratio between [Ca2+] and [CO32−] dissolved in solution (Equation 3). The carbonates might originate from calcite in the immobilizers, including PCC and CCP. The molar ratios between [CO32−], calculated from the stoichiometry equation, and [Me2+], removed from solution via metal-carbonate precipitation, were approximately 1.1–2.4 for Cu, 1.2–1.4 for Pb, and 1.1– 1.7 for Zn (Figure 6). This indicates that sufficient carbonates were dissolved from calcite during the reactions. In addition, approximately 0.35%–2.26%, 0.57%–0.66%, and 0.33%–2.59% of the calcite contained in PCC and CCP were dissolved by Cu, Pb, and Zn during the reactions, respectively (the results were indirectly calculated from the concentration of Ca2+ and/or carbonates released into solution). In the case of DS, although the Ca released from DS was much higher than that from PCC and CCP, [CO32−] could not be calculated according to the equation because atmospheric CO2 dissolutionMetals 2018 was, 8, 666 potentially a substantial source of carbonates. 10 of 17

70 140 (a) (b) 140 (c) 60 120 120 50 100 100 40 80 80 30 60 60

40 20 40 DS 20 10 20 PCC CCP 0 0 0 0 200 400 600 800 1000 1200 1400 1600 0 200 400 600 800 1000 1200 1400 1600 0 200 400 600 800 1000 1200 1400 1600 Concentrationin Ca of solution (mg/L) Reaction time (min) Reaction time (min) Reaction time (min) Figure 5. The concentration of Ca released from immobilizers over time during the immobilization Figure 5. The concentration of Ca released from immobilizers over time during the immobilization experiments for Cu (a), Pb (b), and Zn (c). experiments for Cu (a), Pb (b), and Zn (c).

The mineralogical analysis using XRD did not detect Cu- and Zn-bearing minerals−8.48 due to the The dissolution of calcite is dependent on the solubility product (Ksp of calcite = 10 )[24], lower initial concentration of the metals (100 mg/L); only hydrocerussite (Pb3(CO3)2(OH)2) was which increases with an increasing number of protons (i.e., acidic pH). In addition, the solutions detectedcontaining as minor each peaks metal, (Figure which S1 show; Supplementary different solution material pH, might). A high affecter concentration the dissolution of of metals calcite (1000 mg/L)because were of carbonate their different-bearing acid dissociation minerals, constantincluding (pKa) malachite values of (Cu metal2(OH) complexes2CO3), incerussite the solution. (PbCO3), − hydrocerussite,Although bicarbonate smithsonite (HCO 3 (ZnCO) is the3), predominant and hydrozincite ion in weakly (Zn5(CO alkaline3)2(OH) pH ranges6), which (approximately were detected (Figure8.0–9.0), 2). The based final on thesolution acid dissociation pH values constant for Cu, (pKa)Pb, and for carbonZn after species, 1440 min metal-carbonate were approximately minerals 6.8, 2+ 2− 7.6, areand preferentially 6.9, respectively. precipitated Du et viaal. complexation[26] reported between that the Me applicationand CO3 of razorin the presenceclam shells of metals (aragonite 2− 2− in solution. Therefore, the CO3 concentration ([CO3 ]) consumed by the precipitation of metal 2+ carbonates, regardless of atmospheric CO2 dissolution, can be indirectly calculated from [Ca ] in 2+ 2− solution, based on the molar ratio between [Ca ] and [CO3 ] dissolved in solution (Equation 3). The carbonates might originate from calcite in the immobilizers, including PCC and CCP. 2− 2+ The molar ratios between [CO3 ], calculated from the stoichiometry equation, and [Me ], removed from solution via metal-carbonate precipitation, were approximately 1.1–2.4 for Cu, 1.2–1.4 for Pb, and 1.1–1.7 for Zn (Figure6). This indicates that sufficient carbonates were dissolved from calcite during the reactions. In addition, approximately 0.35%–2.26%, 0.57%–0.66%, and 0.33%–2.59% of the calcite contained in PCC and CCP were dissolved by Cu, Pb, and Zn during the reactions, respectively (the results were indirectly calculated from the concentration of Ca2+ and/or carbonates released into solution). In the case of DS, although the Ca released from DS was much higher than that from 2− PCC and CCP, [CO3 ] could not be calculated according to the equation because atmospheric CO2 dissolution was potentially a substantial source of carbonates. The mineralogical analysis using XRD did not detect Cu- and Zn-bearing minerals due to the lower initial concentration of the metals (100 mg/L); only hydrocerussite (Pb3(CO3)2(OH)2) was detected as minor peaks (Figure S1; Supplementary material). A higher concentration of metals (1000 mg/L) were carbonate-bearing minerals, including malachite (Cu2(OH)2CO3), cerussite (PbCO3), hydrocerussite, smithsonite (ZnCO3), and hydrozincite (Zn5(CO3)2(OH)6), which were detected (Figure2). The final solution pH values for Cu, Pb, and Zn after 1440 min were approximately 6.8, 7.6, and 6.9, respectively. Du et al. [26] reported that the application of razor clam shells (aragonite phase) and oyster shells (calcite phase) as sorbents removed Pb and Zn through the precipitation of cerussite and hydrozincite upon combining with the carbonate ions released from sorbents. Furthermore, Sarioglu et al. [27] reported that the removal mechanism of Cu in artificial drinking water involves the formation of 2− complexes between Cu and carbonate, sulfate (SO4 ), and hydroxide ions in biosolids, such as waste sludge cake derived from wastewater treatment plants. Metals 2018, 8, x FOR PEER REVIEW 10 of 16 phase) and oyster shells (calcite phase) as sorbents removed Pb and Zn through the precipitation of cerussite and hydrozincite upon combining with the carbonate ions released from sorbents. Furthermore, Sarioglu et al. [27] reported that the removal mechanism of Cu in artificial drinking water involves the formation of complexes between Cu and carbonate, sulfate (SO42−), and hydroxide ions in biosolids, suchMetals as2018 waste, 8, 666 sludge cake derived from wastewater treatment11 plants. of 17

