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Evaluation of ferrihydrite-humic acid coprecipitate as amendment to remediate a Cd- and Pb-contaminated soil
Geoderma 361 (2020) 114131
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Evaluation of ferrihydrite-humic acid coprecipitate as amendment to remediate a Cd- and Pb-contaminated soil
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Miaomiao Xu, Zhuanjun Zhao , Yiran Song, Jing Li, Yang You, Jie Li Key Laboratory of Western China’s Environmental Systems (Ministry of Education) and Key Laboratory for Environmental Pollution Prediction and Control, Gansu Province, College of Earth and Environmental Sciences, Lanzhou University, Lanzhou 730000, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: Cd Pb Fh-HA coprecipitate Fractionation TCLP Enzyme activities
Heavy metals will accumulate continuously after entering the soil environment, which will adversely affect soil quality and harm to human health through the food chain. Among many soil remediation methods, stabilization methods for applying amendments to soil are widely used because of their higher cost-effectiveness and lower environmental impact. In order to evaluate the actual remediation effect of soil amendments, more means are needed to confirm. In this paper, the effects of Fh-HA (Ferrihydrite-Humic acid) coprecipitate as amendment on soil environmental quality of Cd- and Pb-contaminated soil was evaluated by combining the changes of heavy metals fractionation, TCLP-leachable concentration and soil enzyme activity. The experimental data showed that the Fh-HA coprecipitate could greatly enhance the stabilization of Cd and Pb, and promote its transformation from labile fractions to stable fractions. And coprecipitate could also reduce both the TCLP-leachable Cd and Pb, with the maximum reduction efficiencies of 50.7% and 74.8%, respectively. The activities of urease, catalase and alkaline phosphatase increased up to 62.7%, 50.8% and 38.5%, respectively, at a coprecipitate addition of 10%. At the same time, the soil pH was almost unchanged. The results indicated that the application of Fh-HA coprecipitate amendment has a positive effect on improving the quality of terrestrial environment, and no effect on soil physical and chemical properties. The Fh-HA coprecipitate can be further investigated as an economical and environmentally friendly amendment.
1. Introduction Chemical industry manufacturing, mining, industrial or domestic wastewater discharges and the irregular use of pesticides and fertilizers in agricultural production have led to a gradual increase in heavy metals in the soil (Cao et al., 2018). Heavy metals (e.g., Cu, Zn, Mn, Cd, Pb and Ni) can accumulate in the soil up to the level of toxicity, leading to problems such as soil ecological function destruction and deterioration of agricultural product quality (Bolan et al., 2014; Nan and Zhao, 2000). Among these heavy metals, Cd and Pb are of particular concern due to their high hazards. Hence, only the content of Cd and Pb in the soil is higher than 0.3 and 70 mg/kg respectively, there may be soil pollution risks such as edible agricultural products that do not meet the quality and safety standards (GB 15618-2018). Cd and Pb are non-essential element of human body. Cd is highly toxic, mobile, and easily absorbed by plants and inhibits the growth and development of crops (Halim et al., 2014). Pb has low solubility and long retention time in soil (Wang et al., 2016). Meanwhile, Cd and Pb can enter the body through the food chain, which could cause kidney
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damage, nervous system disorders, digestive system and male reproductive system dysfunction and bone hematopoietic function block (Cao et al., 2014; Iqbal and Shah, 2014; Johri et al., 2010; Wei et al., 2015). Therefore, the treatment of Cd- and Pb-contaminated soil is of great significance to ensure human health. Among many well-developed technologies, immobilization is widely used because of its low disruptive to soil fertility and ecological environment. Many materials have proven useful as amendments, mainly including biosolids(Elkhatib et al., 2018), municipal solid waste and sewage sludge compost (Garau et al., 2017; Gattullo et al., 2017; Silvetti et al., 2017), lime (Cui et al., 2016; Garau et al., 2007; Wang et al., 2016), sepiolite (Abad-Valle et al., 2016; Sun et al., 2016; Sun et al., 2013), zeolite (El-Eswed et al., 2015; Shi et al., 2009; Wen et al., 2016), phosphate (Cao et al., 2009; Huang et al., 2016; Su et al., 2015; Valipour et al., 2016), cement and metal (Fe, Mn, Al) oxides (Hale et al., 2012; Hua et al., 2012; Komarek et al., 2013; Michalkova et al., 2014; Rahmani et al., 2010). These amendments directly or indirectly reduce the bioavailability and mobility of heavy metals by precipitating or adsorbing them (Abad-
Corresponding author. E-mail address: [email protected] (Z. Zhao).
https://doi.org/10.1016/j.geoderma.2019.114131 Received 13 July 2019; Received in revised form 4 December 2019; Accepted 8 December 2019 0016-7061/ © 2019 Elsevier B.V. All rights reserved.
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The near-surface soil (0–20 cm) was randomly collected, bulked together, homogenized to form a representative single composite soil sample. The soil was naturally air dried and passed through a 2-mm mesh sieve before it was characterized. The total concentrations of Cd and Pb, OM, CEC (cation exchange capacity), pH, total nitrogen (N), available phosphorus (P) and the particle size of soil were analyzed. For total Cd and Pb concentrations, the soil was digested with HNO3, HF and H2O2 and the extract was measured by atomic absorption spectrometer (ZEEnit 700P, Analytik Jena) (Perez-Santana et al., 2007). The OM was measured by potassium dichromate titration (NY/T 1121.62006) and the CEC was determined by calcium acetate extraction (NY/ T 1121.5-2006). Soil pH is measured by pH meter (FiveEasy Plus FE28, Mettler Toledo), and the soil–water ratio was 1:2.5(w/v). Total N and available P were determined by Kjeldahl method and 0.5M sodium bicarbonate (pH 8.5) extraction, respectively. The latter was measured by visible spectrophotometry using ammonium molybdate and ascorbic acid (Martín et al., 2015). The crystal and amorphous Fe contents were determined by oxalate-ascorbic acid and ammonium oxalate extraction, respectively (Crepin and Johnson, 1993). Particle size of soil was analyzed by the laser particle size analyzer (Mastersize 2000, Malvern). General properties of soil are summarized in Table 1.
