Geochemical fractions and modeling adsorption of heavy metals into contaminated river sediments in Japan and Thailand determined by sequential leaching technique using ICP-MS

2.5.1 The adsorption capacities

The adsorption capacities were modified and calculated by the following formula (Saeedi, 2004): $$A_{\text{c}}=(C_{\text{i}}-C_{\text{r}}).\text{v}/\text{m}.1000$$ where C_i and C_r are the initial or spiked and final or residual metal concentrations, respectively (μg/l), m is the amount of sediment (g), v is the volume of the solution (mL), and A_c is the adsorption capacity (μg/g).

2.5.2 The Langmuir isotherm adsorption equation

The Langmuir adsorption isotherm equation is modified and given: $$C_{\text{r}}/A_{\text{c}}=1/\mathit{kb}+C_{\text{r}}/b$$ where

• C_r = residual solution concentration (ppb)

• A_c = adsorption capacity of sediment (μg/g)

• b = adsorption maximum

• k = a constant relating to the binding energy of the sediment.

A plot of C_r/A_c against C_r should yield a straight line of slope 1/b and intercept 1/kb on the C_r/A_c axis.

2.5.3 The Freundlich isotherm equation

When the coefficient correlation (r²) in linear regressions is very low when using the Langmuir isotherm equation, we applied and modified the Freundlich equation which was put forward for describing the adsorption of ions or molecules to sediment. The equation is given as follows: $$A_{\text{c}}={\mathit{kC}}_{\text{r}}^{1/b}$$ where C_r = the equilibrium concentration of the adsorbate after adsorption has occurred or final/residual solution of concentration (μg/l), A_c = the adsorption capacity of the sediment (μg/g) and k, b = the constants for the system between adsorbate and adsorbent. In order to establish when data have a linear confirmation to the Freundlich isotherm equation, the logarithmic for of that equation is applied as follows: $$\text{log}{A}_{\text{c}}=1/b\text{log}{C}_{\text{r}}+\text{log}k$$

A plot of log A_c versus log C_r should give a straight line of slope 1/b and an intercept of log k.

3.1.1 Mobile and exchangeable fractions

Mobile and exchangeable metal ions are ions that are released most readily into the environment (Filgueras et al., 2002). A solution of NH₄NO₃ was the preferred reagent for leaching these ions as the relatively high concentration of the released metal ions indicated that these were water-soluble metals, exchangeable metals, and easily soluble organic complexes (Ratuzny et al., 2009). We modified the technique by using NH₄NO₃ 1 M at pH 7, rather than chloride and acetate salts as nitrate anions prevent metal complexion or coagulation so the exchange mechanism reaction can be performed. The new modified leaching technique using NH₄NO₃ 1 M pH 7 can leach the metals Ca, Pb, Cd, Zn, As, Cu, Fe, Ca, and Mn with the chemical reaction, as follows (Example Ca): $$\text{Sediment}-{\text{Ca}}_{(\text{s})}^{2+}+{\text{NH}}_4{\text{NO}}_{3(\text{aq})}\to \text{Sediment}-{\text{NH}}_{4(\text{s})}^{+}+\text{Ca}{({\text{NO}}_{\text{3}})}_{2(\text{aq})}$$

The metal ions released by ion-exchange processes indicated that the weakly adsorbed metals were retained on the sediment surface by relatively weak electrostatic interactions. Filgueras et al. (2002) reported that changes in the ionic composition caused remobilization of metals from this fraction.