0.018 0.018

0.016 DS 0.016 PCC PCC CCP 0.014 CCP 0.014 0.012 0.012

0.010 0.010

0.008 0.008

0.006 0.006

0.004 0.004

Cu removal Cu (mmol)

Cu in Cu solution (mmol) 0.002 0.002

0.000 0.000 6.0 6.5 7.0 7.5 8.0 8.5 9.0 0.000 0.005 0.010 0.015 0.020 0.025

0.014 0.018

DS 0.016 PCC 0.012 PCC CCP CCP 0.014 0.010 0.012

0.008 0.010

0.006 0.008

0.006 0.004 0.004

Zn removal Zn (mmol)

Zn in solution Zn (mmol) 0.002 0.002

0.000 0.000 6.5 7.0 7.5 8.0 8.5 9.0 0.000 0.001 0.002 0.003 0.004 0.005 0.006 Solution pH Carbonate in solution (mmol)

Figure 6. Summary of the relationship between the removal of Cu/Zn and final solution pH or Figure 6. Summary carbonate of the levels relationship in solution during between the immobilization the removal experiments according of Cu/Zn to reaction and time. final solution pH or carbonate levels in solutionThe formation during of these the carbonate-bearing immobilization minerals experiments corresponded according exactly with theto Pourbaixreaction time. diagrams simulated using the geochemical modeling program (Figure7a-1,b-1,c-1). It might be considered that the reactions in the immobilization processes were carried out at positive Eh (oxidation The formation ofstates). these For the carbonate PCC and CCP-bearing immobilizers, minerals in thermodynamic corresponded equilibrium, theexactly predominant with the Pourbaix diagrams simulated chemicalusing species the ofgeochemical metals in aqueous solution modeling in the presence program of (bi)carbonate (Figure ionss were 7a malachite,-1,b-1,c-1). It might be cerussite, hydrocerussite, hydrozincite, and smithsonite in their respective pH ranges [8,28–31]. In the considered that the results reactions of XRD analysis in the (Figure immobilization2), actually, metal-bearing proce precipitatessses contained were in carried the immobilizer out at positive Eh (oxidation states). Forwere identified the PCC after immobilization and CCP experiments immobilizers, for only a higher in concentration thermodynamic of heavy metals equilibrium, the (1000 mg/L). However, the predominant chemical species of metals in concentrations of 100 and predominant chemical1000 species mg/L might of be themetals same because in aqueous the final solution solution pHs are inin their the respective presence pH ranges of for(bi)carbonate ions formation of the species. In detail, the removal efficiency of Cu in solution was approximately 46.6% were malachite, cerussite,for PCC and hydroce 30.8% for CCPrussite, after 120 hydrozincite min of reaction time, and at a solution smithsonite pH of 6.4 (Figure in 4theird–f). respective pH ranges [8,28–31]. In Subsequently, the results the concentration of XRD of Cuanalysis gradually decreased (Figure until 2), 540 min actually, despite a negligible metal change-bearing precipitates in pH for both PCC and CCP. This indicates that the unstable malachite formed at the beginning of the contained in the immobilizerreaction was aged were for 1440 identified min of reaction time after before immobilization being precipitated onto experimentsthe surface of calcite as for only a higher concentration of heavythe reactionmetals progressed. (1000 Formg/L). Zn, the removal However, in solution the was sharplypredominant increased by approximately chemical 35.1% species