The immobilization performances of Diatomite, Ca(H2PO4)2, CaCO3, Hydroxyapatite (HAP) and Nano-HAP (n-HAP) for Zn, Mn, Pb, and Cd contaminated sediments were investigated by immobilization experiments and evaluated by the European Community Bureau of Reference (BCR) extraction test, toxicity characteristic leaching procedure (TCLP). The result of BCR indicated that HAP and Nano-HAP (n-HAP) had a better immobilization effect on metal contaminated sediments, and the residual fractions of Zn, Mn, Pb, and Cd increased from 30.4, 31.9, 55.49 and 54.27% to 36, 39, 72, and 57%, respectively. The order for immobilized effects of additive was: n-HAP > HAP > CaCO3 > Ca(H2PO4)2 > diatomite. However, the cost-effectiveness of HAP was slightly higher than that of n-HAP, so HAP was more suitable for immobilization of heavy metals in sediment. The TCLP test showed that with HAP as immobilization the leaching amount was reduced by approximately 76, 28, 78, and 85% for Zn, Mn, Pb, and Cd, respectively, compared to the blank group. The results also proved that HAP would be an effective and economical agent for immobilizing heavy metals in sediment, with the optimum mass dosage of 10% (the mass ratios of HAP/sediment (dry weight)) of the sediment.
The order for metal immobilization in sediments was: n-HAP > HAP > CaCO3 > Ca(H2PO4)2 > diatomite.
The optimum dosage of HAP was 10% for immobilization heavy metals in sediment.
With addition of HAP to the sediment, the oxidizable and residual fraction of heavy metals increases.
With increasing waste discharges by anthropogenic activities, such as mining, metal smelting, manufacturing, fertilizer and pesticide use, a large number of rivers and lakes have been severely contaminated with heavy metals, which has become a major environmental concern (Sekulić et al. 2018; Liu et al. 2019; Li et al. 2020).
When the metals enter water bodies, they can be harmful to aquatic ecosystems through a range of biochemical processes. Usually, the metals can also be adsorbed by suspended particles and finally accumulate in sediments (Islam et al. 2018; Jin et al. 2019; Kang et al. 2019). Lake or river sediment is the sink of heavy metals in the water, and it is reported that the sediment could adsorb up to 90% of the dissolved fraction in the water column (Puttiwongrak et al. 2019; Zhao et al. 2019). With the characteristics of heavy metals such as potential toxicity, persistence, non-degradability, and biological accumulation, the heavy metal contamination accumulated in sediment is extremely harmful to the aquatic environment (Zhang et al. 2018; Sodrzeieski et al. 2019). The biogeochemical conditions at the sediment–water interface control the speciation and mobility of dissolved substances, specifically dissolved oxygen (DO), salinity, pH, redox potential (Eh), and temperature play large roles in the release of heavy metals into the overlying water column (Liu & Shen 2014; Dung et al. 2019; Kang et al. 2019). Sediment probably acts as a source of contaminants, and causes the deterioration of water quality (Feng et al. 2019; Moore et al. 2019).
Nowadays, the treatment technologies of heavy metal contaminated sediment can be classified into in situ remediation and ex situ remediation (Cai et al. 2019; Zhang et al. 2019). In situ remediation aimed to prevent the potential release of heavy metals from sediments to the aquatic ecosystem by transferring the metals into stabilizing fractions such as immobilization with amendments, sand cap and phytoremediation (Peng et al. 2009; Liu et al. 2019). Ex situ remediation aimed at extracting or separating heavy metals from sediment, such as washing, electrochemical remediation, flotation and immobilization, and usually applied for the sediment slightly polluted by heavy metals (Peng et al. 2009). Compared with common technologies for polluted sediment remediation, immobilization with amendments is a relatively economic and low-impact technique for the remediation of sediment contaminated with heavy metals and hydrophobic organic contaminant (Ghosh et al. 2011; Li et al. 2019). Immobilization technology could reduce solubility, mobility and bioavailability of heavy metals, lowering the risk of heavy metals exposure by precipitation or sorption (Raicevic et al. 2006; Peng et al. 2009; Khan et al. 2015; Yi et al. 2017).
