Remediation of Heavy Metal (Cu, Pb) Contaminated Fine Soil Using Stabilization with Limestone and Livestock Bone Powder

: Soil environments contaminated with heavy metals by typhoon ﬂooding require immediate remediation. High-pressure soil washing using water could be a viable short-term solution for cleaning soil contaminated with heavy metals. Soil washing employing high-pressure generates heavy metal contaminated ﬁne soil and wastewater. This contaminated ﬁne soil cannot be reused without proper treatment because of the high levels of heavy metal contamination. Stabilization was used for immobilizing heavy metals (Cu, Pb) in the contaminated ﬁne soil. The stabilizing agents used for immobilizing heavy metals (Cu, Pb) in the contaminated ﬁne soil included two types of limestone (Ca-LS and Mg-LS) and livestock bone powder (LSBP). The Ca-LS, Mg-LS, and LSBP were applied to the contaminated ﬁne soil at dosages in the range of 2 wt%~10 wt%. Two different particle sizes (-#10 vs. -#20 mesh) and curing times (1 week vs. 4 weeks) were used to compare the effectiveness of the stabilization. Extractions using 0.1 N HCl were conducted to evaluate the stabilization effectiveness. Heavy metal leachability was signiﬁcantly decreased with higher Ca-LS and LSBP dosages. The LSBP treatment was more effective than the Ca-LS and Mg-LS treatments and the Mg-LS showed the poorest performance. The highest degree of immobilization was attained using a 10 wt% LSBP (-#20 mesh), resulting in an approximate leachability reduction of 99% for Pb and 92% for Cu. The -#20 mesh material and 4 weeks of curing were more effective than the -#10 mesh material and 1 week of curing, respectively. The SEM-EDX results showed that metal precipitates and pyromorphite like phases could be responsible for effective heavy metal immobilization. This study suggests that Ca-LS and LSBP used at an optimum dosage can be effective stabilizing agents for immobilizing Cu and Pb in contaminated ﬁne soils.