of metals in in PCC for 5 min and 67.9% in CCP for 10 min, through the formation of smithsonite in the pH range of concentrations of 1007.1–7.2. and Subsequently, 1000 mg/L removal might reached be 99.9% the after same 1440 min because following the the increase final in pH solution beyond 8.0 pHs are in their and aging with hydrozincite, producing a very stable Zn hydroxycarbonate phase (Ksp of hydrozincite respective pH ranges= 10for−14.9 formation)[32,33]. In contrast, of the with Cuspecies. and Zn, significant In detail, Pb removal the via removal the precipitations efficiency of cerussite of Cu in solution was approximately 46.6%and hydrocerussite for PCC was and observed 30.8% due tofor the CCP presence after of sufficient 120 min carbonates of reaction in solution, andtime the at a solution pH of 6.4 (Figures 4d‒f). Subsequently, the concentration of Cu gradually decreased until 540 min despite a negligible change in pH for both PCC and CCP. This indicates that the unstable malachite formed at the beginning of the reaction was aged for 1440 min of reaction time before being precipitated onto the surface of calcite as the reaction progressed. For Zn, the removal in solution was sharply increased by approximately 35.1% in PCC for 5 min and 67.9% in CCP for 10 min, through the formation of smithsonite in the pH range of 7.1–7.2. Subsequently, removal reached 99.9% after 1440 min following the increase in pH beyond 8.0 and aging with hydrozincite, producing a very stable Zn hydroxycarbonate phase (Ksp of hydrozincite = 10−14.9) [32,33]. In contrast, with Cu and Zn, significant Pb removal via the precipitations of cerussite and hydrocerussite was observed due to the presence of sufficient carbonates in solution, and the maintenance of a high solution pH (above 8.3) until the end of the experiments. Actually, this study suggests only the removal mechanisms of metals by immobilizers derived from concrete sludge in model solutions, thus it should be conducted to evaluate the applicability of the processes to actual wastewater. When the processes are applied in a scaled-up version, a large amount of sludge wastes (by-products) containing heavy metals, which are

Metals 2018, 8, 666 12 of 17

maintenance of a high solution pH (above 8.3) until the end of the experiments. Actually, this study suggests only the removal mechanisms of metals by immobilizers derived from concrete sludge in model solutions, thus it should be conducted to evaluate the applicability of the processes to actual wastewater. When the processes are applied in a scaled-up version, a large amount of sludge wastes Metals(by-products) 2018, 8, x FOR containing PEER REVIEW heavy metals, which are inevitably generated, should be considered to treat11 of 16 through separation with sedimentation and/or ﬁltration. For this, the environmental impacts of heavy inevitably generated, should be considered to treat through separation with sedimentation and/or metals in by-products should be evaluated via detailed characterizations of the metals including filtration.fractionation For this, and mobility/bioaccessibility.the environmental impacts of heavy metals in by-products should be evaluated via detailed characterizations of the metals including fractionation and mobility/bioaccessibility.

