Competitive adsorption of Cu²⁺, Pb²⁺, Cd²⁺, and Zn²⁺ onto water treatment residuals: implications for mobility in stormwater bioretention systems

Water treatment residuals (WTRs) are solid wastes produced from drinking water treatment processes. The worldwide amount of WTRs is more and more substantial due to urban population increase and living quality improvement. As a result, the appropriate disposal of WTRs becomes a primary concern in the water treatment industry. In the past decades, the main disposal method of WTRs was landfilling in most countries. Recently, the sustainability of WTRs landfilling is questioned because this disposal method possibly has environmental risks to nearby ecosystems (Ahmad et al. 2016; Zhao et al. 2018). Therefore, environmental professionals are exploring sustainable, innovative alternative uses for WTRs disposal in the context of developing a more global circular economy (Ahmad et al. 2016; Zhao et al. 2018).

Globally, the main, newly developed, disposal method of WTRs is reuse in land application as the soil amendment to adjust soil pH, to immobilize phosphorous in phosphorus laden soils, and to improve soil structure (Zhao et al. 2018). A few countries have gradually lifted the reuse ratio of WTRs including the Netherlands, Denmark, and Japan with a reuse ratio higher than 98, 75, and 55%, respectively (Zhao et al. 2018). The other sustainable reuse options were summarized by Ahmad et al. (2016) including reuse in wastewater treatment, as the substrate in constructed wetland systems, and in construction and building materials. Previous studies have shown that WTRs have high adsorption capacity for phosphorus (Xu et al. 2020).

Finding the strong affinity of WTRs toward phosphorus in various aqueous solutions enhances the interest of researchers worldwide in applying WTRs to the filling media layer of stormwater bioretention systems with the goal of improving phosphorus removal (Xu et al. 2020). Stormwater bioretention systems cover an area that is used as an engineering strategy in urban water management to treat stormwater runoff, to maximize infiltration, and to mitigate peak flows during rain events. They typically contain vegetation, inlet structure, soil-based media layer, and the underdrain structure at the bottom. The main part is the media layer that removes contaminants by adsorption, precipitation, filtration and plant uptake.

In addition to phosphorus pollution, the presence of heavy metals in stormwater runoff has become another important concern to control non-point source pollution around the world (Genç-Fuhrman et al. 2016). Heavy metals in stormwater runoff can enter the ecological water systems via typical surface water pathways and threaten the health of animals and humans due to their toxicity (Reddy et al. 2014). The sources of heavy metal pollution in stormwater runoff include heavy metal polluted sites caused by industrial pollution, atmospheric deposition, vehicular traffic, domestic fertilizer use, and the use of various metal materials in building and roofing that stormwater runoff flows across (Deng et al. 2016). The dominating heavy metals in stormwater runoff include copper (Cu), lead (Pb), cadmium (Cd), and zinc (Zn) (Deng et al. 2016; Xu et al. 2020). The concentration of heavy metals in road stormwater runoff often fluctuates within a large range (Cu: 22–7,033 μg/L; Pb: 73–1,780 μg/L; Zn: 56–929 μg/L) (Deng et al. 2016). Xu et al. (2020) summarized the removal efficiencies of Cu, Pb, Cd and Zn in stormwater runoff by various media used in stormwater bioretention systems. Overall, WTRs and commercial media have higher removal efficiencies for some heavy metals compared to traditional soil-based media and recycled media, and the removal efficiencies of both types of media are approximately equivalent for each heavy metal tested (Xu et al. 2020). However, WTRs have noteworthy advantages over commercial media due to their low costs. Therefore, the beneficial reuse of WTRs for stormwater bioretention systems as the adsorbent for multiple pollutants is a cost-effective disposal method to replace landfilling.

Research on competitive adsorption of heavy metal ions in stormwater runoff onto WTRs is scarce. The lack of the related information limited the reuse of WTRs in stormwater bioretention systems for the purpose of removing heavy metals since there often exist multiple types of heavy metal ions in stormwater runoff (Al-Ameri et al. 2018; Kluge et al. 2018). Because of the differences of coagulants added in various water treatment processes and methods used in waste handling and management in different water treatment plants, the composition and physiochemical properties of WTRs are different. The combination of polyaluminium chloride (PAC) as the coagulant in water treatment processes and anionic polyacrylamide (APAM) as the coagulant in the dewatering process of water treatment sludge has been increasingly adopted in large-sized municipal water treatment plants around the world and especially in China. The WTRs produced from such a combination are called PAC-APAM WTRs in this paper.