In Sumida River sediments, the level of Pb (0.11 ppm), Zn (17.5 ppm), As (2.96 ppm), Ca (22,281 ppm), and Fe (36.1 ppm) at Asakusa showed a higher mobile fraction when compared with other sites. The Cd (0.18 ppm), Zn (21.8 ppm), Ca (8169 ppm), Fe (28.6 ppm), and Mn (432 ppm) are also recorded to be higher at the site 40 km from the river mouth in the mobile fraction of the Chao Phraya River sediments (Table 1). The higher level of concentrations of Ca, Fe, and Mn may be contributed by the nature of the natural material (e.g., limestone, hematite, and pyrolusite) surrounding the river. Ca, Fe, and Mn are formed as CaCO₃, Fe₂O₃, and MnO₂, respectively. The concentrations of Zn, As, Cd, and Pb indicated that there were high mobility metals in Asakusa and the 40 km site on the Chao Phraya which are readily released to organisms and human beings or they react with other cations in the river by an exchangeable reaction when the conditions are at pH 7.

Pb, Cd, Zn, As, Cu, Fe, Ca, and Mn corresponding to the mobile/exchangeable fraction represent a portion of the total metal content of the sediments in the Sumida (0.15–39.9%) and Chao Phraya Rivers (0.19–35.0%) (Table 2, Fig. 2). Pb, Zn and Fe respectively varied from 0.03 to 011 ppm, 1.97 to 17.5 ppm, and 16.0 to 36.1 ppm in the Sumida River sediment whereas Pb, Zn, and As respectively ranged from 0.01 to 0.27 ppm, 2.09 to 21.4 ppm and 0.01 to 007 ppm in Chao Phraya River sediment. These contribute the least to the portion from their concentrations, indicating that their introduction is the result of a small anthropogenic source. The ranged concentrations of Cu (2.04–12.5 ppm), Cd (0.03–0.36 ppm), As (0.05–2.96 ppm), and Mn (5.63–31.6) in the Sumida River and Fe (14.2–28.6 ppm), Cd (0.02–0.18 ppm), Cu (1.03–11.7 ppm), and Mn (5.63–73.4 ppm) in the Chao Phraya River were low to moderate portions from their total concentrations. The mobile and exchangeable sediment fractions of Cd and As in the Sumida River and Cd and Cu in the Chao Phraya River should be considered for use in monitoring pollution due to the high portion (12–17%) from total concentrations. The higher proportion of Ca concentrations in both of the rivers’ sediments indicated a high mobility of Ca in sediment.

3.1.2 Soluble fraction

Ratuzny et al. (2009) suggested that a solution of CH₃COONH₄ can be used to release metal ions due to the metal complexing capability of acetates, which prevents re-adsorption or precipitation. We modified the technique using CH₃COONH₄ 1 M and adjusted the pH to 4 from the ion exchangeable stage or as specially adsorbed at the solid surfaces. In addition, Filgueras et al. (2002) investigated that ammonium leaches metals bound to carbonates in the surface sediment.

In the soluble fraction of the Sumida River sediment, the high levels of Cd (0.44 ppm), As (0.43 ppm), and Ca (76,519 ppm) are listed at the Asakusa site, whereas the high level of Fe (253 ppm), Zn and Mn (154 and 53.2 ppm), Pb (0.41 ppm) and Cu (14.2 ppm) was recorded at the Continental Hotel, West Side of Kashidoki Bridge, East Side of Kashidoki Bridge, and Hamcho sites, respectively. In the Chao Phraya River sediments showed high levels of Zn (136 ppm) and Cu (14.5 ppm) at 40 km from the river mouth. High concentrations of As (2.06 ppm), Ca (826 ppm), and Mn (337 ppm) were recorded at the site 50 (I) km whereas high levels of Cd (32.6 ppm) and Pb (1.56 ppm) were recorded at 50 (II) and 50 (III) km from river mouth, respectively. The high level of metal contents in the soluble fraction of the sediment in the different sites on the Sumida and Chao Phraya Rivers indicates metals in the carbonate form can be released as metal ions when the condition in the rivers are acidic (pH < 7) and then these ions can affect organisms and human beings.