(a-1) (b-1) (c-1) 6.7 8.4 7.2

4.7 3.2 5.7 25℃ 25℃ 25℃

(a-2) (b-2) (c-2) 9.6 9.1

4.5 3.4 25℃ 25℃ 25℃

FigureFigure 7. 7.PourbaixPourbaix diagrams diagrams for for Cu Cu ( (aa),), Pb Pb ( (b), and ZnZn ((cc)) usingusing the the Lawrence Lawrence Livermore Livermore National National LaboratoryLaboratory (LLNL) (LLNL) thermodynamic thermodynamic database database simulated with thethe Geochemist’sGeochemist’s Workbench Workbench (GWB) (GWB) 2+ softwaresoftware (dashed (dashed lines; lines; stability stability limits limits of of water water at at 1 1atm atm pressure). pressure). Input Input data: data: a[Cu a[Cu2+] =] 0.0016, = 0.0016, and − 2+ − and a[HCO3 ] = 0.0021 (a-1) and 0.0549 (a-2); a[Pb ] = 0.00048, and a[HCO3 ] = 0.00073 (b-1) and a[HCO3−] = 0.0021 (a-1) and 0.0549 (a-2); a[Pb2+] = 0.00048, and a[HCO3−] = 0.00073 (b-1) and 0.0492 (b- 2+ − 0.0492 (b-2); a[Zn ] = 0.0015, and a[HCO3 ] = 0.002 (c-1) and 0.0511 (c-2). Minerals: Malachite, 2); a[Zn2+] = 0.0015, and a[HCO3−] = 0.002 (c-1) and 0.0511 (c-2). Minerals: Malachite, Cu2(OH)2CO3; Cu2(OH)2CO3; Tenorite, CuO; Cuprite, Cu2O; Cerussite, PbCO3; Hydrocerussite, Pb3(CO3)2(OH)2; Tenorite, CuO; Cuprite, Cu2O; Cerussite, PbCO3; Hydrocerussite, Pb3(CO3)2(OH)2; Plattnerite, Pb2O; Plattnerite, Pb2O; Hydrozincite, Zn5(CO3)2(OH)6; Zincite, ZnO. Hydrozincite, Zn5(CO3)2(OH)6; Zincite, ZnO. 3.2.2. Effect of Initial Solution pH and Immobilizer Dosage 3.2.2. Effect of Initial Solution pH and Immobilizer Dosage Initial solution pH is considered as an important variable in the immobilization process via the Initial solution pH is considered as an important variable in the immobilization process via the precipitation of metals. The removal of metals as a function of initial solution pH (1–5) is shown precipitationin Figure8. of In metals. the application The removal of DS, of the metals removal as a efficienciesfunction of ofinitial Cu, Pb,solution and ZnpH in (1 an–5) initial is shown pH in Figureof 1.0 8. were In the approximately application of 2.9%, DS, 12.6%,the removal and 0.0%, efficiencies respectively. of Cu, Clearly, Pb, and the Zn metal in an species initial existpH of in 1.0 werethe approximately ionic form of Me 2.9%,2+ in 12.6%, acidic and conditions; 0.0%, respectively. thus, a much Clearly, lower removalthe metal efficiency species exist of metals in the was ionic formobserved of Me2+ due in acidic to the inhibitionconditions; of thus, metal-hydroxide a much lower precipitation removal efficiency [31,34]. On of metals the contrary, was observed in the case due to ofthe carbonate-bearing inhibition of metal immobilizers,-hydroxide PCCprecipitation and CCP, the[31,34] removal. On the of metals contrary, was i significantlyn the case of enhanced carbonate- bearingdue to immobilizers, a sufficient amount PCC andof carbonates CCP, the released removal from of metals calcite was under significantly the weakly enhancedacidic conditions. due to a sufficientFurthermore, amount the removal of carbonates of metals might released be enhanced from calcite with a under longer reaction the weakly time of acidic 1440 min, conditions. which Furthermore,would contribute the removal to the aging of metals of unstable might precipitates. be enhanced In with addition, a longer large reaction amounts time of carbonate of 1440 min, resulted which − wouldin the contribute inhibition of to a significant the aging pH of decrease unstable through precipitates. the formation In addition, of bicarbonate large amounts (HCO3 ) of as carbonate a buffer resulted in the inhibition of a significant pH decrease through the formation of bicarbonate (HCO3−) as a buffer to protons [35]. Hence, more than 95% of the metals were effectively removed in the initial pH range of 2–5.

