Natural amino acids as potential chelators for soil remediation

Environmental Research 183 (2020) 109140

Contents lists available at ScienceDirect

Environmental Research journal homepage: www.elsevier.com/locate/envres

Natural amino acids as potential chelators for soil remediation a

a

a

b

b

Noam Dolev , Zhanna Katz , Zvi Ludmer , Amos Ullmann , Neima Brauner , Roman Goikhman a b

T a,∗

Faculty of Agriculture Food and Environment, The Hebrew University of Jerusalem, Rehovot, 76100, Israel School of Mechanical Engineering, Faculty of Engineering, Tel-Aviv University, Tel-Aviv, 69978, Israel

A R T I C LE I N FO

A B S T R A C T

Keywords: Amino acid Glycine Soil remediation Toxic metal Chelator

The soils contaminated by toxic metals are often remediated using EDTA and similar non-biodegradable chelators. Most chelators are in fact synthetic amino acid derivatives, whereas natural proteinogenic amino acids (PAAs) have not been systematically explored as remediation agents, despite their well-known metal chelating abilities and environmental benefits. Our study represents a comprehensive research exploring 16 structurally and functionally different PAAs as potential remediating agents, applied to 3 different heavy metal-contaminated samples. The study was mostly focused on extracting Cd, Cu, Ni, and Zn. The reaction parameters were screened and optimized. It was found that the efficiencies of extracting Cu, Ni, and Zn by Threonine, Aspartic acid and Histidine were comparable to those by EDTA, whereas non-polar side chain–containing PAAs demonstrated consistently lower PTM extraction rates compared to other PAAs. The sulfur-containing Cysteine appeared to be efficient to extract Cd (to some extent), Ni and Zn, but not Cu, due to chemical reasons. The structure-functional correlations were identified, described, and found to be independent on the specific samples. Possible molecular mechanisms of metal extraction from soils by PAAs are discussed. In contrast to EDTA, the soil-essential elements are almost not extracted by PAAs. This important feature of the PAAs, along with their availability, observed selectivity, competitive efficiency, non-toxicity and even fertilizing properties, make them particularly soilfriendly, and thus, potentially applicable chelators in certain remediation processes.

1. Introduction Contamination of soils and sediments by potentially toxic metals (PTMs) is a well-recognized current global problem (He et al., 2015; Kumar et al., 2019; Li et al., 2019; Liedekerke et al., 2014; Vareda et al., 2019). Unlike organic contaminants, metals do not undergo microbial or chemical degradation (Bolan et al., 2014) and pose hazards to human health and the ecosystem through the food chains or groundwater (Wuana and Okieimen, 2011). A number of technologies have been developed to remediate the PTM-contaminated soils. Among the remediation techniques, ex-situ soil washing processes have been successfully implemented for many years because of their high PTM removal efficiency, wide applicability, and economic feasibility (Dermont et al., 2008; Ferraro et al., 2016; Wuana and Okieimen, 2011). Powerful synthetic PTM-chelating agents (like EDTA) are widely used in soil washing processes (Dermont et al., 2008; Lestan, 2015). However, residues of non-biodegradable chelators can affect natural biochemical processes (Jelusic and Lestan, 2014; Lestan, 2015). In addition, the aggressive extraction of Ca by EDTA can damage the soil structure (Tsang et al., 2007). Since EDTA extracts soilessential elements (e.g. calcium), the consumption of EDTA by ∗

complexation with the essential metals makes the extraction process less efficient (Manouchehri and Bermond, 2006; Tandy et al., 2004). Recently, synthetic biodegradable chelators have been tested for PTM extraction from soils (Fabbricino et al., 2013; Hauser et al., 2005; Kolodynska, 2011; Pinto et al., 2014). Their extraction efficiency appears to be lower than that of EDTA towards some metals (Tandy et al., 2004); nevertheless, they are used due to ecological considerations and their low affinity for essential metals. To date, the most widely used chelators appear to be modified synthetic alpha-amino acids, i.e. compounds that involve the R2N–CH (R)-COOH molecular fragment. The most common examples include EDTA, DTPA (diethylenetriaminepentaacetic acid), and EDDS (ethylenediaminedisuccinic acid) (Fig. 1 A-C, the alpha-amino acid core is shaded). In contrast, natural amino acids, and particularly, proteinogenic amino acids (PAAs) have hardly been explored as soil remediation chelators despite being known as strong PTM complexants (Farkas and Sovago, 2012; Fleck and Petrosyan, 2014; Laudicina et al., 2013). PTM-PAA interactions are common in living organisms and are vital for life. Metalloproteins that involve the interaction of PAA residues with metal ions comprise over a third of all the known proteins (Hu et al., 2014). Consequently, PTM-PAA complexes (Fig. 1-D) have been

Corresponding author. E-mail address: [email protected] (R. Goikhman).

https://doi.org/10.1016/j.envres.2020.109140 Received 23 September 2019; Received in revised form 29 December 2019; Accepted 14 January 2020 Available online 19 January 2020 0013-9351/ © 2020 Elsevier Inc. All rights reserved.

Environmental Research 183 (2020) 109140

N. Dolev, et al.

Fig. 1. Common chelator structures with featured AA core, and a typical PAA-metal complex.

2. Materials and methods

extensively studied in biochemistry-oriented research (Farkas and Sovago, 2012; Fleck and Petrosyan, 2014; Shimazaki et al., 2009). Moreover, PAAs are used to detoxify living organisms from PTMs (Ralph et al., 2010). Various PAAs are abundantly present in soils (Laudicina et al., 2013; Warren, 2014). They play an important role in plant growth (Warren, 2014), and in some cases are used in agriculture as fertilizers (Liu and Lee, 2013). Therefore, residual PAA adsorption in the soil can result in a rather positive effect on the biosphere since PAAs can serve as a nitrogen source for plants, and can also assist in essential element uptake. PAAs differ in their structure, sizes, solubility, hydrophobic/hydrophilic and acidic/basic properties, side chain coordination ability, etc. (Wu, 2013). Such a variety could make PAAs valuable models for research aimed at a better understanding of their chelating performance in soil remediation. Nevertheless, to our knowledge, only a few related studies were reported, where 1 or 2 PAAs were occasionally tested. Particularly, Glycine (Gly according to the IUPAC abbreviations for the PAAs) alone was tested on separate soil components, demonstrating moderate PTM (Cu, Pb, and Zn) extraction efficiency (Fischer et al., 1992). Gly and Histidine (His) have been tested for the extraction of Cu, Pb, and Zn, and both were found to be competitive to EDDS (Karczewska and Milko, 2010). His was tested for extraction of Cd and Pb from a spiked soil (Chen et al., 2007) and was found to be less efficient than citric or oxalic acid. However, no other PTMs were considered. Cysteine (Cys) was tested in two studies (Fischer, 2002; Vadas and Ahner, 2009) and found to be fairly efficient regarding some PTMs. These few studies provide limited and unsystematic data, mostly inconclusive and rather inapplicable. The lack of relevant studies was pointed out as “surprising” in a recent review on PTM remediation by biomolecules (Vandenbossche et al., 2015). As part of our continuous search for efficient and eco-friendly soil/ sediment remediation processes (Golan et al., 2014; Ludmer et al., 2009; Ullmann et al., 2013), a first systematic study of using readily available, competitively priced, and eco-friendly PAAs as PTM extraction agents is presented. The goals of the present research are (1) to demonstrate the general ability of the PAAs to extract heavy metals from contaminated soils, (2) to find structure-functional correlations based on PAA structures (3) to propose the most promising PAAs as the “green” chelators for soil remediation. Our study was focused on the generality and potential broadness of PAA applications to multi-metal contaminated samples.