Valle et al., 2016). However, these amendments still need to be used with caution. Among them, although phosphate and lime can reduce the exchangeable fraction of Cd and Pb, lime will increase soil pH and the application of phosphate would enrich phosphorus in the soil, which pose a certain threat to the surrounding water. Likewise, the application of some organic materials could enhance the mobility of heavy metals (Yang et al., 2018a). (Co)precipitation or surface adsorption by the formation of inner-surface complexes cause strong association of metal oxides with heavy metals (Scheinost et al., 2001; Sherman and Randall, 2003; Tiberg et al., 2013). Fh is a widely-occurring nano-scale hydrous Fe oxide mineral that has been often proposed as an amendment. (Co)precipitation would occur when Fh is formed in the presence of OM (organic matter) such as HA (humic acid) (Kleber et al., 2015). On the one hand, the formation of the coprecipitate can slow down or inhibit the biodegradation of OM (Eusterhues et al., 2008). On the other hand, OM suppresses the crystallization of Fh and increases its surface electromotive force, thereby enhancing the ability of Fh to bind heavy metal cations (Angelico et al., 2014; Eusterhues et al., 2014; Yang et al., 2018b). Some researchers have been conducted on the role of Fh-HA coprecipitate as heavy metal adsorbent. It has been reported that the adsorption of Cd, Pb, Cu and As by Fh coprecipitated with HA is enhanced compared to pure Fh (Du et al., 2018a; Du et al., 2018b; Ko et al., 2007; Luo et al., 2015). And heavy metals are mainly binding through the bidentate edge-share on pure Fh and the carboxyl groups on HA (Du et al., 2018b). However, little research has been done on Fh-HA coprecipitate as a soil heavy metal amendment, and the effects on soil enzyme activities are still unclear. As the most active soil component, soil enzymes play an important role in soil nutrient cycling and often used as a biomarker of soil quality to assess the effects of various pollutants on the soil environment (Alkorta et al., 2011; Burke et al., 2011). Different studies have been conducted to investigate the ability of chemical modifiers such as sepiolite, apatite, Fh, bentonite phosphate and goethite to restore enzyme activity in heavy metal contaminated soils (Abad-Valle et al., 2015; Abad-Valle et al., 2016; Cui et al., 2017; Qian et al., 2009; Sun et al., 2016). Although Cd and Pb are not the strongest inhibitors of enzymatic activity, it has been shown that these trace elements have decreased the activity of enzymes (Abad-Valle et al., 2016; Chaperon and Sauvé, 2007; Yang et al., 2006). Therefore, the assessment of soil enzyme activity recovery is also the key to determining the actual effect of soil remediation. The main objective of this research is to evaluate the effect of using Fh-HA coprecipitate as amendment to restore Cd- and Pb-contaminated soil. By understanding the influence of Fh-HA coprecipitate on Cd and Pb chemical fractionation, TCLP-leachable (Toxicity Characteristic Leaching Procedure) Cd and Pb concentrations and soil enzyme activities, as well as soil properties, it is expected to provide an environmentally friendly amendments for heavy metal contaminated soil.
2.2. Soil amendment HA was purchased from Shandong West Asia Chemical Industry Co., Ltd., and its carbon content was determined to be 53 wt% by X-ray energy dispersive spectroscopy (JSM-5600LV, Kevex). With regard to the synthesis of Fh-HA coprecipitate (C/Fe = 0.5, molar), FeCl3·6H2O and HA were dissolved in water and 0.4-M NaOH solution, respectively. Then the HA solution was added to the stirred FeCl3 solution and the pH of the mixed solution was adjusted to 7.5 by 0.4-M NaOH solution. After stirring for 2 h, the mixture was allowed to stand for 20 min, the supernatant was siphoned off and the precipitate was centrifuged 5times with deionized water and then freeze-dried.
2.3. Soil treatments The stabilization treatment was carried out by adding Fh-HA coprecipitate to the soil (100 g) to obtain samples having amendment/soil ratio of 0, 3, 5, and 10% (m/m). The samples were placed in a 250-ml plastic container and thoroughly mixed to homogenize it, and then wetted to a humidity of 35%. Subsequently, the container was sealed and stored at room temperature for 2 months. During this period, open the container for several minutes a week and mix the samples to ensure adequate aeration. At the end of the treatment, each group of treated samples was divided into two sub-samples. One was for chemical analysis and the other was for soil enzyme activity analysis. Soil samples without added amendment were studied as control samples.
2. Materials and methods
Table 1 General properties of Cd- and Pb-contaminated soil.
2.1. Soil samples
Control soil
The sampling area of this study is located in the Dongdagou stream basins of Baiyin City, Gansu Province, in the arid and semi-arid regions of Northwest China. Baiyin City is China's main non-ferrous metal mining and smelting base (since the 1950s). Dongdagou was originally a dry river course, which played a role of flood discharge during the flood period. With the continuous development of the industry, many enterprises built along the Dongdagou, making it gradually develop into a sewage drainage channel. Due to the lack of water resources, sewage irrigated farmland appeared in the 1960s (Nan and Zhao, 2000). This behavior resulted in heavy metal content in crops failing to meet national standards, and a large amount of farmland was abandoned. After the study area was abandoned, several non-ferrous metal smelters, tailings ponds and chemical plants were built around it.
pH CEC(cmol(c)/kg) OM(g/kg) Sand (%) Silt (%) Clay (%) Oxide of Fe (g/kg) Crystal Amorphous Total N(g/kg) Available P(mg/kg) Total Cd(mg/kg) Total Pb(mg/kg)
2
7.66 5.98 88.2 36 54 10 36.8 3.12 1.23 97 152 1526
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Fig. 1. The changes of Cd (a) and Pb (b) chemical fractionation in the untreated and treated soils.
3. Chemical analysis of samples
Catalase activity was determined by KMnO4 titration (Stpniewska et al., 2009). Soil samples were added to 40 ml distilled water and 5 ml 0.3% H2O2 (the control group without soil sample was set up). Plug the stopper tightly and shake it on a shaker for 30 min, and then 5 ml 3 mol/L H2SO4 was added. The filtrate was titrated to a reddish color using 0.002 mol/L KMnO4. Each enzyme activity experiments were performed in triplicate.