We also report that NH₄CO₃ 1 M pH 4 leached 38.6% Cd, 56.7% Zn, 55.2% Ca, and 26.5% Mn in the Sumida River sediments and 24.7% Zn and 41.3% Mn in the Chao Phraya River sediments from the total metal concentrations (Table 2, Fig. 2a and b). However, the leached concentrations of Pb, As, and Fe in this fraction were comparatively low in both rivers’ sediments. The high portion ranges of Cd (0.17–0.44 ppm) and Ca (503–76,519 ppm) in the Sumida River sediment and Mn (58.5–237 ppm) in the Chao Phraya River sediment indicated the level of those metals bound in carbonate sediments. The difference of metal binding to carbonates may be dependent upon the concentration of the reacting species (metal ion, metal complex, and solid surface sites), pH, ionic strength, and concentration of competing ions. The largest proportion of total Cd in the Sumida River was associated with the soluble fraction, indicating a high solubility of Cd in the carbonate (Fig. 2a). Filgueras et al. (2002) reported that carbonaceous sediments commonly and easily act as a Cd sink. The average Cd in the soluble fraction was 38.6 ppm. The high solubility of Cd is mainly affected by the pH of the sediments and soils (Ratuzny et al., 2009). The reaction of Cd²⁺ in sediment may be written as follows: $$\text{Sediment}-{\text{CdCO}}_{3(\text{s})}+{\text{CH}}_3{\text{COONH}}_{4(\text{aq})}\to {({\text{NH}}_4)}_2{\text{CO}}_{3(\text{aq})}+\text{Cd}{({\text{CH}}_3\text{COO})}_{2(\mathit{aq})}$$

3.1.3 Reducible fraction

Hydroxylamine hydrochloride is widely preferred for leaching the easily reducible fraction (Filgueiras et al., 2002; Postma et al., 2007; Ratuzny et al., 2009). In general, the reducible fraction is divided into three fractions: the easily reducible fraction (Mn Oxides), the moderately reducible fraction (amorphous Fe oxides (Fe₂O₃)), and the poorly-reducible fraction (crystalline Fe oxides (Fe(OH)₃)). We modified the leaching methods using NH₂OH-HCl solute solution with pH 2.

The high levels of metal contents such as Pb (24.5 ppm), Zn (661 ppm), and Ca (3460 ppm) were recorded at the Asakusa site. As (2.37 ppm) and Fe (7212 ppm) were found to be high at the Continental Hotel site. The higher concentrations of Mn (33.7 ppm), Cd (0.28 ppm) and Cu (20.8 ppm) were recorded at the West side of the Kashidoki Bridge, at the East side of the Kashidoki Bridge, and at Hamcho, respectively. In the Chao Phraya River sediment, the metal contents of Cd (0.25 ppm), Zn (307 ppm) and Cu (28.9 ppm) at the site 40 km from the river mouth were recorded as higher than at other sites (Table 1). As (1.06 ppm) and Mn (162 ppm) were high at 50 (I) km, while Pb (49.2 ppm), Ca (8858 ppm), and Fe (8858 ppm) were found to be higher at site 50 (III) km, when compared to other sites. The variation of the metal contents in the Sumida and Chao Phraya River sites in these fractions was affected by local geology and natural sources when combined with anthropogenic inputs along the river. This was evident when considering the high concentrations of Mn and Fe at each site.

The proportion of metal contents in sediments which could be leached with NH₂OH-HCl at pH 2 is as follows: Zn (56.7%) > Cd (32.9%) > Pb (30.2%) > As (18.6%) > Mn (18.2%) > Cu (12.6%) > Ca (4.02%) > Fe (3.24%) in the Sumida River and Pb (52.1%) > Cd (52.1%) > Zn (45.0%) > Fe (32.5%) > Mn (27.3%) > Ca (16.1%) > Cu (4.81%) > As (2.97%) in the Chao Phraya River (Fig. 2a and b). Pb and Zn were most efficiently leached through the method using NH₂OH-HCl pH 2 when compared with other fractions in the Sumida and Chao Phraya Rivers’ sediments (Table 2). The metal contents in both river sediments may be released from reduction of Fe(III) and Mn(IV) under anoxic conditions by any or a combination of the following mechanisms as described by Li and Thonston (2001): co-precipitation, adsorption, surface complex formation, ion exchange, and penetration of the lattice. Tokalioglu et al. (2000) and Li and Thonston (2001) also confirmed that metal contents in Fe(III) and Mn(IV) oxides in sediment were thermodynamically unstable under anoxic circumstances and were affected by benthic organisms.