2.1. Chemicals and reagents All the reagents were purchased from Fischer chemicals, Acros Organics, and Sigma-Aldrich. The purity of all the chemicals was above 98%. Inorganic acids (HNO3 70%, HCl 32%) were purchased from BioLab Ltd. The water was pretreated with ion-exchange column (Zalion Ltd.) and analyzed using the ICP-AES instrument (axial Inductively Coupled Plasma Atomic Emission Spectrometer, Spectro Arcos) to confirm the absence of the residual metal ions. 2.2. Spiked soil characterization and preparation The spiked soil (abbreviated as SP) was prepared using the collected agricultural soil according to Begum et al. (2013). A solution containing equimolar amounts of Cd(II), Cr(III), Cu(II), Ni(II), Pb(II), and Zn(II) nitrates (4.1–4.2 mmol each) in 2.5 L deionized water was prepared. The solution was mixed with 400 g of the agricultural soil (free from PTM). The resulting slurry was mixed for 10 days at room temperature. After separating the soil from the solution by centrifuge, the prepared spiked soil was washed twice with 2 L deionized water and stirred with deionized water at 60 °C for 1 h to remove free and loosely bound metal cations. The resulting soil was separated from the solution and dried at 100 °C for 2 days prior to further characterization (for more details see SI section 2.1). 2.3. Sampling and characterization of the contaminated media Two different samples were collected from different locations in Israel. These are (1) heavily contaminated sludge that was collected from an abandoned wastewater facility (abbreviated as WS) and (2) moderately contaminated Kishon River Sediment (KS) that was collected from excavated sediment piles in the industrial area. The samples were dried at 105 °C for 2 days (the weight was checked to be stable), then ground and passed through a 2-mm sieve. Physicochemical characteristics of the contaminated soils were conducted using dry combustion technique (for organic matter), calcimeter technique for carbonates, and particle size distribution was conducted using laser diffraction analysis (Mastersizer 2000, Malvern Panalytical LTD). The major sample properties are detailed in Table 1. Metal content was determined by ICP-AES and summarized in Table 2 and in Tables S1 and S2 (SI). Images of the soil structure and particle size before and after the extraction process were obtained using the JEOL IT-100 Scanning Electron Microscopy (SEM) instrument.

Table 1 Selected properties of the contaminated samples. Contaminated media

SP KS WS

Particle size distribution Sand (> 50 μm)

Silt (2–50 μm)

Clay (< 2 μm)

64% 54% 63%

19% 40% 33%

17% 6% 4%

2

Organic matter

Carbonates

pH

3.2% 4.9% 41.4%

3.2% 15.1% 2.4%

6.8 7.7 5.4

Environmental Research 183 (2020) 109140

N. Dolev, et al.

3. Results and discussion

Table 2 Relevant metal content in the contaminated samples. Sample

Cd

Cu

Ni

Zn

(mg Kg−1) SP KS WS Alloweda a

1105 ± 19 22 ± 1 618 ± 17 1

The present work introduces PAAs as possible soil remediation chelators and/or models for further chelator design and research and does not pretend to suggest the ready-to-use optimized technology yet. Therefore, the principal ability of the PAAs to extract PTMs was examined under conditions of the “total chelator-extractable PTMs”, using an excess of chelator to PTM content. The results were compared to extraction with EDTA solutions of equivalent concentrations and to washing with water. The chelator excess was chosen to compensate competition of the essential metal ions, and to observe detectable and experimentally significant differences (above deviation/measurement errors) of the PAA efficiencies. This approach allowed to exclude possible discrepancies in findings, and to detect the best candidates for the further research and development.

Ca (g Kg−1)

367 ± 4 198 ± 1 365 ± 2 100

612 ± 2 55 ± 1 154 ± 1 50

665 ± 33 683 ± 61 2023 ± 38 200

72.067 ± 0.4 111.714 ± 0.8 39.724 ± 0.5 –

Van der Voet et al. (2013).

2.4. Batch metal extraction Screening experiments were performed to optimize PAA concentration, solid to liquid ratio, temperature and reaction time (Figs. S1–S5 in the SI). Reaction time in a given temperature was set, and solid to liquid ratio was based on common soil washing procedures (Wang et al., 2018). Contaminated soil samples were placed in a 50 ml plastic tube. Then, 20 mL of 0.25 M PAA aqueous solution (or 0.125 M EDTA, since an EDTA molecule is equivalent to two PAA fragments) were applied to 3 g of the contaminated sample. However, since PTM concentration was different in the various tested samples, applying the same (standard) amount of the PAAs resulted in different PAA/PTM actual ratios depending on the specific sample tested. Particularly, the PAA:PTM molar ratio was about 10:1 (WS, the most contaminated sample), 45:1 (KS, the least contaminated sample) and 17:1 (SP), considering PTM(PAA)2 molecular configuration. This ratio falls within the commonly tested EDTA:PTM molar ratios (e.g. 8:1 (Lim et al., 2005), 30:1 (Barona et al., 2001)) considering that only part of the chelator in the washing solution is complexed with PTMs. The rest remains in differently protonated forms or complexed with major soil cations, such as Fe, Mn and Ca. The prepared mixtures were stirred at 60 °C for 2 h. The pH of the PAA extraction solutions was adjusted when necessary, although in most cases the soil samples buffered the reaction after a few seconds to that of the contaminated samples’ pH (See SI section 5). Following the reaction, the supernatant and the solid phase were separated by centrifugation, and analyzed by ICP-AES (See SI, section 6). Typically, triplicate was used to obtain reliable results, and the average is presented.