All soil samples from various stabilization treatments (0, 3, 5, and 10% amendment) were analyzed in triplicate for their pH, leaching toxicity and chemical fractionations of Cd and Pb. Soil pH was determined by the methods described above. Leaching toxicity is based on the US Environmental Protection Agency recommended TCLP. According to the procedure described by (Sun et al., 2012), fluid #2 (0.1 mol/L CH3COOH with pH 2.88 ± 0.05) was chosen for TCLPleachable Cd and Pb determination in this study. The fluid #2 was added at a solid-liquid ratio of 1:20 (m/v), and shaken at 32r/min for 18 h. After centrifugation, all extracts were acidified with pH < 2 with HNO3. An improved sequential extraction procedure BCR (European Community Bureau of Reference) was used to analyze different fractionation of Cd and Pb (Kazi et al., 2005). The soil sample was mixed with the each extraction solution and shaken at 25℃ for 16 h, then centrifuged and filtered. The extracts from different extraction steps correspond to acid soluble, reducible, oxidizable and residual fractions. Both extracts were analyzed by a ZEEnit 700P instrument of atomic adsorption spectrometry.
3.2. Statistical analysis The mean and standard deviation were calculated by Microsoft Office Excel 2010. One-way analysis of variance was performed using SPSS25.0. Multiple comparisons were made by the least significant difference test when significant (P < 0.05) differences were observed between treatments. 4. Results and discussion 4.1. Chemical analysis of soil The general characteristics of soil are shown in Table 1. It shows that the soil has high total N and available P, and low OM and CEC. According to the US Department of Agriculture's soil texture classification triangle chart, the soil is a weakly alkaline silt loam. The total Cd and Pb content is 152 mg/kg and 1526 mg/kg, respectively, far exceeding the soil environmental quality risk control standard for soil contamination of agricultural land of Chain (GB 15618-2018). After the addition of the Fh-HA coprecipitate, the soil pH increased slightly with the added amount (3%, 5%, and 10%) to 7.68 ± 0.03, 7.69 ± 0.03, and 7.74 ± 0.02, respectively. Compared with the use of sepiolite, phosphate material, calcium carbonate, lime and Fh as amendments, Fh-HA coprecipitate treatment has almost no effect on soil pH (Abad-Valle et al., 2015; Huang et al., 2016; Sun et al., 2012). This is due to the affinity of Fh to hydrogen ions, while HA is a kind of particle colloid with negative charge (Angelico et al., 2014; Mcbride, 1994). When both exist at the same time, the effect on soil pH may be counteracted. Furthermore, soil buffering also plays a role in regulating acidity and alkalinity (Blake and Goulding, 2002). Figs. 1 and 2 showed the changes of Cd and Pb chemical fractionation and TCLP-leachable Cd and Pb concentrations in each treated sample. The TCLP-leachable Cd concentration in the control samples was 4.32 mg/L, which was far excess the secure threshold of 1 mg/L. The initial data about Cd fraction presented in Fig. 1a showed that in the untreated soil mainly occurs in acid soluble fraction (128 mg/kg),
3.1. Enzyme activities of samples In this study, catalase, urease, alkaline phosphatase and invertase were selected to study the effect of Fh-HA coprecipitate treatment on soil enzyme activity in Cd- and Pb-contaminated soil. To determine soil urease activity, 10 ml 10% urea was used as substrates (Tabatabai 1994). 5.0 g soil was incubated with 1 ml toluene and 20 ml citrate buffer (pH 6.7) for 24 h at 37 °C. After the completion of the culture, the amount of ammonium released was measured at 578 nm by Indophenol Blue Method. For the soil invertase activity, 5.0 g soil was placed in a 100 ml flask. And 1 ml toluene, 15 ml 8% sucrose solution and 5 ml phosphate buffer (pH 5.5) were added. Then incubate in a 37 °C incubator for 24 h (Kandeler et al., 1999). The extracted glucose content was measured at 508 nm using 3,5-DNS method (dinitrosalicylic acid colorimetric method). 5.0 g soil was incubated with 1 ml toluene and 20 ml 0.5% sodium phenyl phosphate for 24 h at 37 °C to determine alkaline phosphatase activity (Tabatabai and Bremner, 1969). After the culture, 0.3% aluminum sulfate solution, boric acid buffer (pH 9.4) and chlorodibromop-benzoimine were added, subsequent quantified by spectrophotometry at 660 nm. For the determination of the above three enzyme activities, a substrate-free control was used for each sample. 3
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Fig. 2. The changes of TCLP-leachable Cd (a) and Pb (b) concentrations in the untreated and treated soils.
Fig. 3. Changes of soil urease (a), catalase (b), Invertase (c) and alkaline phosphatase (d) activities after treatment with different amount of Fh-HA coprecipitates.
4
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invertase activity in soil reflects the maturity and fertility level of soil, and is an important indicator for evaluating soil fertility (Cang et al., 2009). The change of invertase activity in soil treated with Fh-HA coprecipitate is shown in Fig. 3c. As the Fh-HA coprecipitate content increases, the activities of invertase increased first and then decreased. At the addition of 3%, the coprecipitate treatment significantly (P < 0.05) increased the invertase activity and reached the maximum value of 34.0 mg glucose/g soil, with an increase of 18.0%. Alkaline phosphatase is a kind of hydrolytic enzyme which widely exists in soil. The enzymatic action of alkaline phosphatase can accelerate the speed of dephosphorization and improve the availability of soil P (Abad-Valle et al., 2016). Under the stress of Cd and Pb, the alkaline phosphatase activity gradually increased with increasing FhHA coprecipitate concentrations (Fig. 3d). There was no obvious increase in phosphatase activity under 3% coprecipitate treatments. This may be due to the small change of soil pH. It has been showed that alkaline phosphatase activity was positively correlated with soil pH (Abad-Valle et al., 2015). In contrast, at higher coprecipitate concentrations of 5% and 10%, phosphatase activity increased significantly (P < 0.05) by 26.0% and 38.5%, respectively.In addition to the effects of increasing soil pH, this may also be due to the retention of phosphate by coprecipitate (Wang et al., 2015). Phosphatase activity usually increases when there is less phosphate in the soil (Haynes and Swift, 1988; Wang et al., 2008). Table 2 shows the correlation coefficients between soil enzyme activity and Cd and Pb content under different extractants. The leachable Cd and Pb were negatively correlated with urease and catalase activities (P < 0.05), and acid soluble Cd was closely negatively correlated with catalase activity (P < 0.01). In addition, only TCLP-leachable Pb was negatively correlated with alkaline phosphatase activity (P < 0.05). The negative correlation between leachable Cd and Pb and soil enzyme activities indicates that the enzyme activities gradually increased along with the decreasing heavy metal content. It was attributed to the weakening of the toxicity of Cd and Pb to catalase, urease and alkaline phosphatase activities by Fh-HA coprecipitate stabilization. The invertase activity had little change after the Fh-HA coprecipitate treatments and did not have any statistically significant relationship with all leachable Cd and Pb. Researchers found that the invertase activity has a suitable soil pH of 4.2–4.5(Cang et al., 2009). In this study, soil pH above 7.66, which may affect the activity of the invertase. Although the invertase activity decreased slightly as the amount of Fh-HA coprecipitate increased from 3% to 10%, the invertase activity of the treated soil was higher than that of the untreated soil.