Introduction
The contamination of soil with heavy metals such as copper (Cu) and lead (Pb) may be caused by a variety of anthropogenic activities (e.g., mining, smelting) or natural disasters.Copper (Cu) and lead (Pb) are known to be very toxic elements to humans.Specifically, Cu is considered an aquatic toxin [1] and chronic exposure to it can harm the liver and kidneys [2].Lead (Pb) exposure can damage the brain, red blood cells, blood vessels, kidneys, and the nervous system [3,4].Therefore, soil contaminated with high levels of heavy metals requires remedial action.The use of high-pressure soil washing has been reported in a previous study for a soil contaminated with heavy metals (Cu, Pb, Zn) [5].Removal rates for heavy metals (Cu, Pb, Zn) were reported at the optimal operation of the high-pressure soil washing device.Soil washing at high-pressure was designed for emergency response situations when soil contamination occurs from accidental wastewater spills caused by natural disasters such as typhoons or flooding.
In high-pressure soil washing, cavitation flow causes the separation of dispersed soil aggregates into a fine soil and a wash water (wastewater) stream.The separated fine soil contains high levels of heavy metals that preclude disposal or the reuse of this soil "as is" due to possible heavy metal leaching upon exposure to severe environments such as very low pH conditions.Therefore, this heavy metal contaminated soil should be properly remediated in order to prevent severe heavy metal leaching.Among the various remediation technologies (i.e., electrokinetics, phytoremediation, stabilization, etc.), the stabilization process is selected for its cost effectiveness, convenience, and rapid treatment timeframes.In the past, the stabilization process was utilized widely to remediate heavy metal contaminated soil using industrial products and/or waste materials (i.e., Portland cement, cement kiln dust, fly ash, etc.) [6][7][8][9].Mahedi et al. [6] used cement activated fly ash and slag for stabilizing Al, Cu, Fe, and Zn in contaminated soil to evaluate the leaching behavior using the toxicity characteristic leaching procedure (TCLP) test.Accordingly, the leachability of Al, Cu, and Zn followed amphoteric leaching behavior where the concentrations increased in both acidic and basic conditions.Ouhadi et al. [7] used cement stabilization/solidification to remediate heavy-metal-contaminated clay.They reported that a significant reduction in TCLP Pb concentration was attained with the cement treatment [7].Zha et al. [8] also used cement and fly ash to stabilize/solidify heavy metal contaminated soil.They reported that treatment resulted in an increase in the unconfined compressive strength (UCS) and a decrease in the leached ion concentration.Currently, CaCO 3 based natural waste resources such as eggshell, starfish, and oyster shell are used broadly to immobilize heavy metals in contaminated soil and have been proven to be effective in reducing heavy metal leachability [10][11][12][13][14][15][16][17][18].Specifically, Torres-Quiroz et al. [10] used oyster shell powder, zeolite, and red mud as stabilizing agents for toxic metal contaminated soil.They reported that oyster shell powder was the best low-cost adsorbent material to stabilize toxic metals in contaminated soils [10].Zheng et al. [11] used cow bone meal and oyster shell meal to immobilize Cd, Pb, Cu, and Zn in contaminated soil.They reported that cow bone meal and oyster shell meal were effective agents for the remediation of heavy metal contaminated soils.Moreover, Ahmad et al. [12] used eggshell and calcined eggshell to immobilize Pb in contaminated military shooting range soil.They reported that both egg shell and calcined egg shell treatments reduced the exchangeable Pb fraction to about 1% of the total Pb in the soil.Ok et al. [12] also used eggshell as the CaCO 3 source in order to immobilize the Cd and Pb in the contaminated soil.They suggested that eggshell waste could be used as an alternative CaCO 3 source to immobilize heavy metals in soils.Therefore, CaCO 3 -based waste has been successfully demonstrated as an effective stabilizing agent.Thus, limestone was used in this study as the main CaCO 3 material to immobilize heavy metals.Specifically, two types of limestone and livestock bone, available on the market at low cost, were used to treat the heavy metal (Cu, Pb) contaminated fine soils.Moreover, two types of limestone (high Ca limestone vs. high Mg limestone) were used and compared for their stabilization effectiveness.Phosphate rich materials are known to be effective for immobilizing heavy metals in contaminated soils [19][20][21][22][23][24][25][26].Moreover, in a previous study, high phosphorus biochar derived from soybean stover was effective with the immobilization of Pb in contaminated soil [22].Ren et al. [24] also reported that phosphate-induced stabilization was very effective for Pb contaminated soil and that the immobilization rate could be significantly increased depending on the pH of the contaminated soil for other heavy metals such as Zn and Cd [24].Moreover, Zhang et al. [21] showed that phosphorous could decrease Cu and Cd leachability by two to three times in paddy soil with phosphorus modified biochar made by pyrolysis using biomass feedstocks.Andrunik et al. [20] reported that upon phosphate treatment of heavy metal (Pb, Cd, Zn) contaminated soil, new stable mineral substances formed, causing a reduction in leachability as measured by the toxicity characteristic leaching procedure (TCLP).Hence, phosphate rich livestock bone powder was used in this study as a renewable waste material alternative to limestone.To establish the influence of particle size, -#10 and -#20 mesh materials were added to the contaminated soil.Moreover, the curing period effects on heavy metal immobilization were also investigated between 1 week and 4 weeks.
The objective of the study reported herein was to assess the feasibility of using limestone and livestock bone powder as stabilizing agents for immobilizing heavy metals (Cu, Pb) in contaminated fine soil.The stabilization effectiveness was evaluated with 0.1 N HCl extraction methods.X-ray powder diffraction (XRPD) was employed to assess the mineralogical makeup of the soil while the stabilization mechanism was studied by scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM-EDX).

Heavy Metal Contaminated Soil
Upon treatment with high-pressure soil washing, the soil samples contaminated with heavy metals were collected.As reported in a previous study, cavitation flow induced by high-pressure soil washing caused dispersion of the soil aggregates [5].The resulting fine solid suspension and wastewater were discharged from one of the two outlets [5].The copper and lead levels in the fine soil were about 668 and 515 mg/kg, respectively.These levels exceeded the Soil Contamination Warning Standards of 500 mg/kg for Cu and 400 mg/kg for Pb (Region 2).The collected contaminated fine soil was air-dried, thoroughly mixed, and used in the stabilization process.Quality characteristics of the contaminated soil including mineralogical information are listed in Table 1.The chemical composition of the contaminated soil and stabilizing agents determined by X-ray fluorescence (XRF, ZSX100e, Rigaku, Japan) is presented in Table 2.