3.1. Screening of the PAA metal extraction efficiency on the spiked soil To estimate the general ability of PAAs to extract PTMs from soil, preliminary screening of the PAAs was carried out using the carefully prepared spiked soil (SP) for testing the extraction of PTMs and of naturally present Ca. The PAA extraction efficiencies were compared to those obtained by EDTA and blank (deionized water) (Fig. 3, the PAAs are organized and colored according to their groups in Fig. 2 and Table 3). It was observed that different PAAs extract different percentages of PTMs and the efficiency of some PAAs are comparable to that of EDTA. Deionized water was able to extract only negligible amounts of metal ions. For example, Cd extraction varied from 16% (Val) to 47% (Thr) and 69% (Cys); the latter result was practically equal to that of EDTA (70%). Cu extraction for most of the PAAs was about 30–35%, similar to that of EDTA. Noteworthy, Cys demonstrated the lowest activity towards Cu (13%). Ni extraction by the PAAs varied from 16 to 17% to over 30%. Nearly all but three PAAs demonstrated an equal or better Ni extraction result compared to EDTA (23%). Zn extraction by PAAs ranged from 9% (Ala) to 38% (Cys), depending on the specific PAA. Cys demonstrated better results than EDTA (38% vs 30%), whereas extraction with Phe, Thr, Asp, and Trp reached or exceeded 20%. Extraction of Pb by PAAs was generally less than 10%, except 63–70% in the case of Cys, that was equal to EDTA result (71%). Cr extraction by the PAAs also appeared to be limited (2–12%). However, these results were comparable to the EDTA result (6%) (for Pb and Cr extraction results see Table S3 in SI, the corresponding graphs are omitted here for clarity). The above-described results with spiked soil indicate that the activity of some PAAs might be comparable to that of EDTA. On the other hand, neither specific PAA, nor specific PAA group demonstrated an obvious preference as a metal extracting agent. Structurally and chemically different Phe, Thr, Asp, and Cys (with the exception of Cu) demonstrated better metal extraction activity compared to other PAAs. Noteworthy is that most PAAs extracted less than 5% Ca (except acidic Asp), compared to ~ 40% Ca extracted by EDTA, which suggests that PAAs are potentially highly soil-friendly reagents. These spiked soil experiments demonstrated that the PAAs are potentially capable of extracting certain heavy metals similarly to EDTA while the essential metals are less affected (Table S4 in the SI). These promising results encouraged us to test the PAAs on the real contaminated samples (KS and WS) because the spiked soil can serve as a model only (Ferraro et al., 2016). The further research was focused at demonstrating the PAAs’ ability to extract heavy metals from different real contaminated media, searching for the general structure-functional correlations, and determining the best-performing PAAs that can serve as potential “green” chelators in soil remediation.

2.5. Choice and classification of PAAs There are 21 known PAAs, 16 of which were chosen for the experiments. Highly priced selenocysteine, poorly water-soluble tyrosine and leucine were excluded. Lysine and arginine were not studied due to low efficiency in the preliminary experiments (SI, section 7, Tables S3, S5 and S7). Structures and names of the explored PAAs are depicted in Fig. 2. Official IUPAC – recognized three-letter abbreviations of the PAAs are used in Fig. 2 and thereafter. PAAs are usually classified by their structure, side-chain nature, electronic, hydrophilic, or steric properties according to the specific research needs, and many PAA classifications exist (Wu, 2013). In this paper, PAAs are divided into five groups according to their structural/ chemical similarity (Fig. 2): (1) PAAs with non-functional alpha-hydrocarbon substituent; (2) hydroxyl-containing; (3) side chain carboxyls and their derivatives; (4) sulfur-containing; (5) aromatic. Being both hydrocarbon-containing and aromatic, Phe is included in, and compared to, both groups 1 and 5. The relevant stability constants of PAA-metal complexes and PAAs’ isoelectric point values pI (i.e., the pH at which the particular molecule is electrically neutral) are presented in Table 3 (Berthon, 1995; Kiss et al., 1991; Pettit, 1984; Powell, 2000; Sovago et al., 1993; Tang and Skibsted, 2016; Yamauchi and Odani, 1996).

3

Environmental Research 183 (2020) 109140

N. Dolev, et al.

Fig. 2. Structures of the explored PAAs, divided into 5 groups.

functional) alpha-hydrocarbon substituent, including Phe. In this group, hydrophobicity increases (and solubility in water decreases) in the order Gly < Ala < Val < Ile ≈ Phe. The steric bulk generally increases with the alpha substituent size in the order Gly < Ala < Phe < Val < Ile (Fauchère et al., 1988). However, stability constants of the PAA-metal complexes only weakly depend on the PAA's steric bulk and hydrophobicity (Table 3). Gly and Pro tend to form the most stable transition metal complexes in this group. Despite the mentioned PAA structural differences, the extraction results of applying this PAA group to the WS and KS samples (Fig. 4A and B) did not reveal any “group leader”. For instance, the small and hydrophilic Gly and the much bulkier hydrophobic Phe demonstrated very similar results of Cu and Zn extraction from the WS sample. Also, Ala and Ile demonstrated negligible (mostly 0–3%) differences in the PTM extraction from both samples despite the considerable difference in their molecular sizes (Fig. 4A and B, and Tables S5 and S7 in SI). In general, PTM extraction efficiency by this PAA group was below 50%. Extraction results of Cr and Pb by the PAAs are omitted here and in the following results due to low efficiency (generally less than 5%) but are presented in Tables S5 and S7 in the SI. Noteworthy is that EDTA was inefficient towards Cr extraction as well. Relative Cd extraction efficiency from KS and WS samples differs considerably (here and thereafter) due to low Cd content in the KS sample compared to the WS sample (22 ppm vs 618 ppm), therefore a comparison of Cd extraction efficiency based on the chosen samples is ambivalent. To conclude the first PAA group results, neither the bulk hindrance nor hydrophobic or nucleophilic properties have any significant effect on the PTM extraction efficiencies. However, the similar PAA efficiencies correlate with the similarity of M(PAA)2 stability constants. These facts indicate that the main mechanism of the PTM extraction

Table 3 PAAs’ pI, and reliable (rounded to 0.1) ML2 stability constants (M = divalent metal cation, L = PAA anion).