Diatomite, a natural mineral, was applied for metal adsorption from wastewater due to its cost-effectiveness, non-toxicity, large reserves and high pore volume and surface area (Li et al. 2018). Metal remediation in aqueous solution and contaminated soil can be achieved by using MgO-supported diatomite (Kumar et al. 2018). The liming materials CaCO3, and the phosphate induced materials Ca(H2PO4)2 and HAP were the typical inorganic matter amendments used for the in situ immobilization in order to decrease the metal concentration in soil solution, to decrease metal mobility and leachability and to transform from soluble fraction to residual fraction (Guo et al. 2006). The P-containing material can interact with metals to form metal phosphates precipitation to decrease bioavailable heavy metals (Seshadri et al. 2017; Li et al. 2018). The combination of CaCO3 and Ca(H2PO4)2 significantly reduce the extractable metal concentration and successfully immobilize heavy metals (Wang et al. 2001). HAP was found to be effective in immobilizing and extracting heavy metals in sediment and water due to its moderate solubility and low cost (Zhang et al. 2010; Kadouche et al. 2012a). Currently, the rapid development of nanotechnology and the wide application of nanomaterials in soil restoration provide a new direction for sediment remediation (Cai et al. 2019). Nano-materials have high surface area and reactivity and the ability to disperse in aqueous solution. They also have high activity and adsorption capacity, and broad application in a variety of areas (Smičiklas et al. 2008; Kadouche et al. 2012a). Nano-hydroxyapatite is a potential material that could reduce the bioavailable fraction that can be used to remediate Pb and Cd in sediment effectively (Zhang et al. 2010).
The objective of this study was to investigate the immobilizing performance of diatomite, Ca(H2PO4)2, CaCO3,HAP and n-HAP as immobilization materials to remediate Zn, Mn, Pb and Cd in sediment. The remediation performance of immobilization materials was also studied by assessing the bioavailability of heavy metals in sediment using the European Community Bureau of Reference (BCR) extraction test and toxicity characteristic leaching procedure (TCLP).
MATERIALS AND METHODS
Diatomite (AR) and Ca(H2PO4)2 (AR) were acquired from Kermel Chemical Reagent Co., Ltd (Tianjin, China). CaCO3 (AR) was acquired from Sinopharm Chemical Reagent Co., Ltd (Shanghai, China). HAP, with a particle size of about 100 μm, and n-HAP (average particle size ≤40 nm) were purchased form Nanjing Emperor Nano Material Co., Ltd (Nanjing, China). Heavy metal contaminated sediments used in the experiment were taken from Shihe River in Jinan City (Shandong, China). In the 1990s, many large industrial and mining enterprises nearby discharged Zn, Mn, Pb and Cd contaminated wastewater to the river, which blackened the silt and accumulated in the water. The surface sediment of 15 cm was collected with a wooden shovel. After natural air drying, the samples were cleaned by removing stones and sticks, and were ground to pass through 100 mesh sieves to determine the basic physical and chemical properties such as moisture content, pH, TP, heavy metal content, and organic matter (OM). Table 1 shows the physicochemical properties of the experimental sediment.
|Parameter .||Value .|
|Moisture content (%)||29.2|
|TP (mg kg–1)||1.93|
|Zn (mg kg–1)||6,869|
|Mn (mg kg–1)||738|
|Pb (mg kg–1)||318|
|Cd (mg kg–1)||4.03|
|Organic matter (%)||12.9|
|Parameter .||Value .|
|Moisture content (%)||29.2|
|TP (mg kg–1)||1.93|
|Zn (mg kg–1)||6,869|
|Mn (mg kg–1)||738|
|Pb (mg kg–1)||318|
|Cd (mg kg–1)||4.03|
|Organic matter (%)||12.9|
Sediment immobilization experiments with different amendments
Diatomite, Ca(H2PO4)2, CaCO3, HAP and n-HAP were mixed with the sediment sample (50 g), and the adding dosage was according to the mass ratios of immobilization materials/sediment (dry weight) of 1, 5, and 10%, respectively. The sediment was mixed with deionized water with the mass ratios of water/sample 10:1, and was stirred once a day for 7 d, then kept stationary for 7 d. After air drying, the sample was taken by quartering method and refrigerated. The metal fraction in sediment with different amendments was evaluated by BCR, and the leachability of the sediments after immobilization was determined by TCLP. The BCR method and TCLP program refer to previous research (Zhang et al. 2019). The content of Zn, Mn, Pb and Cd were measured by Atomic Absorption Spectrophotometer (TAS-990, Beijing Puxi Company, China).