accounting for 84.5% of the total Cd content. After adding Fh-HA coprecipitate, mutual transformation occurred between the various fractions. The transformation of acid soluble fraction into other three fractions was mainly manifested, and the amount of transformation increases with the increase of the addition of the coprecipitates. At an addition of 10% (m/m), the acid soluble fraction was significantly (P < 0.05) reduced by 53.7%, and the reducible, residual, and oxidizable fractions were significantly (P < 0.05) increased to 38.4, 48.6, and 5.5 mg/kg, respectively. Simultaneously, as shown in Fig. 2a, the TCLP-leachable Cd concentration also showed the same trend: as the amount of Fh-HA coprecipitate increased, the TCLP-leachable Cd concentration in the soil significantly (P < 0.05) decreased, and decreased by 50.7% in 10% addition. Different from the Cd content in the soil, Pb mainly exists in the reducible fraction (1016 mg/kg), accounting for 66.6% of the total Pb content (Fig. 1b). After the coprecipitate added, both the acid soluble and reducible fractions were transformed into other fractions, and the amount of transformation is increased as the addition increases. When the addition is 10% (m/m), the acid soluble and reducible fraction significantly (P < 0.05) decreased from 167 and 1016 mg/kg to 92.4 and 789 mg/kg, respectively. At the same time, the oxidizable fraction and the residual fraction significantly (P < 0.05) increased from 51.9 to 96.7 mg/kg and 291 to 547 mg/kg, respectively. The change of TCLP-leachable Pb concentration in the soil was shown in Fig. 2b. It can be seen in Fig. 2b that the original TCLP-leachable Pb concentration of the soil was about 2.46 mg/L (below the secure threshold of 5 mg/L), and it was significantly (P < 0.05) decreased to 0.62 mg/L after 60 days of treatment (about 74.8% reduction, 10% addition). The macroscopically significant changes in Cd and Pb fractionations and TCLP-leachable Cd and Pb concentrations indicate that Fh-HA coprecipitate have a positive effect on the stabilization of Cd- and Pb-contaminated soil, and could reduce the bioavailability of Cd and Pb. It has been found that Cd and Cu were adsorbed by bidentate edge-sharing on the Fh fraction, and on the other hand by adsorption with the carboxyl group on the HA (Du et al., 2018b; Moon and Peacock, 2012). In addition, the addition of Fh-HA coprecipitates has almost no effect on soil pH. Indicate that the Fh-HA coprecipitate has great potential for the remediation of contaminated soil. Furthermore, Fh and HA are widely distributed in pristine soils, sediments, aquatic and terrestrial soil systems (Adriano, 1986; Cornell and Schwertmann, 1996). The high environmental friendliness and availability of the material is also a factor supporting its use in such an environment. 4.2. Soil enzyme activities
5. Conclusion The changes of soil enzyme activity before and after the addition of Fh-HA coprecipitate are show in Fig. 3. It can be seen from the changes of catalase, urease, alkaline phosphatase and invertase activity that the coprecipitate has improved soil enzyme activity. Ammonium is products of enzymatic reaction of urease. The activity of urease reflects the ability of soil organic N to transform into inorganic N and the supply of soil inorganic N (Guo et al., 2015). Although the value of urease activity increased slightly when the amount of Fh-HA coprecipitate increased to 10%, it was significantly (P < 0.05) increased as the amount of coprecipitate added increased. The urease activity increased from 4.22 to 6.86 mg NH3-N/g soil after the addition of the coprecipitate, which reflected the sensitivity of urease to Cd and Pb stabilization treatment. Soil catalase can promote the decomposition of hydrogen peroxide, which is beneficial to prevent the poisoning effect of hydrogen peroxide on organisms (Stpniewska et al., 2009). The trend of catalase activity is similar to that of urease activity (Fig. 3b). The soil catalase activity treated by coprecipitates was significant (P < 0.05) higher than that in the untreated soil. At additions of 3%, 5% and 10%, the catalase activity increased by 30.5%, 43.0% and 50.8%, respectively. Glucose, products of enzymatic reaction of invertase, is the nutrient source of plants and microorganisms (Hu et al., 2010). Therefore, the
Stabilization of Fh-HA coprecipitate as amendment is an effective technique to restore Cd- and Pb-contaminated soils. Treatment with coprecipitates transformed the labile fractions of Cd and Pb to stable fractions, and reduced both the TCLP-leachable Pb and Cd concentrations. The increase in soil enzyme activity indicates that the metabolism of soil has improved after coprecipitates treatment. The activities of urease and catalase can be significant increased by more than half at a coprecipitate addition of 10%. This is mainly related to the decrease of bioavailability of Cd and Pb. At the same time, this treatment has no Table 2 Correlation between soil enzyme activities and leachable Cd/Pb.
Acid soluble Cd Acid soluble Pb TCLP-leachable Cd TCLP-leachable Pb
Urease
Catalase
Invertase
Alkaline phosphatase
−0.986* −0.989* −0.987* −0.935
−0.991** −0.973* −0.967* −0.911
−0.317 −0.014 −0.03 −0.203
−0.832 −0.933 −0.914 −0.967*
* Significant at P < 0.05 ** Significant at P < 0.01 5
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significant effect on soil pH. This indicates that the application of FhHA coprecipitate amendment can improve the status of contaminated soil without affecting soil physical and chemical properties. It provides an economical and environmentally friendly solution for the remediation of Cd- and Pb-contaminated soils.