3.2. Performance of metal extraction by PAAs using real contaminated samples Divided into the following five groups, the PAAs demonstrated different PTM extraction activity. 3.2.1. The first (alpha-hydrocarbon) PAA group The first PAA group consists of PAAs with a non-polar (and non4

Environmental Research 183 (2020) 109140

N. Dolev, et al.

Fig. 3. Cd, Cu, Ni, Zn and Ca extraction results from SP sample by PAAs, water, and EDTA.

were not observed with the first PAA group.

in the case of the first PAA group involves a reversible dissociation of the metal cations from the solid matrix to the solution (Lucia et al., 2003), followed by a complexation of these dissociated metal cations by the dissolved PAA (i.e. “dissociative” mechanism (Chauhan et al., 2015a, 2015b)). In the case of an alternative (“associative”) mechanism, where PAA molecules attack the soil-trapped metal cations and substitute the soillocated ligands (Chauhan et al., 2015a, 2015b; Zhang and Tsang, 2013), one would expect some PAA structure-dependent trends, which

3.2.2. The second (side hydroxyl) PAA group The second PAA group involves side hydroxyl-containing Thr and Ser. They are slightly more acidic and are obviously more polar than the sterically similar Val and Ala. Nevertheless, the stability constants of the PTM complexes of the four herein mentioned PAAs are very similar (Table 3). The extraction efficiencies of these PAAs (Fig. 5) demonstrate a 5

Environmental Research 183 (2020) 109140

N. Dolev, et al.

clear superiority of Thr and Ser over Val and Ala in the treatment of both WS and KS samples. Also, Ser showed slightly (but consistently) lower efficiency than Thr with regards to all the PTMs. Particularly, in the case of the WS sample (Fig. 5A), Thr was able to extract almost twice more Cu, about 3 times more Ni (reaching 53%), and about 5 times more Zn than Val. Similarly, in the case of the KS sample (Fig. 5B), Thr extracted more Cu and Ni, twice as much Zn, and 3.5 times more Cd than Val. Extraction of Cu and Zn with Thr reached 50%. Noteworthy is that despite better results for PTMs, Thr and Ser did not extract more Ca than the other PAAs (Fig. 5). The plausible explanation for the superior activity of Thr and Ser compared to OH-lacking aliphatic PAAs is discussed below. Side chain hydroxyl group of Ser and Thr can contribute to the metal chelation process, making these PAAs tridentate chelators (Ye et al., 2008). In addition, alcoholic hydroxyl groups form strong bonds with carbonates (which are always present in soils) (Okhrimenko et al., 2013), that presumably allow hydroxyl-containing Ser and Thr to be reversibly adsorbed onto the soil surface (Yeasmin et al., 2014). This can lead to two consequences: (1) Bearing three polar groups in the molecule, Thr and Ser can easily form multilayers near the polar surfaces owing to PAA-surface and PAA-PAA hydrogen bonding (Rahsepar et al., 2016). As a result, local PAA concentration increases near the targeted surface. This, in turn, can facilitate formation of the PAA-metal complexes according to Le Chatelier's principle (increasing concentration of the starting reagent shifts the equilibrium towards the formation of the products). (2) When both the PAA and the targeted metal ion are adsorbed on the surface, the surface can “catalyze” their interaction due to the forced proximity of the reacting centers. Similar “catalytic” enhanced metal complex formation on the surface vs in the solution was reported and explained (Holzwarth et al., 1978).

Fig. 4. Metal extraction (%) by the 1st PAA group from (A) WS, (B) KS.

It is possible that either one or both pathways contribute to the superiority of the second PAA group over the first one. Further investigation is required to ascertain the involved molecular mechanism. 3.2.3. The third (side carboxyl and amide) PAA group The third tested PAA group consists of the acidic Asp and Glu, and their amide derivatives Asn and Gln. PTM complexes of Asp are slightly stronger than those of Glu (Table 3) because the side chain carboxyl group of Asp forms a fairly stable six-membered ring with the metal cation, and makes Asp a tridentate chelator (Fleck and Petrosyan, 2014). Fig. 6 compares pH-dependent metal extraction from the KS sample. When Asp and Glu were used at their natural initial pH, 2.3 and 2.8 respectively, Ca extraction reached ~20%. However, when the pH was elevated to 7 (adjusted with NaOH), Ca extraction dropped to a negligible amount of 1%. In contrast to Ca extraction, PTM extraction increased at a higher pH. Particularly, PTM extraction values by Asp at pH 7.0 (vs pH 2.3) were 1.5 times higher for Cu, and 2.5 times higher for Zn. Glu showed a similar pH-dependent trend. These results demonstrate a selective, pH-controlled metal extraction. Under acidic conditions, the PAAs dissolve carbonates and extract essential metals, but PAA chelating abilities (i.e. nucleophilicity) with regards to PTMs are weak (Tables S7 and S8 in the SI). On the other hand, at a neutral pH, PAAs lose their acidity but increase their nucleophilicity due to deprotonating carboxyl groups, and thus become more selective towards PTM extraction. The considerably better PTM extraction efficiency of Asp over Glu in both KS and WS cases (Figs. 6 and 7) correlates with the stability constants of the PAA-metal complexes (Table 3). Asp and Glu were further compared to Asn and Gln at pH 7. Both amide derivatives are slightly weaker complexants than their corresponding acids (see Table 3). However, Asn and Gln were nearly as

Fig. 5. Metal extraction (%) by the 2nd vs the 1st PAA group from (A) WS (B) KS.

6

Environmental Research 183 (2020) 109140

N. Dolev, et al.

Fig. 6. Metal extraction (%) by the 3rd PAA group at different pH values from KS.

either (Tables S5 and S7 in SI). In contrast to Cys, Met extracted 44% and 14% Cu from KS and WS, respectively. The failure of Cys to extract Cu can be explained by the fact that Cys instantly reacts with Cu(II) producing water-insoluble products: Cu(I) compounds and cystine (Berthon, 1995). These products are not removed from the sample by washing. Comparing –SH vs –OH side chain groups, Cys extracted 1.5 times more Cd, Ni, and Zn from the KS sample, and 3 times more Zn from WS sample than Ser.

efficient on average as Asp and Glu, correspondingly (Figs. 6 and 7). Asp and Asn were also clearly superior to Glu and Gln with regards to PTM extraction from both samples, which is consistent with the stability constants of the appropriate PTM complexes. Thus, the third PAA group exhibited higher PTM extraction efficiency at neutral vs acidic pH, which supports the dominance of the chelating mechanism of the metal extraction over the acid-promoted dissolution of the PTM-containing soil particles (such as carbonates). The importance of the chelating factor is also supported by Asp superiority in this group (Figs. 6 and 7), since Asp has an additional (side chain) chelating group in the molecule.

3.2.5. The fifth (aromatic) PAA group The fifth (aromatic) PAA group includes heteroaromatic His, Trp, and homoaromatic Phe (Fig. 2). The strength of the PTM complexes increases in a row Phe < Trp < His (Table 3). His is able to strongly coordinate metal cations by the imidazole nitrogen, whereas the indole group of Trp does not participate in chelation (Yamauchi et al., 2002). Indeed, His exhibited high PTM extraction efficiency, whereas Trp and Phe demonstrated poorer results (especially, in the treatment of WS sample), (Fig. 9a-b).