Dose optimization of HAP in metal immobilization
HAP was mixed with sediment sample (50 g), and the dosage was determined according to the mass ratios of immobilization materials/sediment (dry weight) which were 5, 8, 10, 12 and 14%, respectively. Deionized water was then added (the mass ratios of water/sample were 10:1), it was stirred once a day for 7 d, and then kept stationary for 7 d. After air drying, the sample was taken by quartering method and refrigerated. The immobilization effect of solidified sediment was evaluated by BCR and TCLP.
One or two-way analysis of variance (ANOVA) was performed on all data to determine if significant difference existed between treatments. Significance was tested at the 0.05 probability levels.
RESULTS AND DISCUSSION
Effect of different immobilization materials on heavy metal fraction
To determine the changes of immobilization additives on heavy metal fraction in sediment, raw and additive added contaminated sediment samples were analyzed by BCR sequential extraction (Castillo et al. 2011; Liang et al. 2012). Taking 10% HAP as an example, the BCR recovery ratio are shown in Table 2.
|.||Zn .||Mn .||Pb .||Cd .|
|.||Zn .||Mn .||Pb .||Cd .|
The mobility of heavy metal in sediment is affected by the speciation fraction. The acid extractable fraction is the most unstable and mobile; the reducible and oxidizable are stable in normal sediment conditions, but they can be mobilized under the acidification and low Eh; and the residual fraction is the most stable (Lu et al. 2018).
Figure 1 shows the fraction changes of Zn in the sediment by adding different kinds of immobilizing additives. When 1% of immobilizing additives was applied (Figure 1(a)), each additive had a certain immobilized effect on the Zn in sediments. Compared to the control sediment, the acid extractable Zn reduced by 3–6%, the reducible Zn increased by 3–6%, and the change range of the oxidizable and residual fraction by only about 1%. It can be found that 1% of immobilizing additives only affected the fraction of extractable Zn.
From Figure 1(b), with adding 5% additive to the sediment, the acid extractable fraction of Zn decreases, and the reducible fraction increases. The reason could be that immobilizing additives had an obvious effect on promoting the transformation of the acid extractable Zn to the reducible Zn. When 5% of n-HAP was applied, the acid extractable fraction reduced by nearly 8%, and the reducible and residual fraction increased by 5 and 3%, respectively. It can be observed that the acid extractable Zn could be transformed into reducible Zn by n-HAP.
From Figure 1(c), with 10% of additive dosage the acid extractable fraction of Zn in sediment decreased by 9.07, 7.83 and 7.81% with the diatomite, Ca(H2PO4)2, CaCO3 additive. Correspondingly, the reducible fraction Zn increased by 5.69, 2.59 and 3.83%, respectively, and the residual fraction Zn increased to 34.16, 35.40 and 34.16%, respectively. It can also be found that the oxidizable fraction Zn had no obvious change with the HAP and n-HAP additive, but the acid extractable fraction Zn decreased by 9.69% and 9.07%, the residual fraction Zn increased by 5.62% and 5.01%, and the oxidizable fraction Zn did not obviously increase.
It can be observed from Figure 1 that the immobilized effect of HAP and n-HAP had no significant difference, but compared with the other three additives, they were better. The stabilizing fraction of Zn is in direct proportion to the dosage of immobilizing additives.
It can be seen from Figure 1 that there is a decrease of the oxidizable Zn and an increase of the residual with the additives; similar results were found with previous studies (Lucchini et al. 2014; Lu et al. 2018). In addition, the mechanism of surface complexation on HAP surface immobilized Zn was described and found that pH had an effect on the metal complexation on HAP (Wu et al. 1991; Zhang et al. 2019).
From Figure 2(b) it was found that when the additive dosage was 5%, the effect of the additive was more obvious than when the dosage was 1%. After adding the additive of HAP and n-HAP, the acid extractable fraction reduced to 30.90 and 29.69%, respectively, compared to the control group, the fraction decreased by 4.07 and 4.51%. While adding the additive of diatomite, Ca(H2PO4)2 and CaCO3, the acid extractable fraction decreased by 2.04, 3.09 and 3.43%, respectively, there was a slight difference with the additive of HAP and n-HAP. Compared with 1% of additive dosage, the residual fraction increased by 3–6%, among them, the residual fraction increased to 38.39, 38.18 and 38.79%, respectively, with the additive of CaCO3, HAP and n-HAP.