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Evaluation of ferrihydrite-humic acid coprecipitate as amendment to remediate a Cd- and Pb-contaminated soil
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Miaomiao Xu, Zhuanjun Zhao , Yiran Song, Jing Li, Yang You, Jie Li Key Laboratory of Western China’s Environmental Systems (Ministry of Education) and Key Laboratory for Environmental Pollution Prediction and Control, Gansu Province, College of Earth and Environmental Sciences, Lanzhou University, Lanzhou 730000, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: Cd Pb Fh-HA coprecipitate Fractionation TCLP Enzyme activities
Heavy metals will accumulate continuously after entering the soil environment, which will adversely affect soil quality and harm to human health through the food chain. Among many soil remediation methods, stabilization methods for applying amendments to soil are widely used because of their higher cost-effectiveness and lower environmental impact. In order to evaluate the actual remediation effect of soil amendments, more means are needed to confirm. In this paper, the effects of Fh-HA (Ferrihydrite-Humic acid) coprecipitate as amendment on soil environmental quality of Cd- and Pb-contaminated soil was evaluated by combining the changes of heavy metals fractionation, TCLP-leachable concentration and soil enzyme activity. The experimental data showed that the Fh-HA coprecipitate could greatly enhance the stabilization of Cd and Pb, and promote its transformation from labile fractions to stable fractions. And coprecipitate could also reduce both the TCLP-leachable Cd and Pb, with the maximum reduction efficiencies of 50.7% and 74.8%, respectively. The activities of urease, catalase and alkaline phosphatase increased up to 62.7%, 50.8% and 38.5%, respectively, at a coprecipitate addition of 10%. At the same time, the soil pH was almost unchanged. The results indicated that the application of Fh-HA coprecipitate amendment has a positive effect on improving the quality of terrestrial environment, and no effect on soil physical and chemical properties. The Fh-HA coprecipitate can be further investigated as an economical and environmentally friendly amendment.
1. Introduction Chemical industry manufacturing, mining, industrial or domestic wastewater discharges and the irregular use of pesticides and fertilizers in agricultural production have led to a gradual increase in heavy metals in the soil (Cao et al., 2018). Heavy metals (e.g., Cu, Zn, Mn, Cd, Pb and Ni) can accumulate in the soil up to the level of toxicity, leading to problems such as soil ecological function destruction and deterioration of agricultural product quality (Bolan et al., 2014; Nan and Zhao, 2000). Among these heavy metals, Cd and Pb are of particular concern due to their high hazards. Hence, only the content of Cd and Pb in the soil is higher than 0.3 and 70 mg/kg respectively, there may be soil pollution risks such as edible agricultural products that do not meet the quality and safety standards (GB 15618-2018). Cd and Pb are non-essential element of human body. Cd is highly toxic, mobile, and easily absorbed by plants and inhibits the growth and development of crops (Halim et al., 2014). Pb has low solubility and long retention time in soil (Wang et al., 2016). Meanwhile, Cd and Pb can enter the body through the food chain, which could cause kidney
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damage, nervous system disorders, digestive system and male reproductive system dysfunction and bone hematopoietic function block (Cao et al., 2014; Iqbal and Shah, 2014; Johri et al., 2010; Wei et al., 2015). Therefore, the treatment of Cd- and Pb-contaminated soil is of great significance to ensure human health. Among many well-developed technologies, immobilization is widely used because of its low disruptive to soil fertility and ecological environment. Many materials have proven useful as amendments, mainly including biosolids(Elkhatib et al., 2018), municipal solid waste and sewage sludge compost (Garau et al., 2017; Gattullo et al., 2017; Silvetti et al., 2017), lime (Cui et al., 2016; Garau et al., 2007; Wang et al., 2016), sepiolite (Abad-Valle et al., 2016; Sun et al., 2016; Sun et al., 2013), zeolite (El-Eswed et al., 2015; Shi et al., 2009; Wen et al., 2016), phosphate (Cao et al., 2009; Huang et al., 2016; Su et al., 2015; Valipour et al., 2016), cement and metal (Fe, Mn, Al) oxides (Hale et al., 2012; Hua et al., 2012; Komarek et al., 2013; Michalkova et al., 2014; Rahmani et al., 2010). These amendments directly or indirectly reduce the bioavailability and mobility of heavy metals by precipitating or adsorbing them (Abad-
Corresponding author. E-mail address: [email protected] (Z. Zhao).
https://doi.org/10.1016/j.geoderma.2019.114131 Received 13 July 2019; Received in revised form 4 December 2019; Accepted 8 December 2019 0016-7061/ © 2019 Elsevier B.V. All rights reserved.
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The near-surface soil (0–20 cm) was randomly collected, bulked together, homogenized to form a representative single composite soil sample. The soil was naturally air dried and passed through a 2-mm mesh sieve before it was characterized. The total concentrations of Cd and Pb, OM, CEC (cation exchange capacity), pH, total nitrogen (N), available phosphorus (P) and the particle size of soil were analyzed. For total Cd and Pb concentrations, the soil was digested with HNO3, HF and H2O2 and the extract was measured by atomic absorption spectrometer (ZEEnit 700P, Analytik Jena) (Perez-Santana et al., 2007). The OM was measured by potassium dichromate titration (NY/T 1121.62006) and the CEC was determined by calcium acetate extraction (NY/ T 1121.5-2006). Soil pH is measured by pH meter (FiveEasy Plus FE28, Mettler Toledo), and the soil–water ratio was 1:2.5(w/v). Total N and available P were determined by Kjeldahl method and 0.5M sodium bicarbonate (pH 8.5) extraction, respectively. The latter was measured by visible spectrophotometry using ammonium molybdate and ascorbic acid (Martín et al., 2015). The crystal and amorphous Fe contents were determined by oxalate-ascorbic acid and ammonium oxalate extraction, respectively (Crepin and Johnson, 1993). Particle size of soil was analyzed by the laser particle size analyzer (Mastersize 2000, Malvern). General properties of soil are summarized in Table 1.