3.2.4. The fourth (sulfur-containing) PAA group The fourth (sulfur-containing) PAA group involves Cys and Met. Cys bears a free thiol group, whereas Met possesses a longer and less reactive thioether group (Fig. 2). Generally, metal complexes of Cys are far more stable (see Table 3) because the non-hindered thiol group can coordinate to the transition metal ions, making Cys a tridentate ligand (Berthon, 1995). Cys was compared to Met, and to its oxygen-containing analog Ser (see Fig. 8). Cys demonstrated clear superiority towards extracting Ni and Zn from both the WS and KS samples. However, Cu was not extracted at all by Cys from either KS or WS. Raising the pH from natural 5.3 to 8 did not promote Cu extraction

3.3. Comparison of the best performing PAAs Fig. 10 summarizes and compares PTM extraction efficiencies of the PAAs that demonstrated the best performance in each PAA group. These

Fig. 7. Metal extraction (%) by the 3rd PAA group at pH 7 from WS. 7

Environmental Research 183 (2020) 109140

N. Dolev, et al.

are Gly, Thr, Asp, Cys, and His. Disodium EDTA results are added for comparison as a standard chelating agent. In general, the first (aliphatic) PAA group exhibits the lowest results. This suggests that the side-chain functional groups play an important role in PTM extraction. As discussed in paragraph 3.2.2 the polar or charged side chain group presumably provides an additional opportunity for the formation of electrostatic and/or hydrogen bonding with the soil surface, and also assists in the formation of PAA multilayers near the targeted surfaces. Such interactions, though reversible, increase the local concentration of the PAAs near the contaminated soil surface. This, in turn, can shift the PAA-soil competition for the metal ions towards complexation with the PAAs. In the case of the heavily contaminated WS sample (Fig. 10-A), Ni extraction by Asp, Cys, and His reached the EDTA efficiency values. Extraction of Cu by Thr and Asp, and extraction of Zn by His and Cys exceeded 50% of the EDTA efficiency values. The Cd extraction efficiencies by the PAAs were unfortunately much lower than those obtained by EDTA. On the other hand, the PAAs extracted insignificant amounts (4–12%) of Ca and other essential elements, whereas EDTA extracted 56% Ca, and about 40% Mn and P (Table S6 in SI). In the case of the KS sample (Fig. 10-B), which is moderately contaminated and has a low organic matter content, the PAAs appeared to be highly competitive to EDTA towards certain metals. The results of Cu, Ni and Zn extraction by the leading PAAs were generally close to or even better than those of EDTA. For example, Thr, Asp, and His extracted 53–64% Cu compared to 38% by EDTA. Extraction of Zn by Asp and Cys also exceeded that of EDTA and reached 60%. Cd extraction by Thr and Cys reached 50–75% of EDTA's extraction value. The results of the authentically contaminated samples were compared to the SP sample, which contained amounts of Cd, Cu, Ni, and Zn that were comparable to the real samples (1105, 367, 612, and 665 ppm respectively). It was found that the observed trends with Cu, Ni, and Zn extraction repeat in the SP case, i.e. Thr and Asp extract the same or close amounts of these metals compared to EDTA. The noticeable difference between SP and WS was observed in the Cd case: extraction reached 50% by Thr and Asp, and 70% by Cys and EDTA. Also, Pb extraction by Cys from the SP sample (that contained 1200 ppm Pb) reached the EDTA value of 70%, whereas, in the case of the WS sample (288 ppm Pb), Cys extracted only 1–2% Pb, compared to 52% by EDTA (Table S5 in SI). It is noteworthy that, alike other samples, EDTA extracted large amount of Ca from SP sample (almost 40%), compared to 1–3% extracted by PAAs. These results indicate that the PAAs can be competitive “green” chelators in the cases of moderate Cu, Ni, and Zn contamination. These metals dominate in a number of contaminated sites worldwide and present a serious problem (Slukovskaya et al., 2018). However, in the case of Cd and Pb, the PAA efficiency should be tested on a specific case basis. In fact, SP results, although similar to some of the real soil results and trends, cannot be considered completely reliable as a model for any authentically polluted media (Ferraro et al., 2016). The fact that the PAA efficiency of some PTM extraction is comparable to that of EDTA under the same conditions can be explained by an observation that EDTA extracts considerable amounts of Ca which competes with PTM for complexation (Manouchehri and Bermond, 2006; Tandy et al., 2004). Since even a small percentage of extracted Ca usually exceeds the amount of extractable PTM by hundreds of times, such competition affects EDTA efficiency. In contrast to EDTA, the PAAs exhibit low Ca extractability, hence the Ca factor does not visibly affect the PAA efficiency. Therefore, despite PAAs are weaker chelators than EDTA, their PTM extraction efficiencies are comparable. This point is an additional argument that supports the potential applicability of the PAAs as chelators.

Fig. 8. Metal extraction (%) by the 4th PAA group from (A) WS (B) KS.

Fig. 9. Selected PTM extraction (%) by the 5th PAA group from (A) WS (B) KS.

3.4. Estimation of PAA adsorption on the soil after treatment To estimate the percentage of the PAAs adsorbed on the treated soil, 8

Environmental Research 183 (2020) 109140

N. Dolev, et al.

Fig. 10. Metal extraction (%) from the WS (A) KS (B) and SP (C) samples by the leading PAAs from each group compared to EDTA results.

9

Environmental Research 183 (2020) 109140

N. Dolev, et al.

Table 4 Met concentration in the resulting solution before and after applying to the contaminated samples. Methionine solution concentration (mM) Sample

Before extraction

After extraction

SP KS WS

250 250 250

214 227 210

Table 5 Sand size distribution (> 50 μm, according to USDA (United States Department of Agriculture)) for the treated WS and KS samples.

Met left in solution

Treatment

86% 91% 84%

Water Thr EDTA HCl

Met (as a PAA representative) was applied to the soil samples, and the concentration of Sulfur in the solution was measured by ICP before and after the treatment. Table 4 summarizes the average results from triplicate (standard deviation did not exceed 5%). The results indicate that PAA adsorption on the soil samples falls in a range of 9–16%. Considering that EDTA adsorption to soil can reach up to 64% (20% in average, Jez and Lestan, 2016), the PAA adsorption percentage seems to be acceptable. However, in contrast to synthetic chelators, PAA adsorption can result in a positive effect on the biosphere as was noted in the introduction. Further research of the residual PAA impact on the soil chemistry and biosphere is necessary to ascertain the benefits of PAA use.

Sand (> 50 μm) population (%) KS

WS

50.8 44.7 44.5 10.1

57.2 31.9 32.2 8.1

or aggregates, the other samples exhibit an increasingly greater presence of smaller particles (less than 50 μm). The observed sample particle sizes seem to get smaller after the treatments in the order of Blank > Thr ≈ EDTA > HCl. Further support of this observation was obtained by quantitative PSD results using laser diffraction analysis. Both KS and WS samples were treated as above (using deionized water, Thr, EDTA, and HCl), and their PSD was analyzed. The results are shown in Table 5, and in Fig. 12 (for KS sample) and Fig. 13 (for WS sample). Particularly, the population of the sand-sized particles (50 μm and bigger) slightly decreased from 50% to ~44.6% after applying EDTA and Thr to the KS sample, compared to the destructive shift from 50% to 10% after applying HCl. In the case of the WS sample, the sand-sized particle population decreased from 57% to 32% after applying the chelators, compared to dropping down to 8% after HCl treatment. These observations support our findings that Thr (as a PAA representative) appears to have a similar effect on the soil structure compared to the widely used conventional chelator EDTA, whereas HCl mostly destroys the soil particles.