From Figure 2(c), the effect of 10% additive dosage was more obvious. Compared to 5% dosage, there was a slight decrease in the acid extractable fraction by 10%. There was no significant change to be observed except that the residual in sediment with HAP and n-HAP increased. In addition, the oxidizable fraction in sediment with additives increased by nearly 2%, except with n-HAP which decreased.
Figure 3 shows the fraction changes of Pb in sediment with different additives dosage. From Figure 3(a), when the dosage of additives was 1% HAP and n-HAP, it was found that the oxidizable and residual fraction increased by nearly 8%. Compared to 1% HAP and n-HAP, when the dosage was 5% (Figure 3(b)), the oxidizable and residual fraction increased by nearly 15%, and an inevitable decrease of the acid extractable and reducible fraction could be found. In particular, the acid extractable fraction was lower than 0.5% with additives, but it was 0.71% with diatomite.
When the dosage of additives was 10%, as shown in Figure 3(c), the stabilizing fraction was maintained above 65%. Among them, the residual fraction in sediment with phosphorous additives (Ca(H2PO4)2, HAP and n-HAP) reached 68.58, 70.73 and 72.56% respectively. It can also be observed that the phosphorous additives had a good stabilization effect on the lead in heavy metal contaminated sediment. This is because the P amendments significantly reduced Pb water solubility and the Pb immobilization probably attributed to the formation of insoluble Pb phosphate minerals (Cao et al. 2009). Jiang et al. (2012) investigated the mobility of Pb in sediments with added rice straw biochar and found consistent results with this study with little change of the residual fraction.
As shown in Figure 4, the immobilized effect of additives applied from 1 to 10% on Cd in sediment was not obvious. Compared to the control group, all the fractions except acid extractable in sediment increased slightly, but not obviously. When the dosage of additive was 10%, as shown in Figure 4(c), the oxidizable and residual fraction in the sediment applied with CaCO3, HAP and n-HAP increased to 62.73, 62.77 and 62.83%, respectively, which was 4–5% higher than that in the blank control group. The acid extractable Cd was decreased by 5.6–14.1%, but the residual fraction changed little with amending biochar (Jiang et al. 2012).
Evaluation of heavy metals immobilization
Figure 5 shows the TCLP leaching amount of heavy metals applied by different additive conditions.
As shown in Figure 5, the Pb and Cd initial leaching amount was much lower than Zn and Mn. The reason was that the stabilizing components of Pb and Cd in sediment account for nearly 60%, and because of the low acid extractable fraction. The results of heavy metal leaching amount fraction were in accordance with the BCR results of the unstable fraction (Zhang et al. 2019). It can be seen from Figure 5 that after adding the additive, the leaching amount of heavy metals decreased significantly, and with the increase of the additive dosage, the leaching amount decreased significantly. From Figure 5(a), the TCLP-extractable leaching amount of Zn in sediments with HAP and n-HAP was rather higher than that with diatomite and Ca(H2PO4)2. When the dosage of additives was 10%, the TCLP-extractable leaching amounts with HAP and n-HAP were 228.57 and 239.29 mg kg–1, much lower than that with diatomite and Ca(H2PO4)2. From Figure 5(b), the TCLP-extractable leaching amount of Mn in sediments with 10% HAP was the lowest leaching amount, which indicates that HAP had the best immobilized effect. As for Pb, from Figure 5(c), it can be obviously observed that the heavy metal immobilization of 10% Ca(H2PO4)2 was the best, and HAP and n-HAP had better immobilization than diatomite and CaCO3. From Figure 5(d), the order for Cd immobilized effects of additive was: HAP > CaCO3 > Ca(H2PO4)2 > n-HAP > diatomite, and when the HAP dosage was 10%, the leachability of Zn, Mn, Pb and Cd decreased by nearly 75.95, 28.17, 78.17 and 85.35%. Used Fe3O4/biochar composites reduced Zn, Pb and Cd leachability by 15, 28 and 11% (Lu et al. 2018).