Metals 2018, 8, 666 13 of 17

Metals 2018to, 8 protons, x FOR PEER [35]. Hence,REVIEW more than 95% of the metals were effectively removed in the initial pH range12 of 16 of 2–5.

DS-Metal Concentration PCC-Metal Concentration CCP-Metal Concentration DS-pH PCC-pH PCC-Carbonates CCP-pH CCP-Carbonates 110 12 0.7 (a-1) 100 (a-2) 0.6 90 10

80 0.5 8 70 0.4 60 6 50 0.3 40 4 30 0.2

Finalsolution pH 20

Cu in Cu solution (mg/L) 2 0.1 10

0 0 0.0 Carbonatesin solution (mmol) 1 2 3 4 5 1 2 3 4 5 110 12 0.6

100 (b-1) (b-2)

90 10 0.5

80 8 0.4 70

60 6 0.3 50

40 4 0.2 30

Finalsolution pH 20 Pbin solution (mg/L) 2 0.1 10

0 0 0.0 Carbonatesin solution (mmol) 1 2 3 4 5 1 2 3 4 5 110 12 0.6 (c-1) (c-2) 100

90 10 0.5

80 8 0.4 70

60 6 0.3 50

40 4 0.2 30

Finalsolution pH 20 Zn in solution Zn (mg/L) 2 0.1 10

0 0 0.0 Carbonatesin solution (mmol) 1 2 3 4 5 1 2 3 4 5 Initial solution pH Initial solution pH

Figure 8. Summary of metal removal, final solution pH, and carbonate level in solution after the Figure 8.immobilization Summary of experiments metal removal, for Cu (a ), final Pb (b ), solution and Zn (c pH,) as a and function carbonate of initial solution level in pH solution (experimental after the immobilizationconditions: experiments initial metal concentration, for Cu (a), 100 Pb mg/L; (b), reactionand Zn time, (c) 1440 as a min; function liquid/solid of initial ratio, 100). solution pH (experimental conditions: initial metal concentration, 100 mg/L; reaction time, 1440 min; liquid/solid ratio, 100).Based on the experimental results, the mechanism of metal removal in aqueous solution involves precipitation onto the surface of immobilizers, which is strongly inﬂuenced by the concentration of Basedcarbonates on the experimental and the equilibrium results, pH. Therefore,the mechanism metal removal of metal is heavilyremoval dependent in aqueous on the solution immobilizer involves precipitationdosage. onto Figure the9 showssurface the of removal immobilizers, of metals atwhich various is ratiosstrongly of liquid influenced to solid inby the the immobilization concentration of carbonatesexperiments. and the As equilibrium expected, the removal pH. Therefore, of metals was metal highly removal dependent is on heavily immobilizer dependent dosage due on to the changes in pH and the release of carbonates. It was clearly observed that the application of a smaller immobilizer dosage. Figure 9 shows the removal of metals at various ratios of liquid to solid in the immobilization experiments. As expected, the removal of metals was highly dependent on immobilizer dosage due to changes in pH and the release of carbonates. It was clearly observed that the application of a smaller immobilizer dosage did not enhance the removal of Cu and Zn, due to the low content of carbonates and the insignificant increase in solution pH.