3.5. Impact of the applied PAAs on the soil structure: Scanning Electron Microscopy (SEM) and particle size distribution (PSD) results The evidence that applying PAAs does not significantly affect the soil structure was supported by SEM and particle size distribution (PSD) results. As an example, four treated KS samples were compared using SEM. These are blank (deionized water), and equivalent amounts of Thr, EDTA, and HCl that were applied at the standard conditions (see section 2.4). The resulting SEM patterns of the treated and dried KS samples are depicted in Fig. 11. While the KS Blank sample consists of primarily large soil particles

4. Conclusions A systematic study of the efficiencies of PTM extraction from a

Fig. 11. SEM patterns of the KS sample after treating with 1. Water 2. Thr 3. EDTA 4. HCl. 10

Environmental Research 183 (2020) 109140

N. Dolev, et al.

Fig. 12. Particle size distribution of the KS sample conducted by laser diffraction analysis.

Fig. 13. Particle size distribution of the WS sample conducted by laser diffraction analysis.

properties were found to be the more dominating factor for PTM extraction than the acidity of the chelators. These findings can help to hone the future search for more efficient and more selective chelators. (2) Thr, Asp, His, and Cys appeared to be as efficient as EDTA with regards to certain PTM extraction from all three different tested contaminated samples (spiked soil, real contaminated soil, and sediment), thus demonstrating their general potential applicability. Cys was highly active towards PTMs except for Cu. Low extraction of Ca by the PAAs (i.e. higher selectivity) makes their use more soilfriendly and even more efficient compared to EDTA due to the lack of competition between Ca and PTMs over the complexation. The most efficient PAAs can be potentially used as green chelating agents for remediating moderately contaminated soils, considering the fact that the allowable residual PTM levels vary greatly in different countries (He et al., 2015).

spiked soil sample and two different real contaminated media samples by applying 16 PAAs was conducted. The PAAs were classified into five groups based on their structural similarity. The most efficient PAA was determined in each group and compared to the leading PAAs in other groups, and to EDTA as a standard. The conclusions refer to (1) scientific aspects (2) applicative aspects (3) further considerations. (1) The research results suggest that hydrophobic (as was demonstrated with the 1st PAA group), nucleophilic, and steric properties are not the factors that determine the PAA metal-extracting efficiency. However, the reactive (bond-forming) side-chain functional groups play an important role in PTM extraction. Particularly, an unexpected pronounced effect of the side chain hydroxyl (alcoholic) groups was clearly observed (and elaborated). Also, the superiority of the potentially tridentate Asp and His over the similar bidentate Glu and Trp was demonstrated. Moreover, metal-chelating 11

Environmental Research 183 (2020) 109140

N. Dolev, et al.

(3) The advantages, limitations and observed structure-functional correlations of the PAAs when considered as potential chelators for PTM removal from soils were studied. In future research for practical application, more specific studies of PAA applicability should be conducted with regards to the soil/sediment/sludge types, composition, physical/chemical properties, nature and degree of contamination, as well as optimization of the chelator amount. Addressing the commercial feasibility, the use of technical grade PAAs or their mixtures, and PAA recycling-reuse can be considered. The latter concept was demonstrated by our group using Gly-spiked soil model (Dolev et al., 2019) and appears to be a promising approach to pursue.