Valle et al., 2016). However, these amendments still need to be used with caution. Among them, although phosphate and lime can reduce the exchangeable fraction of Cd and Pb, lime will increase soil pH and the application of phosphate would enrich phosphorus in the soil, which pose a certain threat to the surrounding water. Likewise, the application of some organic materials could enhance the mobility of heavy metals (Yang et al., 2018a). (Co)precipitation or surface adsorption by the formation of inner-surface complexes cause strong association of metal oxides with heavy metals (Scheinost et al., 2001; Sherman and Randall, 2003; Tiberg et al., 2013). Fh is a widely-occurring nano-scale hydrous Fe oxide mineral that has been often proposed as an amendment. (Co)precipitation would occur when Fh is formed in the presence of OM (organic matter) such as HA (humic acid) (Kleber et al., 2015). On the one hand, the formation of the coprecipitate can slow down or inhibit the biodegradation of OM (Eusterhues et al., 2008). On the other hand, OM suppresses the crystallization of Fh and increases its surface electromotive force, thereby enhancing the ability of Fh to bind heavy metal cations (Angelico et al., 2014; Eusterhues et al., 2014; Yang et al., 2018b). Some researchers have been conducted on the role of Fh-HA coprecipitate as heavy metal adsorbent. It has been reported that the adsorption of Cd, Pb, Cu and As by Fh coprecipitated with HA is enhanced compared to pure Fh (Du et al., 2018a; Du et al., 2018b; Ko et al., 2007; Luo et al., 2015). And heavy metals are mainly binding through the bidentate edge-share on pure Fh and the carboxyl groups on HA (Du et al., 2018b). However, little research has been done on Fh-HA coprecipitate as a soil heavy metal amendment, and the effects on soil enzyme activities are still unclear. As the most active soil component, soil enzymes play an important role in soil nutrient cycling and often used as a biomarker of soil quality to assess the effects of various pollutants on the soil environment (Alkorta et al., 2011; Burke et al., 2011). Different studies have been conducted to investigate the ability of chemical modifiers such as sepiolite, apatite, Fh, bentonite phosphate and goethite to restore enzyme activity in heavy metal contaminated soils (Abad-Valle et al., 2015; Abad-Valle et al., 2016; Cui et al., 2017; Qian et al., 2009; Sun et al., 2016). Although Cd and Pb are not the strongest inhibitors of enzymatic activity, it has been shown that these trace elements have decreased the activity of enzymes (Abad-Valle et al., 2016; Chaperon and Sauvé, 2007; Yang et al., 2006). Therefore, the assessment of soil enzyme activity recovery is also the key to determining the actual effect of soil remediation. The main objective of this research is to evaluate the effect of using Fh-HA coprecipitate as amendment to restore Cd- and Pb-contaminated soil. By understanding the influence of Fh-HA coprecipitate on Cd and Pb chemical fractionation, TCLP-leachable (Toxicity Characteristic Leaching Procedure) Cd and Pb concentrations and soil enzyme activities, as well as soil properties, it is expected to provide an environmentally friendly amendments for heavy metal contaminated soil.
2.2. Soil amendment HA was purchased from Shandong West Asia Chemical Industry Co., Ltd., and its carbon content was determined to be 53 wt% by X-ray energy dispersive spectroscopy (JSM-5600LV, Kevex). With regard to the synthesis of Fh-HA coprecipitate (C/Fe = 0.5, molar), FeCl3·6H2O and HA were dissolved in water and 0.4-M NaOH solution, respectively. Then the HA solution was added to the stirred FeCl3 solution and the pH of the mixed solution was adjusted to 7.5 by 0.4-M NaOH solution. After stirring for 2 h, the mixture was allowed to stand for 20 min, the supernatant was siphoned off and the precipitate was centrifuged 5times with deionized water and then freeze-dried.
2.3. Soil treatments The stabilization treatment was carried out by adding Fh-HA coprecipitate to the soil (100 g) to obtain samples having amendment/soil ratio of 0, 3, 5, and 10% (m/m). The samples were placed in a 250-ml plastic container and thoroughly mixed to homogenize it, and then wetted to a humidity of 35%. Subsequently, the container was sealed and stored at room temperature for 2 months. During this period, open the container for several minutes a week and mix the samples to ensure adequate aeration. At the end of the treatment, each group of treated samples was divided into two sub-samples. One was for chemical analysis and the other was for soil enzyme activity analysis. Soil samples without added amendment were studied as control samples.
2. Materials and methods
Table 1 General properties of Cd- and Pb-contaminated soil.
2.1. Soil samples
Control soil
The sampling area of this study is located in the Dongdagou stream basins of Baiyin City, Gansu Province, in the arid and semi-arid regions of Northwest China. Baiyin City is China's main non-ferrous metal mining and smelting base (since the 1950s). Dongdagou was originally a dry river course, which played a role of flood discharge during the flood period. With the continuous development of the industry, many enterprises built along the Dongdagou, making it gradually develop into a sewage drainage channel. Due to the lack of water resources, sewage irrigated farmland appeared in the 1960s (Nan and Zhao, 2000). This behavior resulted in heavy metal content in crops failing to meet national standards, and a large amount of farmland was abandoned. After the study area was abandoned, several non-ferrous metal smelters, tailings ponds and chemical plants were built around it.
pH CEC(cmol(c)/kg) OM(g/kg) Sand (%) Silt (%) Clay (%) Oxide of Fe (g/kg) Crystal Amorphous Total N(g/kg) Available P(mg/kg) Total Cd(mg/kg) Total Pb(mg/kg)
2
7.66 5.98 88.2 36 54 10 36.8 3.12 1.23 97 152 1526
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Fig. 1. The changes of Cd (a) and Pb (b) chemical fractionation in the untreated and treated soils.
3. Chemical analysis of samples
Catalase activity was determined by KMnO4 titration (Stpniewska et al., 2009). Soil samples were added to 40 ml distilled water and 5 ml 0.3% H2O2 (the control group without soil sample was set up). Plug the stopper tightly and shake it on a shaker for 30 min, and then 5 ml 3 mol/L H2SO4 was added. The filtrate was titrated to a reddish color using 0.002 mol/L KMnO4. Each enzyme activity experiments were performed in triplicate.