Metals 2018, 8, 666 14 of 17

Metals 2018immobilizer, 8, x FOR PEER dosage REVIEW did not enhance the removal of Cu and Zn, due to the low content of carbonates 13 of 16 and the insigniﬁcant increase in solution pH.

L/S ratio; 100 - Metal concentration L/S ratio; 200 - Metal concentration L/S ratio; 500 - Metal concentration L/S ratio; 1000 - Metal concentration L/S ratio; 100 - pH L/S ratio; 100 - Carbonates L/S ratio; 200 - pH L/S ratio; 200 - Carbonates L/S ratio; 500 - pH L/S ratio; 500 - Carbonates L/S ratio; 1000 - pH L/S ratio; 1000 - Carbonates 120 12 25 (a-1) (a-2) 100 10 20

80 8 15

60 6

10 40 4

Finalsolution pH 5

Cu in Cu solution (mg/L) 20 2

0 0 0 Carbonatesin solution (μ mol) DS PCC CCP PCC CCP 0.16 12 10 (b-1) (b-2) 0.14 10 8 0.12 8 0.10 6

0.08 6

4 0.06 4 0.04 Finalsolution pH 2

Pbin solution (mg/L) 2 0.02

0.00 0 0 Carbonatesin solution (μ mol) DS PCC CCP PCC CCP 5 12 30 (c-1) (c-2) 10 25 4

8 20 3

6 15

2 4 10

1 Finalsolution pH

Zn in solution Zn (mg/L) 2 5

0 0 0 Carbonatesin solution (μ mol) DS PCC CCP PCC CCP

Figure 9. Summary of metal removal, final solution pH, and carbonate level in solution after Figurethe 9. Summary immobilization of metal experiments removal, for Cu final (a), so Pblution (b), and pH, Zn and (c) at carbonate various ratios level of in liquid solution to solid after the immobilization(experimental experiments conditions: initial for Cu concentration (a), Pb (b of), metals, and Zn100 mg/L; (c) at reaction various time, ratios 1440 of min) liquid. to solid (experimental conditions: initial concentration of metals, 100 mg/L; reaction time, 1440 min). In addition, the amount of carbonates released in solution increased with increasing immobilizer Indosage. addition, The the removal amount of of Pb carbonates was not directly released affected in solution by immobilizer increased dosage with because increasing higher immobilizer final solution-pH ranges of 8.8–10.2 were maintained during the experiments. In the application of dosage. The removal of Pb was not directly affected by immobilizer dosage because higher final CCP, it was confirmed that the removal mechanism of metals from aqueous solution involved the solutionprecipitation-pH ranges of of metals, 8.8–10.2 according were to maintained pH and the amountduring ofthe carbonate experiments. released In from the CCP, application as opposed of CCP, it was to confirmed calcite-coating that onto the the removal surface of mechanism sludge particles. of metals from aqueous solution involved the precipitation of metals, according to pH and the amount of carbonate released from CCP, as opposed to calcite-coating onto the surface of sludge particles.

Metals 2018, 8, 666 15 of 17

3.3. Estimation of Environmental Impact In this study, we estimated the environmental impact of the application of metal-immobilization processes using materials derived (DS) and manufactured (PCC and CCP) from concrete sludge, based on the metal-immobilization experimental results (Table3). For a volume of 3600 m 3/day generated from the plant, approximately 345.6 tons/day of DS can be achieved through the wet-sieving procedure. As demonstrated, the direct use of DS as a metal-immobilizer was effective in utilizing vast amounts of waste and removing Cu, Pb, and Zn from contaminated aqueous solutions. In contrast, CCP (3.8 tons/day) produced through carbon mineralization could signiﬁcantly reduce environmental loads, in terms of CO2 sequestration (3.4 tons/day), wastewater/waste recycling, and the treatment of contaminated solutions. Therefore, concrete sludge can be utilized for the reduction of environmental loads. In addition, further studies are required to understand the effect of increasing the pulp density of sludge wastes on the production of CCP through carbon mineralization, to enhance the recycling rate for sludge waste.