The reactions of metals in the reducible fraction in the Chao Phraya River sediment may be described as one of three possibilities, as follows (for example Pb):

• Sediment-MnO₂-Pb_(s) + 2 NH₂OH.HCl_(aq) → Mn²⁺_(aq) + Cl_2(g) + Pb²⁺_(aq) + H₂O_(l) + 2NH_3(g)

• Sediment-Fe₂O₃-Pb_(s) + 2NH₂OH.HCl_(aq) → 2Fe²⁺_(aq) + Cl_2(g) + Pb²⁺_(aq) + 2H₂O_(l) + 2NH_3(g)

• Sediment-Fe(OH)₃-crystal-Pb_(s) + 2NH₂OH.HCl_(aq) → 2Fe²⁺_(aq) + Cl_2(g) + Pb²⁺_(aq) + H₂O_(l) + 2NH_3(g)

3.1.4 Oxidizable fraction

Metals associated with oxidizable fractions are assumed to remain in sediments for longer periods, and are possibly mobilized by decomposition processes (Filgueiras et al., 2002). Ratuzny et al. (2009) reported that oxdizable metals may be associated with a stable and high molecular weight humic substance that releases metals in a slow manner. Degradation of organic material such as living organisms, detritus, or coatings on mineral particles under oxidizing conditions can contribute to metals bound to the component. The amount of metals bound to organic matter and sulfides might be extracted during leaching by H₂O₂ in an acid medium. Postma et al. (2007) confirmed that H₂O₂ could be used for the oxidation method and successfully attacked organic matter, resulting in minimum alteration of the silicates. The organic matter in sediment has been subdivided into four categories: anthropogenic, humic acids, fulvic acids, and yellow organic acids (Ratuzny et al., 2009). The carboxylate and phenol groups in the humic acids easily form complexes with metal ions. We modified the procedure using the common oxidant H₂O₂ 35% by adding a heated process and HNO₃ 15 M for decomposing the oxidizable fractions in sediment.

Metal contents in the oxidizable fraction were recorded at high levels at the West side of Kashidoki Bridge site on the Sumida River as follows: Pb (13.3 ppm), Zn (28.9 ppm), As (1.53 ppm), Cu (72.0 ppm), Ca (242 ppm), Fe (2776 ppm), and Mn (39.5 ppm). At site 40 km, on the Chao Phraya River metal contents such as Zn (43.8 ppm), As (21.4 ppm), Cu (593 ppm), and Mn (21.4 ppm) were much higher than at other sites (Table 1). The high metal contents recorded at that site indicated that those metals were trapped in humic acids in the sediment. Metals found in this fraction will be released into the river when they react with domestic or industrial waste containing peroxide.