Environ. Sci. Biotechnol. 15, 111–145. https://doi.org/10.1007/s11157-015-9378-2. Fischer, K., 2002. Removal of heavy metals from soil components and soil by natural chelating agents. Part I : displacement from clay minerals and Peat by L -cysteine and L-penicillamine. Water. Air Soil Pollut. 137, 267–286. Fischer, K., Rainer, C., Bieniek, D., Kettrup, A., 1992. Desorption of heavy metals from typical soil components (clay, peat) with glycine. Int. J. Environ. Anal. Chem. 46 (1–3), 53–62. Fleck, M., Petrosyan, A.M., 2014. Salts of Amino Acids: Crystallization, Structure and Properties. Springer International Publishing, Cham, Switzerland. https://doi.org/10. 1007/978-3-319-06299-0. Golan, T., Dahan, G., Ludmer, Z., Brauner, N., Ullmann, A., 2014. Heavy metals extraction with the SRPTE process from two matrices – industrial sludge and river sediments. Chem. Eng. J. 236, 47–58. https://doi.org/10.1016/j.cej.2013.09.062. Hauser, L., Tandy, S., Schulin, R., Nowack, B., 2005. Column extraction of heavy metals from soils using the biodegradable chelating agent EDDS. Environ. Sci. Technol. 39, 6819–6824. https://doi.org/10.1021/es050143r. He, Z., Shentu, J., Yang, X., Baligar, V., Zhang, T., Stoffella, P., 2015. Heavy metal contamination of soils: sources, indicators, and assessment. J. Environ. Indic. 9, 17–18. Holzwarth, J., Knoche, W., Robinson, B.H., 1978. Catalysis of metal complex formation on micelle surfaces. The reaction between divalent metal ions and PADA in the presence of sodium dodecyl sulphate. Berichte der Bunsengesellschaft für Phys. Chemie 82, 1001–1005. https://doi.org/10.1002/bbpc.19780820963. Hu, C., Chan, S.I., Sawyer, E.B., Yu, Y., Wang, J., 2014. Metalloprotein design using genetic code expansion. Chem. Soc. Rev. 43, 6498–6510. https://doi.org/10.1039/ c4cs00018h. Jelusic, M., Lestan, D., 2014. Effect of EDTA washing of metal polluted garden soils. Part I: toxicity hazards and impact on soil properties. Sci. Total Environ. 475, 132–141. https://doi.org/10.1016/j.scitotenv.2013.11.049. Jez, E., Lestan, D., 2016. EDTA retention and emissions from remediated soilo. Chemosphere 151, 202–209. https://doi.org/10.1016/j.chemosphere.2016.02.088. Karczewska, A., Milko, K., 2010. Effects of chelating agents on copper , lead an zinc solubility in polluted soils and tailings produced by copper industry. Ecol. Chem. Eng. 17, 395–403. Kiss, T., Sovago, I., Gergely, A., 1991. Critical survey of stability constants of complexes of glycine. Pure Appl. Chem. 63, 597–638. Kolodynska, D., 2011. Chelating agents of a new generation as an alternative to conventional chelators for heavy metal ions removal from different waste waters. In: Ning, R.Y. (Ed.), Expanding Issues in Desalination. InTech, pp. 339–370. https://doi. org/10.5772/826. Kumar, S., Prasad, S., Kumar, K., Shrivastava, M., Gupta, N., Nagar, S., Bach, Q., Kamyab, H., Khan, S.A., Yadav, S., 2019. Hazardous heavy metals contamination of vegetables and food chain : role of sustainable remediation approaches - a review. Environ. Res. 179, 108792. https://doi.org/10.1016/j.envres.2019.108792. Laudicina, V.A., Palazzolo, E., Badalucco, L., 2013. Natural organic compounds in soil solution: potential role as soil quality indicators. Curr. Org. Chem. 17, 2991–2997. https://doi.org/10.2174/13852728113179990120. Lestan, D., 2015. Remediation of toxic metal-contaminated soil using EDTA soil washing. In: Varma, A., Sherameti, I. (Eds.), Heavy Metal Contamination of Soils. Springer International Publishing Switzerland, pp. 395–429. https://doi.org/10.1007/978-3319-14526-6. Li, C., Zhou, K., Qin, W., Tian, C., Qi, M., 2019. A review on heavy metals contamination in soil: effects, sources, and remediation techniques. Soil Sediment Contam. An Int. J. 28, 380–394. https://doi.org/10.1080/15320383.2019.1592108. Liedekerke, M. Van, Prokop, G., Rabl-Berger, S., Kibblewhite, M., Geertui, L., 2014. Progress in the Management of Contaminated Sites in Europe. https://doi.org/10. 2788/4658. JRC Reference Reports, EUR 26376 EN. Lim, T., Chui, P., Goh, K., 2005. Process evaluation for optimization of EDTA use and recovery for heavy metal removal from a contaminated soil. Chemosphere 58, 1031–1040. https://doi.org/10.1016/j.chemosphere.2004.09.046. Liu, X.Q., Lee, K.S., 2013. Effect of mixed amino acids on crop growth. In: Aflakpui, D.G. (Ed.), Agriculture Science. InTech, pp. 119–158. https://doi.org/10.5772/37461. Lucia, M., Silveira, A., Reynaldo, L., Alleoni, F., Guimarães, L.R., 2003. Bio solids and heavy metals in soils. Sci. Agric. 60, 793–806. Ludmer, Z., Golan, T., Ermolenko, E., Brauner, N., Ullmann, A., 2009. Simultaneous removal of heavy metals and organic pollutants from contaminated sediments and sludges by a novel technology, sediments remediation phase transition extraction. Environ. Eng. Sci. 26, 419–430. https://doi.org/10.1089/ees.2007.0198. Manouchehri, N., Bermond, A., 2006. Study of trace metal partitioning between soil – EDTA extracts and Chelex-100 resin. Anal. Chim. Acta 557, 337–343. https://doi. org/10.1016/j.aca.2005.10.038. Okhrimenko, D.V., Nissenbaum, J., Andersson, M.P., Olsson, M.H.M., Stipp, S.L.S., 2013. Energies of the adsorption of functional groups to calcium carbonate polymorphs: the importance of − OH and − COOH groups. Langmuir 29, 11062–11073. https://doi. org/10.1021/la402305x. Pettit, L.D., 1984. Critical survey of formation constants of complexes of histidine, phenylalanine, tyrosine, L-Dopa and tryptophan. Pure Appl. Chem. 56, 247–292. Pinto, I.S.S., Neto, I.F.F., Soares, H.M.V.M., 2014. Biodegradable chelating agents for industrial, domestic, and agricultural applications—a review. Environ. Sci. Pollut. Res. 21, 11893–11906. https://doi.org/10.1007/s11356-014-2592-6. Powell, K.J., 2000. IUPAC Stability Constants Database. SC-Database. Rahsepar, F.R., Moghimi, N., Leung, K.T., 2016. Surface-mediated hydrogen bonding of proteinogenic α-amino acids on silicon. Acc. Chem. Res. 49, 942–951. https://doi. org/10.1021/acs.accounts.5b00534. Ralph, D.M., Robinson, S.R., Campbell, M.S., Bishop, G.M., 2010. Histidine, cystine, glutamine, and threonine collectively protect astrocytes from the toxicity of zinc. Free Radic. Biol. Med. 49, 649–657. https://doi.org/10.1016/j.freeradbiomed.2010.05.

The reported herein scientific findings (particularly observed structure-functional correlations) help to better understand chelator acting mechanisms and can encourage search and design of new efficient and selective chelators in the broad area of chelator applications. The applicative findings can contribute to further research towards developing new environmentally-friendly soil remediation technologies. Acknowledgments The study was funded by the Israel Science Foundation (ISF, http:// www.isf.org.il/) (grant no. 1355/13), and the Ministry of Science and Technology, Israel (research no. 0605404481). Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.envres.2020.109140. References Barona, A., Aranguiz, I., Elías, A., 2001. Metal associations in soils before and after EDTA extractive decontamination: implications for the effectiveness of further clean-up procedures. Environ. Pollut. 113, 79–85. https://doi.org/10.1016/S0269-7491(00) 00158-5. Begum, Z.A., Rahman, I.M.M., Sawai, H., Mizutani, S., Maki, T., Hasegawa, H., 2013. Effect of extraction variables on the biodegradable chelant-assisted removal of toxic metals from artificially contaminated european reference soils. Water Air Soil Pollut. 224. https://doi.org/10.1007/s11270-012-1381-4. Berthon, G., 1995. The stability constants of metal complexes of amino acids with polar side chains. Pure Appl. Chem. 67, 1117–1240. https://doi.org/10.1351/ pac199567071117. Bolan, N., Kunhikrishnan, A., Thangarajan, R., Kumpiene, J., Park, J., Makino, T., Kirkham, M.B., Scheckel, K., 2014. Remediation of heavy metal(loid)s contaminated soils - to mobilize or to immobilize? J. Hazard Mater. 266, 141–166. https://doi.org/ 10.1016/j.jhazmat.2013.12.018. Chauhan, G., Pant, K.K., Nigam, K.D.P., 2015a. Conceptual mechanism and kinetic studies of chelating agent assisted metal extraction process from spent catalyst. J. Ind. Eng. Chem. 27, 373–383. https://doi.org/10.1016/j.jiec.2015.01.017. Chauhan, G., Pant, K.K., Nigam, K.D.P., 2015b. Chelation technology: a promising green approach for resource management and waste minimization. Environ. Sci. Process. Impacts 17, 12–40. https://doi.org/10.1039/C4EM00559G. Chen, S., Sun, L.N., Chao, L., Zhou, Q.X., Sun, T.H., 2007. Influence of organic acid and amino acid on cadmium and lead desorption from soil. Aust. J. Soil Res. 45, 554–558. https://doi.org/10.1071/SR07029. Dermont, G., Bergeron, M., Mercier, G., Richer-Laflèche, M., 2008. Soil washing for metal removal: a review of physical/chemical technologies and field applications. J. Hazard Mater. 152, 1–31. https://doi.org/10.1016/j.jhazmat.2007.10.043. Dolev, N., Katz, Z., Ludmer, Z., Ullmann, A., Brauner, N., Goikhman, R., 2019. New insights into the chelator recycling by a chelating Resin : from molecular mechanisms to applicability. Chemosphere 215, 800–806. Fabbricino, M., Ferraro, A., Del Giudice, G., D'Antonio, L., 2013. Current views on EDDS use for ex situ washing of potentially toxic metal contaminated soils. Rev. Environ. Sci. Biotechnol. 12, 391–398. https://doi.org/10.1007/s11157-013-9309-z. Farkas, E., Sovago, I., 2012. Metal complexes of amino acids and peptides. Amino Acids Pept. Protein 37, 66–118. https://doi.org/10.1039/9781849734677-00066. Fauchère, J.-L., Charton, M., Kier, L.B., Verloop, A., Pliska, V., 1988. Amino acid side chain parameters for correlation studies in biology and pharmacology. Int. J. Pept. Protein Res. 32, 269–278. https://doi.org/10.1111/j.1399-3011.1988.tb01261.x. Ferraro, A., Fabbricino, M., van Hullebusch, E.D., Esposito, G., Pirozzi, F., 2016. Effect of soil/contamination characteristics and process operational conditions on aminopolycarboxylates enhanced soil washing for heavy metals removal: a review. Rev.