Table 3. Estimation of environmental impact in the immobilization processes for metals in aqueous solution using concrete sludge, based on the experimental results.

Output Production 2 CO sequestration 6 Treatment 7 Materials 2 (m3/d) (tons/d) (tons/d) (m3/d) Wastewater 3330.0 1 --- Concrete (Liquid phase) sludge Waste 270.0 1 --- (Solid phase) DS - 345.6 3 - 34,560.0 PCC - 3.6 4 3.4 359.6 CCP - 3.8 5 3.4 376.3 1 Moisture content: 92.5% based on 3,600 m3/day from the ﬁeld-resource recycling plant of construction waste selected in this study; 2 waste recycling; 3 portion of <25 µm in waste: 53.4 wt%, and bulk density of concrete: 2400 kg/m3; 4 PCC production: 1.08 g PCC/L wastewater during carbon mineralization process; 5 CCP production: mixture with DS-to-wastewater ratio of 200 (v/w) in PCC-producing process; 6 conditions of carbon mineralization: 8 mL CO2/min (99.9% CO2 purity) for 65 min until the solution pH dropped to 8.5 (CO2 sequestration: 100.0%); 7 treatment of Cu-, Pb-, and Zn-contaminated aqueous solution in a concentration of 100 mg/L: liquid/solid ratio of 100 (v/w) for 1440 min without pH adjustments (metal removal of 100.0%).

4. Conclusions The main purpose of this study was to evaluate the applicability of immobilizers derived from concrete sludge for the removal of Cu, Pb, and Zn from aqueous solution. For immobilizers, dried sludge (DS) and calcite-predominant (precipitated calcium carbonate, PCC; calcite-coated particles, CCP) particles were produced via a wet-sieving method using a 25 µm sieve and a carbon mineralization process, respectively. It was conﬁrmed that DS was able to remove metals in aqueous solution through the coprecipitation of metal hydroxides onto the surface, depending on pH. In the applications of PCC and CCP, the main mechanism of metal removal involved the formation of metal carbonates, which was heavily dependent on both increases in pH and the amount of carbonates released from the immobilizers. A longer reaction time could contribute to the stability of unstable metal-carbonate precipitations. In particular, Pb was completely removed via the formation of stable chemical species with an increase in pH within the shortened contact time, regardless of immobilizer type. In this study, the applicability of concrete sludge as an immobilizer for the removal of metals including Cu, Pb, and Zn in aqueous solution was demonstrated through the application of traditional techniques of pH control and coprecipitation. In addition, carbon mineralization process using abandoned concrete sludge for production of immobilizers is expected to solve environmental issues including wastewater/waste discharge, resource depletion, and CO2 emission. Metals 2018, 8, 666 16 of 17

Supplementary Materials: The following are available online at http://www.mdpi.com/2075-4701/8/9/666/s1, Figure S1: The results of XRD analysis for PCC and CCP after immobilization experiments in a metal concentration of 100 mg/L; C—calcite (CaCO3); H—hydrocerussite (Pb3(CO3)2(OH)2). Author Contributions: Conceptualization, H.S.; Methodology, H.S., J.Y., and S.J.; Validation, J.Y. and S.J.; Formal Analysis, J.Y.; Investigation, J.Y., H.S., and S.J.; Resources, H.S. and S.J.; Data Curation, J.Y.; Writing—Original Draft Preparation, J.Y.; Writing—Review & Editing, H.S. and S.J.; Visualization, J.Y.; Supervision, H.S.; Project Administration, H.S.; Funding Acquisition, H.S. Funding: This research received no external funding. Acknowledgments: This research was supported by the Basic Research Project (18-3422) of the Korea Institute of Geoscience and Mineral Resources (KIGAM). Conﬂicts of Interest: The authors declare no conﬂict of interest.

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