The partitioning of metal contents into H₂O₂ solution in the oxidizable factions is also shown in Table 2 and Fig. 2. The average values including the range of metal contents leached by H₂O₂ are listed in Table 2. The level of total concentrations sequences were as follows: Cu > Pb > As > Mn > Fe > Cd > Zn > Ca in the Sumida River and Cu > As > Pb > Cd > Zn > Fe > Ca > Mn in the Chao Phraya River’s sediments. Compared to other fractions, the ranged concentrations of Cd (0.01–0.14 ppm) and Mn (5.49–39.5 ppm) in the Sumida River and Cd (0.01–0.09 ppm), Ca (5.00–1679 ppm), and Mn (1.36–21.4 ppm) in the Chao Phraya River were low (Table 2). The levels of medium to high metal contents were Pb (19.4%), Cd (5.71%), Zn (4.22%), Ca (0.53%), Fe (10.2 %), As (15.3%) and Cu (48.3%) when compared to their fractions in the Sumida River sediment (Fig. 2a). Pb (11.6%), Zn (5.92 %), Fe (4.56%), As (2.53%), and Cu (61.2 %) also showed medium to high levels in the Chao Phraya River sediment (Fig. 2b). Cu was most successfully leached H₂O₂ 35% in both rivers’ sediments. The high Cu content in this fraction may be the result of a high affinity for complexing with organic matter. This was consistent with the findings of other studies (Yu et al., 2001; Ratuzny et al., 2009). In addition, Yu et al. (2001) confirmed that there was a positive correlation between the increasing concentrations of Cu with the increasing organic matter content from humic acid in the river sediment. The reaction of Cu in humic acid (HA) with H₂O₂ in sediment may be written as follows: $$\text{Sediment}-\text{HA}-{\text{Cu}}_{(\text{s})}+{\text{H}}_2{\text{O}}_{2(\text{l})}+\text{2}{\text{H}}_{(\text{aq})}^{+}\to \text{Sediment}-{\text{HA}}_{(\text{s})}+{\text{Cu}}_{(\text{aq})}^{2+}+2{\text{H}}_2{\text{O}}_{(\text{l})}$$

3.1.5 Residual fraction

According to Filgueras et al. (2002), concentrated nitric acid can be used to dissolve metals released in the residual material fraction, such as in refractory organic matter and undissolved amorphous oxides. We combined the concentrated HNO₃ and HF to strongly dissolve metals which were bound to clays and other silicate minerals. Using HF solution in sediment produces silicon tetrafluoride gas and a solution/slurry containing aluminum fluoride and undigested oxides.

On the Sumida River, the highest concentrations of the heavy metals Pb (31.0 ppm), Cd (0.08 ppm), Zn (23.4 ppm), and Fe (11,634 ppm) were observed at the West side of Kashidoki Bridge; high concentrations of Cu (28.5 ppm), Ca (151 ppm), and Mn (71.3 ppm) were recorded at Hamcho and As (4.73 ppm) was also recorded at Continental Hotel (Table 1). In the Chao Phraya River, the concentrations of Pb (51.0 ppm), Cd (0.65 ppm), Zn (152 ppm), As (40 ppm), Cu (111 ppm), and Mn (40.0 ppm) were highest at the site 40 km from the river mouth. Metal contents in this fraction can be released into river when they react with fluoride compounds which originate from domestic sewage or industrial chemical waste.

The proportion of metal contents in the HNO₃ + HF leached fraction is shown in Fig. 2. The highest portions of Pb (49.6%), As (47.1%), Fe (55.9%), and Mn (29.2%) in the Sumida River and Pb (62.6%), Cd (57.1%), As (71.7%), Ca (42.2%) and Fe (62.7%) in the Chao Phraya River’s sediments are released from the residual fraction, suggesting that these metals are bound to SiO₂ and Al₂O₃ (Fig. 2a and b). The highest concentrations of Pb, Fe, and As in both rivers’ sediments may also indicate incomplete dissolution of their oxides by the reducing reagent. However, we assume that Pb, Cd, As, and Fe within the residual fraction mainly originated from the parent materials of the sediment. In addition, Filgueiras et al. (2002) reported that the average metal contents of the HF leached fraction are closely identical to their average contents in granitic rocks.

The reaction of metals in this fraction may be written as follows (Example Pb):

• Sediment-SiO₂-Al₂O₃-Pb_(s) + 8 HNO_3(aq) → Pb(NO₃)_2(aq) + 2 Al(NO₃)_3(aq) + Sediment-SiO₂-Pb_(s) + 8H⁺_(aq)

• Sediment-SiO₂-Pb + 4 HF → Pb²⁺_(aq) + SiF_4(g) + 2H₂O_(l)