12

Environmental Research 183 (2020) 109140

N. Dolev, et al.

Vareda, J.P., Valente, A.J.M., Durães, L., 2019. Assessment of heavy metal pollution from anthropogenic activities and remediation strategies : a review. J. Environ. Manag. 246, 101–118. https://doi.org/10.1016/j.jenvman.2019.05.126. Wang, G., Zhang, S., Zhong, Q., Xu, X., Li, T., Jia, Y., Zhang, Y., Peijnenburg, W.J.G.M., Vijver, M.G., 2018. Effect of soil washing with biodegradable chelators on the toxicity of residual metals and soil biological properties. Sci. Total Environ. 625, 1021–1029. https://doi.org/10.1016/j.scitotenv.2018.01.019. Warren, C.R., 2014. Organic N molecules in the soil solution: what is known, what is unknown and the path forwards. Plant Soil 375, 1–19. https://doi.org/10.1007/ s11104-013-1939-y. Wu, G., 2013. Discovery and chemistry of amino acids. In: Amino Acids Biochemistry and Nutrition. CRC press, pp. 1–32. Wuana, R.A., Okieimen, F.E., 2011. Heavy metals in contaminated soils: a review of sources, chemistry, risks and best available strategies for remediation. ISRN Ecol 2011, 1–20. https://doi.org/10.5402/2011/402647. Yamauchi, O., Odani, A., 1996. Stability constants of metal complexes of amino acids with charged side chains- part I : positively charged side chains. Pure apply Chem 68, 469–496. Yamauchi, O., Odani, A., Takani, M., 2002. Metal–amino acid chemistry. Weak interactions and related functions of side chain groups. Dalton Trans. 3411–3421. https:// doi.org/10.1039/b202385g. Ye, S.J., Clark, A.A., Armentrout, P.B., 2008. Experimental and theoretical investigation of alkali metal cation interactions with hydroxyl side-chain amino acids. J. Phys. Chem. B 112, 10291–10302. Yeasmin, S., Singh, B., Kookana, R.S., Farrell, M., Sparks, D.L., Johnston, C.T., 2014. Influence of mineral characteristics on the retention of low molecular weight organic compounds: a batch sorption – desorption and ATR-FTIR study. J. Colloid Interface Sci. 432, 246–257. Zhang, W., Tsang, D.C.W., 2013. Conceptual framework and mathematical model for the transport of metal – chelant complexes during in situ soil remediation. Chemosphere 91, 1281–1288. https://doi.org/10.1016/j.chemosphere.2013.02.034.

023. Shimazaki, Y., Takani, M., Yamauchi, O., 2009. Metal complexes of amino acids and amino acid side chain groups. Structures and properties. Dalton Trans. 7854–7869. https://doi.org/10.1039/b905871k. Slukovskaya, M.V., Kremenetskaya, I.P., Drogobuzhskaya, S.V., Ivanova, L.A., Mosendz, I.A., Novikov, A.I., 2018. Serpentine mining wastes—materials for soil rehabilitation in Cu-Ni polluted wastelands. Soil Sci. 183, 141–149. https://doi.org/10.1097/ss. 0000000000000236. Sovago, I., Kiss, T., Gergely, A., 1993. The stability constants of complexes of aliphatic amino acids. Pure Appl. Chem. 65, 1029–1080. Tandy, S., Bossart, K., Mueller, R., Ritschel, J., Hauser, L., Schulin, R., Nowack, B., 2004. Extraction of heavy metals from soils using biodegradable chelating agents. Environ. Sci. Technol. 38, 937–944. Tang, N., Skibsted, L.H., 2016. Calcium binding to amino acids and small Glycine peptides in aqueous solution: toward peptide design for better calcium bioavailability. J. Agric. Food Chem. 64, 4376–4389. https://doi.org/10.1021/acs.jafc.6b01534. Tsang, D.C.W., Zhang, W., Lo, I.M.C., 2007. Copper extraction effectiveness and soil dissolution issues of EDTA-flushing of artificially contaminated soils. Chemosphere 68, 234–243. https://doi.org/10.1016/j.chemosphere.2007.01.022. Ullmann, A., Brauner, N., Vazana, S., Katz, Z., Goikhman, R., Seemann, B., Marom, H., Gozin, M., 2013. New biodegradable organic-soluble chelating agents for simultaneous removal of heavy metals and organic pollutants from contaminated media. J. Hazard Mater. 260, 676–688. https://doi.org/10.1016/j.jhazmat.2013.06.027. Vadas, T.M., Ahner, B.A., 2009. Extraction of lead and cadmium from soils by cysteine and glutathione. J. Environ. Qual. 38, 2245–2252. https://doi.org/10.2134/jeq2008. 0524. Van der Voet, E., Salminen, R., Eckelman, M., Mudd, G., Norgate, T., Hischier, R., 2013. Environmental Risks and Challenges of Anthropogenic Metals Flows and Cycles. UNEPhttps://doi.org/10.1227/01.NEU.0000108643.94730.21. Vandenbossche, M., Jimenez, M., Casetta, M., Traisnel, M., 2015. Remediation of heavy metals by biomolecules: a review. Crit. Rev. Environ. Sci. Technol. 45, 1644–1704. https://doi.org/10.1080/10643389.2014.966425.

13