All soil samples from various stabilization treatments (0, 3, 5, and 10% amendment) were analyzed in triplicate for their pH, leaching toxicity and chemical fractionations of Cd and Pb. Soil pH was determined by the methods described above. Leaching toxicity is based on the US Environmental Protection Agency recommended TCLP. According to the procedure described by (Sun et al., 2012), fluid #2 (0.1 mol/L CH3COOH with pH 2.88 ± 0.05) was chosen for TCLPleachable Cd and Pb determination in this study. The fluid #2 was added at a solid-liquid ratio of 1:20 (m/v), and shaken at 32r/min for 18 h. After centrifugation, all extracts were acidified with pH < 2 with HNO3. An improved sequential extraction procedure BCR (European Community Bureau of Reference) was used to analyze different fractionation of Cd and Pb (Kazi et al., 2005). The soil sample was mixed with the each extraction solution and shaken at 25℃ for 16 h, then centrifuged and filtered. The extracts from different extraction steps correspond to acid soluble, reducible, oxidizable and residual fractions. Both extracts were analyzed by a ZEEnit 700P instrument of atomic adsorption spectrometry.
3.2. Statistical analysis The mean and standard deviation were calculated by Microsoft Office Excel 2010. One-way analysis of variance was performed using SPSS25.0. Multiple comparisons were made by the least significant difference test when significant (P < 0.05) differences were observed between treatments. 4. Results and discussion 4.1. Chemical analysis of soil The general characteristics of soil are shown in Table 1. It shows that the soil has high total N and available P, and low OM and CEC. According to the US Department of Agriculture's soil texture classification triangle chart, the soil is a weakly alkaline silt loam. The total Cd and Pb content is 152 mg/kg and 1526 mg/kg, respectively, far exceeding the soil environmental quality risk control standard for soil contamination of agricultural land of Chain (GB 15618-2018). After the addition of the Fh-HA coprecipitate, the soil pH increased slightly with the added amount (3%, 5%, and 10%) to 7.68 ± 0.03, 7.69 ± 0.03, and 7.74 ± 0.02, respectively. Compared with the use of sepiolite, phosphate material, calcium carbonate, lime and Fh as amendments, Fh-HA coprecipitate treatment has almost no effect on soil pH (Abad-Valle et al., 2015; Huang et al., 2016; Sun et al., 2012). This is due to the affinity of Fh to hydrogen ions, while HA is a kind of particle colloid with negative charge (Angelico et al., 2014; Mcbride, 1994). When both exist at the same time, the effect on soil pH may be counteracted. Furthermore, soil buffering also plays a role in regulating acidity and alkalinity (Blake and Goulding, 2002). Figs. 1 and 2 showed the changes of Cd and Pb chemical fractionation and TCLP-leachable Cd and Pb concentrations in each treated sample. The TCLP-leachable Cd concentration in the control samples was 4.32 mg/L, which was far excess the secure threshold of 1 mg/L. The initial data about Cd fraction presented in Fig. 1a showed that in the untreated soil mainly occurs in acid soluble fraction (128 mg/kg),
3.1. Enzyme activities of samples In this study, catalase, urease, alkaline phosphatase and invertase were selected to study the effect of Fh-HA coprecipitate treatment on soil enzyme activity in Cd- and Pb-contaminated soil. To determine soil urease activity, 10 ml 10% urea was used as substrates (Tabatabai 1994). 5.0 g soil was incubated with 1 ml toluene and 20 ml citrate buffer (pH 6.7) for 24 h at 37 °C. After the completion of the culture, the amount of ammonium released was measured at 578 nm by Indophenol Blue Method. For the soil invertase activity, 5.0 g soil was placed in a 100 ml flask. And 1 ml toluene, 15 ml 8% sucrose solution and 5 ml phosphate buffer (pH 5.5) were added. Then incubate in a 37 °C incubator for 24 h (Kandeler et al., 1999). The extracted glucose content was measured at 508 nm using 3,5-DNS method (dinitrosalicylic acid colorimetric method). 5.0 g soil was incubated with 1 ml toluene and 20 ml 0.5% sodium phenyl phosphate for 24 h at 37 °C to determine alkaline phosphatase activity (Tabatabai and Bremner, 1969). After the culture, 0.3% aluminum sulfate solution, boric acid buffer (pH 9.4) and chlorodibromop-benzoimine were added, subsequent quantified by spectrophotometry at 660 nm. For the determination of the above three enzyme activities, a substrate-free control was used for each sample. 3
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Fig. 2. The changes of TCLP-leachable Cd (a) and Pb (b) concentrations in the untreated and treated soils.
Fig. 3. Changes of soil urease (a), catalase (b), Invertase (c) and alkaline phosphatase (d) activities after treatment with different amount of Fh-HA coprecipitates.
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invertase activity in soil reflects the maturity and fertility level of soil, and is an important indicator for evaluating soil fertility (Cang et al., 2009). The change of invertase activity in soil treated with Fh-HA coprecipitate is shown in Fig. 3c. As the Fh-HA coprecipitate content increases, the activities of invertase increased first and then decreased. At the addition of 3%, the coprecipitate treatment significantly (P < 0.05) increased the invertase activity and reached the maximum value of 34.0 mg glucose/g soil, with an increase of 18.0%. Alkaline phosphatase is a kind of hydrolytic enzyme which widely exists in soil. The enzymatic action of alkaline phosphatase can accelerate the speed of dephosphorization and improve the availability of soil P (Abad-Valle et al., 2016). Under the stress of Cd and Pb, the alkaline phosphatase activity gradually increased with increasing FhHA coprecipitate concentrations (Fig. 3d). There was no obvious increase in phosphatase activity under 3% coprecipitate treatments. This may be due to the small change of soil pH. It has been showed that alkaline phosphatase activity was positively correlated with soil pH (Abad-Valle et al., 2015). In contrast, at higher coprecipitate concentrations of 5% and 10%, phosphatase activity increased significantly (P < 0.05) by 26.0% and 38.5%, respectively.In addition to the effects of increasing soil pH, this may also be due to the retention of phosphate by coprecipitate (Wang et al., 2015). Phosphatase activity usually increases when there is less phosphate in the soil (Haynes and Swift, 1988; Wang et al., 2008). Table 2 shows the correlation coefficients between soil enzyme activity and Cd and Pb content under different extractants. The leachable Cd and Pb were negatively correlated with urease and catalase activities (P < 0.05), and acid soluble Cd was closely negatively correlated with catalase activity (P < 0.01). In addition, only TCLP-leachable Pb was negatively correlated with alkaline phosphatase activity (P < 0.05). The negative correlation between leachable Cd and Pb and soil enzyme activities indicates that the enzyme activities gradually increased along with the decreasing heavy metal content. It was attributed to the weakening of the toxicity of Cd and Pb to catalase, urease and alkaline phosphatase activities by Fh-HA coprecipitate stabilization. The invertase activity had little change after the Fh-HA coprecipitate treatments and did not have any statistically significant relationship with all leachable Cd and Pb. Researchers found that the invertase activity has a suitable soil pH of 4.2–4.5(Cang et al., 2009). In this study, soil pH above 7.66, which may affect the activity of the invertase. Although the invertase activity decreased slightly as the amount of Fh-HA coprecipitate increased from 3% to 10%, the invertase activity of the treated soil was higher than that of the untreated soil.