3.1.6 Total level concentrations

The total level concentrations are the summations of the concentrations of metal contents in the mobile/exchangeable, soluble, reducible, oxidizable, and residual fractions. The concentrations of Cd and Zn of the sampled sediment in the Sumida River exceeded the Japanese background sediment values (Cd = 0.14 ppm; Zn = 109 ppm) observed from Wijaya et al. (2012). The contents of Pb, As, Fe and Mn in the Sumida River were low compared with the same metals in the Chao Phraya River (Table 2). The contents of Pb (62.6 ppm), Cd (0.59 ppm), Zn (220 ppm), and Cu (78.4 ppm) in the Chao Phraya River were high when compared with the same metals in the background Chao Phraya River sediment at the site 81 km from the river mouth (Pb = 18.0 ppm; Cd = 0.14 ppm; Zn = 19.7 ppm; Cu = 9.72 ppm) as reported by Wijaya et al. 2013. We suggest that the high level of pollution of the Chao Phraya River is anthropogenic and is associated with runoff from Bangkok. Carman et al. (2007) reported that the total Pb contamination was relatively significant compared to other metals in sediment to assess the environmental pollution in surrounding areas. Table 3 shows the coefficients of correlation between Pb, Cd, Zn, As, and Cu with Fe, Ca, and Mn concentrations in river sediments. The high coefficient of correlation for Fe is only listed for Pb and As concentrations. It may be suggested that Pb and As were found to be the most extracted from the Fe-oxides in sediment. Most As and Fe were in the residual fractioned sediment, suggesting natural inputs from the source of their parent materials (Fig. 2).

The comparison of the total level of metal contents in the Sumida, Chao Phraya, and other river sediments from other countries is presented in Table 4. The different levels of metal contents may be affected by both geogenic and anthropogenic factors. The averages of heavy metal contents in the Ganges and Nemunas River sediments were based on the analysis of grained river sediments (<63 μm). For this study, we used the solid fraction of <50 μm river sediment in our sample. This solid fraction is recognized as the most chemically active sediment phase (Postma et al., 2007). As listed in Table 4, the total fractions of Pb (62.6 ppm), Zn (240 ppm), As (27.2 ppm), Fe (16,636 ppm) and Mn (419 ppm) in the <50 μm grain sediments from the Chao Phraya River showed higher values when compared with those of the fine and bulk sediments in the Sumida, Ganges, Nemunas, and Han Rivers, indicating more anthropogenic activities in the Bangkok. In addition, the levels of Pb (79.8 ppm), Cd (0.80 ppm), Zn (502 ppm), and Cu (184 ppm) of the <20 μm grained sediment of the Buriganga River were higher compared to other river sediments (Table 4). This may be due to more human activities in Bangladesh’s cities and the size of the fraction analyzed. However the total levels of Pb, Cd, Zn, and Cu in the Sumida and Chao Phraya Rivers’ sediments exceeded those in the average for global sediments (Pb: 20.0 ppm, Cd: 0.30 ppm, Zn: 2–1000 ppm, Cu: 0–192 ppm) as well as the unpolluted sediments from the Canadian sites using the standard analytical procedures for the assessment of contamination of the sediments (Pb: 22.0 ppm, Cd: 0.30 ppm, Zn: 97.0 ppm, Cu: 34 ppm) as reported by Wijaya et al. (2012, 2013). Compared with the World Health Organization’s (WHO) recommended guidelines for Pb (31 ppm), Zn (123 ppm), and Cu (16 ppm) in sediment (Ozturk et al., 2009; Li, 2014), the levels of heavy metals in the Sumida and Chao Phraya Rivers sediments were reported as being higher. These data can be used as a guide for the governments to focus more on possible sources of heavy metal contamination surrounding the rivers.