By comparing the effects of different additive conditions on the immobilization of heavy metals in sediment, it can be found that HAP and n-HAP have significant effects on immobilizing heavy metal in sediment, but n-HAP cannot be mixed unevenly with the sediment because of agglomeration phenomenon (Fernando et al. 2015; Ruphuy et al. 2016), resulting in the reduction of the immobilization effect. In addition, the cost of the immobilized effect is three times higher than that of HAP, and there is no significant difference in the immobilized effect of heavy metals in sediment, so HAP is selected as the immobilizing additive for heavy metal contaminated sediment.
Effect of HAP dosage on heavy metal fractions
From Figure 6 the effect of different HAP dosage on heavy metal fraction and immobilization in sediment can be observed. It can be seen from Figure 6(a) that with the increase of HAP dosage, the acid extractable fraction of Zn in sediment decreased and the stabilizing fraction, such as the oxidizable and residual fraction, increased. However, when the additive dosage was more than 10%, the decrease of acid extractable Zn and the increase of stabilizing Zn were lower, and the Zn content of various fractions were not very obvious under different dosage conditions. It can also be seen from Figure 6(b) that when the HAP dosage was more than 10%, the leaching amount of Zn in the sediment was about 220 mg kg−1, and the change was not obvious.
It can be observed from Figure 6(c) and 6(d) that there is no significant difference on the immobilizing effect of the additive dosage on Mn in sediment between 8% or more than 8% of the HAP dosage. The stabilizing Mn in the sediment only increased by 7 mg kg−1 compared with that when the HAP dosage was 14%. In addition, the results of TCLP showed that the change was not obvious when the HAP dosage was more than 10%.
It can be found from Figure 6(e) and 6(f) that there was no obvious effect on Pb fractions in the sediment when the additive dosage exceeded 8%. With the increase of HAP dosage, the results of TCLP showed that the Pb leaching amount in the sediment became lower. When the HAP dosage was more than 10%, the leaching amount of Pb in the immobilized sediment was 0.42, 0.41 and 0.39 mg kg−1, respectively, which made little difference.
From Figure 6(g), the HAP dosage had no obvious effect on the conversion of the acid extractable and reducible Cd to the stable Cd in the sediment, and only a small amount of the acid extractable Cd was converted to the reducible. It can be seen from Figure 6(h) that when the HAP dosage was 5, 8, 10, 12 and 14%, the leaching amount of Cd in sediment was reduced by 82.73, 84.55, 85.45, 85.45 and 86.36%, respectively. It can be observed that when the HAP dosage was more than 10%, the immobilizing effect had little change.
The leaching amounts of heavy metals are directly proportionate to the dosage of immobilization additives. The more HAP the higher the number of retention sites on the surface of the immobilizing agents, and the HAP are natural mineral products of calcium apatite with a significant specific surface area (Karapinar & Donat 2009; Kadouche et al. 2012b). Therefore, compared with the other four additives, HAP has higher adsorption efficiency for heavy metals in sediment (Zhang et al. 2019).
The immobilizing effects on Zn, Mn, Pb, and Cd in sediment with diatomite, Ca(H2PO4)2, CaCO3, HAP and n-HAP as amendments were investigated. The results showed that HAP and n-HAP had a better effect on immobilizing heavy metals in sediment. The order for immobilized effects of additive was: n-HAP> HAP > CaCO3> Ca(H2PO4)2 > diatomite. Compared with the control, the residual fraction of Zn, Mn, Pb and Cd in sediment with HAP and n-HAP increased by about 5.6, 7.1, 16.51, and 1.6%, respectively. Considering the agglomeration phenomenon of n-HAP, HAP was more suitable for immobilizing heavy metals in sediment.
According to the TCLP, adding additives, especially HAP and n-HAP, can effectively decrease the leaching amount of heavy metals in sediments. Compared to the control, the leaching amounts of Zn, Mn, Pb and Cd with 10% HAP as amendment decreased by about 76, 28, 78, and 85%, respectively.
This research was supported by the Natural Science Foundation of Shandong Province (No. ZR2018MEE045), Foundation of remediation of contaminated sediment in Shandong Province (No. 2017-HCZBLY-003), and Shandong Key Scientific and Technical Innovation Project (No. 2018YFJH0902).
DATA AVAILABILITY STATEMENT
All relevant data are included in the paper or its Supplementary Information.