accounting for 84.5% of the total Cd content. After adding Fh-HA coprecipitate, mutual transformation occurred between the various fractions. The transformation of acid soluble fraction into other three fractions was mainly manifested, and the amount of transformation increases with the increase of the addition of the coprecipitates. At an addition of 10% (m/m), the acid soluble fraction was significantly (P < 0.05) reduced by 53.7%, and the reducible, residual, and oxidizable fractions were significantly (P < 0.05) increased to 38.4, 48.6, and 5.5 mg/kg, respectively. Simultaneously, as shown in Fig. 2a, the TCLP-leachable Cd concentration also showed the same trend: as the amount of Fh-HA coprecipitate increased, the TCLP-leachable Cd concentration in the soil significantly (P < 0.05) decreased, and decreased by 50.7% in 10% addition. Different from the Cd content in the soil, Pb mainly exists in the reducible fraction (1016 mg/kg), accounting for 66.6% of the total Pb content (Fig. 1b). After the coprecipitate added, both the acid soluble and reducible fractions were transformed into other fractions, and the amount of transformation is increased as the addition increases. When the addition is 10% (m/m), the acid soluble and reducible fraction significantly (P < 0.05) decreased from 167 and 1016 mg/kg to 92.4 and 789 mg/kg, respectively. At the same time, the oxidizable fraction and the residual fraction significantly (P < 0.05) increased from 51.9 to 96.7 mg/kg and 291 to 547 mg/kg, respectively. The change of TCLP-leachable Pb concentration in the soil was shown in Fig. 2b. It can be seen in Fig. 2b that the original TCLP-leachable Pb concentration of the soil was about 2.46 mg/L (below the secure threshold of 5 mg/L), and it was significantly (P < 0.05) decreased to 0.62 mg/L after 60 days of treatment (about 74.8% reduction, 10% addition). The macroscopically significant changes in Cd and Pb fractionations and TCLP-leachable Cd and Pb concentrations indicate that Fh-HA coprecipitate have a positive effect on the stabilization of Cd- and Pb-contaminated soil, and could reduce the bioavailability of Cd and Pb. It has been found that Cd and Cu were adsorbed by bidentate edge-sharing on the Fh fraction, and on the other hand by adsorption with the carboxyl group on the HA (Du et al., 2018b; Moon and Peacock, 2012). In addition, the addition of Fh-HA coprecipitates has almost no effect on soil pH. Indicate that the Fh-HA coprecipitate has great potential for the remediation of contaminated soil. Furthermore, Fh and HA are widely distributed in pristine soils, sediments, aquatic and terrestrial soil systems (Adriano, 1986; Cornell and Schwertmann, 1996). The high environmental friendliness and availability of the material is also a factor supporting its use in such an environment. 4.2. Soil enzyme activities
5. Conclusion The changes of soil enzyme activity before and after the addition of Fh-HA coprecipitate are show in Fig. 3. It can be seen from the changes of catalase, urease, alkaline phosphatase and invertase activity that the coprecipitate has improved soil enzyme activity. Ammonium is products of enzymatic reaction of urease. The activity of urease reflects the ability of soil organic N to transform into inorganic N and the supply of soil inorganic N (Guo et al., 2015). Although the value of urease activity increased slightly when the amount of Fh-HA coprecipitate increased to 10%, it was significantly (P < 0.05) increased as the amount of coprecipitate added increased. The urease activity increased from 4.22 to 6.86 mg NH3-N/g soil after the addition of the coprecipitate, which reflected the sensitivity of urease to Cd and Pb stabilization treatment. Soil catalase can promote the decomposition of hydrogen peroxide, which is beneficial to prevent the poisoning effect of hydrogen peroxide on organisms (Stpniewska et al., 2009). The trend of catalase activity is similar to that of urease activity (Fig. 3b). The soil catalase activity treated by coprecipitates was significant (P < 0.05) higher than that in the untreated soil. At additions of 3%, 5% and 10%, the catalase activity increased by 30.5%, 43.0% and 50.8%, respectively. Glucose, products of enzymatic reaction of invertase, is the nutrient source of plants and microorganisms (Hu et al., 2010). Therefore, the
Stabilization of Fh-HA coprecipitate as amendment is an effective technique to restore Cd- and Pb-contaminated soils. Treatment with coprecipitates transformed the labile fractions of Cd and Pb to stable fractions, and reduced both the TCLP-leachable Pb and Cd concentrations. The increase in soil enzyme activity indicates that the metabolism of soil has improved after coprecipitates treatment. The activities of urease and catalase can be significant increased by more than half at a coprecipitate addition of 10%. This is mainly related to the decrease of bioavailability of Cd and Pb. At the same time, this treatment has no Table 2 Correlation between soil enzyme activities and leachable Cd/Pb.
Acid soluble Cd Acid soluble Pb TCLP-leachable Cd TCLP-leachable Pb
Urease
Catalase
Invertase
Alkaline phosphatase
−0.986* −0.989* −0.987* −0.935
−0.991** −0.973* −0.967* −0.911
−0.317 −0.014 −0.03 −0.203
−0.832 −0.933 −0.914 −0.967*
* Significant at P < 0.05 ** Significant at P < 0.01 5
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significant effect on soil pH. This indicates that the application of FhHA coprecipitate amendment can improve the status of contaminated soil without affecting soil physical and chemical properties. It provides an economical and environmentally friendly solution for the remediation of Cd- and Pb-contaminated soils.
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