Stabilization Agents
Two types of commercially available limestone were used in this study.One is Ca-rich (98% CaO) limestone (Ca-LS, -#10 mesh materials) and the other is Mg-rich (28% MgO) limestone (Mg-LS, -#10 mesh materials).Livestock bone powder (LSBP, -#10 mesh materials) was also used as an alternative renewable waste material to non-renewable limestone.The chemical makeup of the stabilizing agents (Ca-LS, Mg-LS, and LSBP) quantified by XRF is presented in Table 2.

Stabilization Experiments
The stabilizing agents (Ca-LS, Mg-LS, and LSBP) were added to the contaminated soil at 2~10 wt%.A control treatment without any stabilizing agents was also prepared to serve as a no-treatment baseline for stabilization effectiveness.A mixture using DI water prepared at a 20:1 liquid to solid ratio was sufficient to ensure full hydration.After stabilization, all of the samples were allowed to cure for durations of 1 week and 4 weeks in sealed plastic containers under normal conditions (20 • C, 25% humidity).The effectiveness of heavy metal stabilization was assessed using a rigorous leaching test with a 0.1 N HCl extraction agent.The experimental conditions for all stabilizing agents (Ca-LS, Mg-LS, and LSBP) are shown in Table 3.A flowchart of the stabilization process using Ca-LS, Mg-LS, and LSBP as stabilizing agents is shown in Figure 1.

Mineralogical Characterization
The mineralogical composition of the contaminated soil and stabilizing agents (Ca-LS, Mg-LS, and LSBP) was performed by X-ray powder diffraction (XRPD).Prior to analysis, the samples were pulverized to clear the -#200 mesh (0.075 mm).A XRPD diffractometer (X'Pert PRO MPD, PANalytical, Almelo, The Netherlands) was used to collect the step-scanned diffraction patterns.The instrument was equipped with a beam graphite

Mineralogical Characterization
The mineralogical composition of the contaminated soil and stabilizing agents (Ca-LS, Mg-LS, and LSBP) was performed by X-ray powder diffraction (XRPD).Prior to analysis, the samples were pulverized to clear the -#200 mesh (0.075 mm).A XRPD diffractometer (X'Pert PRO MPD, PANalytical, Almelo, The Netherlands) was used to collect the step-scanned diffraction patterns.The instrument was equipped with a beam graphite monochromator with Cu radiation operated at 40 kV and 40 mA.Diffraction patterns were collected at a 2θ range of 5-60 • , a step size of 0.02 • , and a count time of 3 s/step.Mineral characterizations were accomplished by using Jade software v. 7.1 [29] supplemented with the PDF-2 reference database [30].

Stabilization Mechanism Analytics
The stabilization mechanism was assessed by scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM-EDX) on the treated samples exhibiting the lowest heavy metal leachability, namely, the 10 wt% Ca-LS (-#20 mesh material) and 10 wt% LSBP (-#20 mesh material) samples.Prior to SEM testing (Hitachi S-4800 SEM, Horiba EMAX EDX system), the collected samples were secured on a plate with double-sided Pt-coated carbon tape.

Physicochemical Testing
The pH tests for the soil and stabilizing agent samples were performed at a liquid to solid (L:S) ratio of 5:1 (mass basis) according to the Korean Standard Test (KST) method [31].The National Academy of Agricultural Science (NAAS) method was used to measure the electrical conductivity (EC) [32].An aqua regia extraction agent was used for the total heavy metal determinations.The dissolved heavy metal concentration was tested using an inductively coupled plasma-optical emission spectrometer (ICP-OES, Optima 8300DV, PerkinElmer, Waltham, CT, USA).All ICP-OES determinations were reported as the mean values of triplicate samples (less than 10% measurement error).For QA/QC purposes, the devised protocol entailed three quality-control standards and spiking with a standard solution for every 10 samples analyzed (recovery rate >95%).
(PDF# 14-0164) were the main mineral phases identified in the contaminated soil.Calcite (CaCO3, PDF# 47-1743) was identified as the main phase in Ca-LS while dolomite (CaMg(CO3)2, PDF# 26-0426) was observed in the Mg-LS sample.This indicated that Ca-LS and Mg-LS had different main phases based on the elemental content.The main phase of LSBP was hydroxylapatite (Ca5(PO4)3(OH), PDF# 09-0432), and this was most probably due to the high phosphorus content.Relative Intensity (%) .