Total level concentrations in sediment samples were leached using the sequential leaching technique. This method could explain the concentration levels in each sediment fraction clearly. The previous surveys showed that the sampling sites were affected by anthropogenic inputs using partial leaching (Wijaya et al. 2012; Wijaya et al. 2013) (Fig. 3). We compared the sequential leaching technique with their studies. Two kinds of chemical leaching solutions were compared in the Sumida and Chao Phraya River sediments: (1) The chemical sequential leaching of this study (Fig. 3 and Table 2) and (2) Partial leaching (Wijaya et al., 2012) (Fig. 3). We compared those methods in the collected sediment with the same type, date, and location. This sequential leaching technique showed that the partial leaching performed by Wijaya et al. (2012) could not clearly explain which part of the sediment fraction was most contaminated by heavy metals. The partial leaching as reported by Wijaya (2012) was preferred and chosen just for the assessment and evaluation of heavy metal contents for monitoring pollution in river sediment. Fig. 3a and b shows the comparisons of the methods to leach heavy metals in sediment. The subsequent leaching technique clearly shows the heavy metals in each fraction through their partition level concentrations. Fig. 3 shows that the partial leaching (Wijaya et al. 2012) extracted the heavy metals at only 70–90% from the total concentrations when compared to the sequential leaching technique. The partial leaching can totally dissolve heavy metals in the mobile and soluble fractions and part of the reducible and oxidizable fractions. It cannot dissolve the metals from the residual fraction. The partial leaching could dissolve the metals that came from anthropogenic input, and the sequential leaching technique could distinguish the portion of metals associated with anthropogenic and natural sources. The largest partitioning of Pb, Cd, Zn, and Cu in river sediment was associated with the oxidizable and reducible fractions (>50%) (Fig. 3). The metals in the mobile/exchangeable, soluble, reducible, and oxidizable partitions seemed to result mainly from anthropogenic inputs whereas the residual fractions were mainly affected by natural sources.

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Figure 3

The comparison of Pb, Zn, Cu, (a) and Cd (b) concentrations in the five fractions obtained by the sequential leaching technique and partial leaching (PL) (Wijaya et al. (2012, 2013)) of the Sumida and Chao Phraya River sediments. SR and CP indicate Sumida River and Chao Phraya River, respectively.

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3.2.1 Langmuir and Freundlich isotherms in sediment

In order to provide insight into the adsorption of heavy metals by sediment, we described it in terms of the Langmuir and Freundlich adsorption equations. In this case, we focused on the data of the maximum heavy metal adsorption capacities at the 5 h time contact. The calculations of the Langmuir and Freundlich equations are listed in Table 6. The results of the plots of C_r/A_c against C_r and log A_c against log C_r, including b and k, are calculated and listed in Table 7.

The five sets of C_r/A_c and C_r data corresponding to the spiked metal ion concentrations of 0.25, 0.50, 0.75, 1.00, and 10.0 ppb of five metal ions were fitted to the linear regression of Langmuir’s equation in each case. The results from the calculations of A_c in the Langmuir equation are almost uniform, with a range of 0.07–4.41 ug/g, while C_r/A_c varied depending on the residual metal concentrations (Table 6). The b and k from the treated data of metal adsorptions are calculated from the slope and intercept of each curve with its equation (Table 7). Among the five metals, only Cd fits with the Langmuir equation. The correlation coefficients (r² = 0.94), b (0.467) and k (7137) indicate that the adsorption data of Cd show significant conformity to the Langmuir’s equation (Table 7). Relatively high values of b indicate that the contaminated sediment studied has a substantial maximum adsorption capacity of Cd ions. The values of k are also high, suggesting strong binding between sediments and contaminations of Cd.

Because the correlation coefficients of Cr, Cu, Zn, and Pb using the Langmuir equation were very low, we continued applying the relationship between log A_c and log C_r and then found r² and calculated b and k. The treated data of the adsorption of heavy metals using the Freundlich equation are listed in Table 7. The results showed a positive correlation coefficient (r²_Cr = 0.46, r²_Cu = 0.61, r²_Zn = 0.71, and r²_Pb = 0.64) to be higher when compared with the Langmuir equation, suggesting that metals adsorbed by sediment followed the significant conformity to the Freundlich equation instead.