Stabilization Treatment Effectiveness
The heavy metal (Cu, Pb) leachability results with an extraction solution of 0.1 N HCl for the Ca-LS (-#10 mesh, -#20 mesh), Mg-LS (-#10 mesh, -#20 mesh), and LSBP (-#10 mesh, -#20 mesh) treatments upon 7 days and 28 days of curing are shown in Figures 4-11.Evidently, an increasing stabilizing agent dosage results in decreased heavy metal leachability.Upon 7 days of curing, the Cu and Pb leachability was significantly decreased with an increasing dosage for the -#10 mesh Ca-LS treatment sample (Figure 4).The Mg-LS treatment resulted in the low immobilization of Cu and Pb.This indicates that the pH increase in the soil system upon the addition of Mg-LS compared to the Ca-LS treatment was not sufficient to provide the effective immobilization of heavy metals.In the past, CaCO3-based stabilization was utilized in numerous research papers [10][11][12][13][14][15][16][17][18].Various CaCO3-based stabilization agents have been previously used including eggshell, oyster shell, limestone, starfish, etc.It has been reported that the exchangeable fractions of heavy metals could be decreased and the residual fraction increased upon eggshell treatment [15].Upon the addition of CaCO3 to contaminated soil, the soil pH increased.This can be strongly associated with an increase in the metal sorption on the calcite surface of the soil [33][34][35].When the soil pH increased, the surface negative charge increased.This could be the result of an increase in cation adsorption [34,36].It has been reported that the Pb ad- Upon 7 days of curing, the Cu and Pb leachability was significantly decreased with an increasing dosage for the -#10 mesh Ca-LS treatment sample (Figure 4).The Mg-LS treatment resulted in the low immobilization of Cu and Pb.This indicates that the pH increase in the soil system upon the addition of Mg-LS compared to the Ca-LS treatment was not sufficient to provide the effective immobilization of heavy metals.In the past, CaCO 3 -based stabilization was utilized in numerous research papers [10][11][12][13][14][15][16][17][18].Various CaCO 3 -based stabilization agents have been previously used including eggshell, oyster shell, limestone, starfish, etc.It has been reported that the exchangeable fractions of heavy metals could be decreased and the residual fraction increased upon eggshell treatment [15].Upon the addition of CaCO 3 to contaminated soil, the soil pH increased.This can be strongly associated with an increase in the metal sorption on the calcite surface of the soil [33][34][35].When the soil pH increased, the surface negative charge increased.This could be the result of an increase in cation adsorption [34,36].It has been reported that the Pb adsorption on the calcite surface is enhanced at high pH condition [37].According to Naidu et al. [34], hydroxy species of metal cations could be formed at high pH, which would result in a higher affinity for adsorption sites compared to the metal cations alone [38].Moreover, the precipitation of metal as metal hydroxides can occur when the pH is high.Ok et al. [39] have reported that the dissolution of CaCO 3 in water at high pH conditions can be described as follows: This alkaline condition can expedite metal precipitation as metal hydroxides as follows: where M denotes metal [39].Ok et al. [39] and Torres-Quiroz et al. [10] reported significant heavy metal reduction upon natural oyster shell application, where CaCO 3 was the main phase in the treatment.Moreover, Torres-Quiroz et al. [10] reported that the preference of sorption with oyster shell was in the following order: Pb 2+ > Cu 2+ > Zn 2+ > Cd 2+ > Ni 2+ for Pb and Cu contaminated silty sand and sandy soil samples.Similar results were obtained upon Ca-LS treatment in this study.Therefore, a combination of the aforementioned mechanisms may be accountable for the high degree of heavy metal immobilization observed in the Ca-LS treatments.The Ca-LS -#20 mesh size particles caused higher Cu and Pb leachability reduction than the -#10 mesh materials.This was most probably due to the larger surface area of the finer particles, which is strongly associated with high reactivity.Upon 28 days of curing, Cu and Pb leachability was found to be lower compared to the samples cured for 7 days for the -#10 mesh Ca-LS treatment.This suggests that the curing time duration is also important in reducing heavy metal leachability.Therefore, an optimal curing period should be applied to the contaminated soil during the design phase of the stabilization process.
Even upon curing for 28 days, the Mg-LS treatment was not effective in significantly reducing the heavy metal leachability compared to the Ca-LS treatment.This indicates that the high Mg content present in the Mg-LS sample did not play an effective role in the stabilization process.Moreover, the extraction pH values for the Mg-LS samples were very low, indicating a low buffering capacity for acid neutralization.Specifically, this indicates that 50.9% of the CaO content in the Mg-LS sample, as shown by the XRF analysis, may not be sufficient to endure the strong acid leaching conditions (0.1 N HCl extraction), and therefore high levels of heavy metals could be leached from the stabilized-contaminated soil.
Similar to the 7 days of curing, the -#20 mesh stabilizers showed better immobilization compared to the -#10 mesh materials, most probably due to its larger surface area.Particle size influence on the immobilization of heavy metals was more pronounced than the effect exhibited by the curing duration.The lowest heavy metal leachability was obtained for the -#20 mesh materials for each Ca-LS and Mg-LS treatment.This indicates that increased surface area is important for reducing heavy metal leachability.
Upon 7 days of curing for the -#10 mesh materials, the LSBP treatment outperformed the Ca-LS and Mg-LS treatments in reducing the Cu and Pb leachability.This indicates that high P content (40.58% as P 2 O 5 ) plays an important role in significantly reducing the heavy metal leachability.Similar to the -#10 mesh materials, the LSBP treatment with the -#20 mesh materials showed better immobilization of the heavy metals compared to the Ca-LS and Mg-LS treatments with -#20 mesh materials.This suggests that the reduced particle size was more reactive in reducing the heavy metal leachability.
Similar to the 7 days of curing for -the #10 mesh materials, the LSBP treatment upon 28 days of curing outperformed the Ca-LS and Mg-LS treatments in terms of leachability when the dosage was higher than 4 wt%.Moreover, the trend in the increased stabilization at higher treatment dosages for the LSBP treatment was more pronounced in stabilizing Pb rather than Cu.Specifically, a Pb leachability reduction of higher than 98% was attained with the 6 wt% LSBP treatment.It has been reported that phosphate-induced stabilization is highly effective for reducing Pb and Cu leachability [22,40].A significant reduction in Pb leachability was most probably due to the formation of hydroxypyromorphite [Pb 5 (PO 4 ) 3 OH, Ksp = 10 −76.8 ] [40].Since the hydroxypyromorphite formed during stabilization, the soluble Pb could not be leached from the contaminated soil by virtue of its very low solubility [19].In the XRPD analysis of LSBP, the main high intensity phase in the LSBP was hydroxylapatite (Ca 5 (PO 4 ) 3 (OH)).This indicates that an ample amount of a P source could have contributed to the formation of pyromorphite-like and/or metal phosphate compounds.Moreover, reduced Cu leachability may be strongly associated with the formation of Cu(H 2 PO 4 ) 2 , Cu 3 (PO 4 ) 3 , and CuP 4 O 11 , which were identified in the phosphorus-modified biochar treatment of Cu contaminated paddy soil [41].Zhang et al. [41] reported a two to threefold increase in Cu immobilization efficiency upon the 10 wt% phosphorus-modified biochar treatment.Xu et al. [42] also reported that metal phosphates such as M 3 (PO 4 ) 2 (M = Cd, Pb, Cu, and Zn) could be the responsible compound for the reduction in the soluble content of the heavy metals.Moon et al. [43] also reported that pyromorphite-like phases and the products of pozzolanic reactions (i.e., CSH, CAH) were responsible for the effective stabilization of calcined oyster shell and waste cow bone treatment in firing range soil.
Similar to the Ca-LS and Mg-LS treatment, for LSBP, the treatment particle size was more influential in immobilizing heavy metals than the curing duration.The lowest heavy metal leachability value was attained from the -#20 mesh materials in the LSBP treatment.
Overall, treatment with Ca-LS and LSBP generated efficient heavy metal immobilization.More specifically, for a curing duration of 28 days, the heavy metal immobilization effectiveness was ranked according to the following order: LSBP (-#20 mesh) > Ca-LS (-#20 mesh) > LSBP (-#10 mesh) > Ca-LS (-#10 mesh) Mg-LS (-#20 mesh) > Mg-LS (-#10 mesh) This indicates that the Ca and P content of the stabilizing agent along with the particle size are prevailing factors leading to more effective heavy metal immobilization.Moreover, for practical field applications, the level of heavy metal contamination is also important in addressing the dosage of the stabilizing agents.Furthermore, if the treatment results with -#10 mesh materials show compliance with the regulatory limit, a particle size reduction to -#20 mesh materials would not need to be considered due to the economic feasibility.

SEM-EDX Results
The results of the SEM-EDX analyses are presented in Figure 12a,b for the lowest heavy metal leachability samples (10 wt% Ca-LS -#20 mesh and 10 wt% LSBP -#20 mesh).For the Ca-Ls sample, Pb and Cu were observed in the SEM-EDX image along with Ca, Al, Si, and O.This finding establishes that heavy metal immobilization is likely to be related to the formation of metal precipitates [39], which can be adsorbed on the soil particles.The net negative charge could be increased when the soil pH is high.Ahmad et al. [44] confirmed the theoretical formation of insoluble Pb phases with visual MINTEQ, which was responsible for effective immobilization at high pH conditions.For the 10 wt% LSBP sample, Pb and Cu were observed along with Ca, Al, Si, P, and O.This indicates that pyromorphite-like compounds and/or metal phosphate formations [22,[40][41][42] could be responsible for effective heavy metal immobilization.lated to the formation of metal precipitates [39], which can be adsorbed on the soil particles.The net negative charge could be increased when the soil pH is high.Ahmad et al. [44] confirmed the theoretical formation of insoluble Pb phases with visual MINTEQ, which was responsible for effective immobilization at high pH conditions.For the 10 wt% LSBP sample, Pb and Cu were observed along with Ca, Al, Si, P, and O.This indicates that pyromorphite-like compounds and/or metal phosphate formations [22,[40][41][42] could be responsible for effective heavy metal immobilization.

Conclusions
Two types of limestone (Ca-LS and Mg-LS) and livestock bone powder (LSBP) were used as stabilizing agents for immobilizing Cu and Pb in contaminated fine soil collected after treatment with high-pressure soil washing.The treatment dosage of stabilizing agents to contaminated soil varied in the range of 2 wt% to 10 wt%.The effects of curing duration (1 week vs. 4 weeks) and particle size (-#10 mesh vs. -#20 mesh) on stabilization were also studied.Following the curing period, the effectiveness of heavy metal stabilization in the treated samples was evaluated after extraction with 0.1 N HCl.The stabilization results indicated that a notable reduction in Cu and Pb leachability was achieved upon the Ca-Ls and LSBP treatments.The -#20 mesh material outperformed the -#10 mesh materials.Moreover, a curing period of 4 weeks was more effective in reducing the heavy metal leachability than the 1 week curing period.Finally, the particle size was more influential than the curing time duration for heavy metal immobilization.
The results suggest that the effectiveness of heavy metal immobilization upon 28 days of curing is ranked according to the following order: LSBP (-#20 mesh) > Ca-LS (-#20 mesh) > LSBP (-#10 mesh) > Ca-LS (-#10 mesh) Mg-LS (-#20 mesh) > Mg-LS (-#10 mesh) The SEM-EDX results showed that cation adsorption/metal precipitation may be the immobilization mechanism associated with the Ca-Ls treatment.Moreover, pyromorphitelike/metal phosphate phases may probably be associated with a high degree of heavy metal immobilization.Future studies will be conducted with heavy metal contaminated field soil obtained from emergency natural disaster recovery operations caused by typhoons or flooding, in order to validate the results obtained in this study.

Figure 1 .
Figure 1.Flowchart of the stabilization process using Ca-LS, Mg-Ls, and LSBP.

Figure 2 .
Figure 2. XRPD pattern of the heavy metal contaminated fine soil.

Table 1 .
Characteristics of the heavy metal contaminated soil.

Table 3 .
Treatment matrix for the contaminated fine soil.