The objectives of this study were to investigate and compare the competitive adsorption behavior of Cu²⁺, Pb²⁺, Cd²⁺, and Zn²⁺ onto PAC-APAM WTRs in mono-component systems, binary systems, ternary systems, and quaternary systems at varying pH, temperature, initial concentration, and time. The overall objective of this study was to gain insight into the potential competitive adsorption mechanism of heavy metal ions frequently occurring in stormwater runoff by PAC-APAM WTRs and provide implications for their mobility in stormwater bioretention systems with PAC-APAM WTRs as a media layer.

PAC-APAM WTRs samples were collected from a municipal water treatment plant in Taiyuan city, Shanxi Province, China with the production capacity of 800,000 m³/day, and prepared as performed in our previous studies (Duan & Fedler 2021a, 2021b, 2021c). The final particle size of PAC-APAM WTRs was up to 0.25 mm in equivalent spherical diameter. Scanning electron microscope (SEM) images and detailed analytical results of the physicochemical characteristics of PAC-APAM WTRs can be found in previous studies including bulk density, Brunauer–Emmett–Teller (BET) surface area, pH, the pH value at the point of zero charge (pH_pzc), the electrical conductivity (EC), cation exchange capacity (CEC), contents of C, N, H, O, and S elements, and contents of Al, Fe, Ca, Mg, Cu, Pb, Cd, and Zn elements (Duan & Fedler 2021c).

Analytical grade reagents were used in this study. Stock Cu²⁺, Pb²⁺, Cd²⁺, and Zn²⁺ solutions were prepared by dissolving PbCl₂, CuSO₄·5H₂O, CdCl₂, or ZnCl₂ in deionized distilled (DDI) water. All working Cu²⁺, Pb²⁺, Cd²⁺, and Zn²⁺ solutions with designed concentrations were freshly prepared immediately before the adsorption experiments were conducted. The pH adjustments prior to the adsorption experiments were made by using 0.1 mol/L HCl and 0.1 mol/ L NaOH solutions.

The ratios of molar concentrations of different types heavy metal ions in binary systems, ternary systems, and quaternary systems were 1:1, 1:1:1, and 1:1:1:1, respectively. Each competitive adsorption system contained 0.1 g dry PAC-APAM WTRs and 100 mL heavy metal ion solution with different combinations of each metal. After adjusting the pH, competitive adsorption reactors were put into a rotary shaker in darkness and shaken at 140 r/min at different temperature for 6 h. The supernatant was sampled, centrifuged at 3,000 r/min, and filtered through 0.45-μm syringe filters. The filtrates were analyzed by following ASTM-D1688-17 standard test methods for copper in water, ASTM-D3559-15 standard test methods for lead in water, ASTM-D3557-17 standard test methods for cadmium in water, or ASTM-D1691–17 standard test methods for zinc in water. Every competitive adsorption experiment was conducted in triplicate.

For comparison of adsorption behaviors, the mono-component adsorption systems were set up similarly to the multi-component adsorption systems.

The effects of pH on the competitive adsorption were determined at pH of 2, 4, 6, or 8 at 30 °C with an initial concentration of 2 mmol/L for each heavy metal ionic species and a PAC-APAM WTRs dosage of 1.0 g/L in mono-component systems, binary systems and ternary systems, and at pH of 2–9 in the quaternary system.

The temperature effects were investigated at pH of 6 at 20 °C, 30 °C, or 40 °C with each heavy metal ion initial concentration of 1 mmol/L and PAC-APAM WTRs dosage of 1.0 g/L in mono-component systems and the three multi-component systems. The calculation of q_e,i and AR_t,i was conducted by using Equations (1) and (2).

In the process of evaluating the affinity of different adsorbates, there were simultaneously two, three, or four models with shared parameter, η_j, to model fit Equation (5) for each binary system, ternary system, or quaternary system, respectively.

Analysis of variance (ANOVA) or t-test was used to compare means of a variable at P-value less than 0.05 level after the normality of the data distribution was tested and confirmed by the Kolmogorov–Smirnov analysis at P-value < 0.05 level.

It is well known that the forms of heavy metal species (supposing M²⁺ = Cu²⁺, Pb²⁺, Cd²⁺, or Zn²⁺) existing in aqueous solutions primarily include M²⁺, M(OH)⁺, M(OH)₂⁰, and M(OH)_2(s) (Srivastava et al. 2009). M²⁺ is widely regarded as the main form at pH ≈6.0 or pH < 6 in aqueous solutions because the solubility of M(OH)_2(s) is sufficiently high (Srivastava et al. 2009). Hence, combined with the results of pH effects on the adsorption in this study, pH 6 was selected as the initial pH value in the remaining experiments to avoid significant heavy metal precipitation.

The patterns of pH effects on the removal of heavy metal ions by PAC-APAM could be attributed to various causes. The protonation of H⁺ in the aqueous solutions results in more H⁺ ions competing with Cu²⁺, Pb²⁺, Cd²⁺, or Zn²⁺ for available active sites on the surface of the adsorbent at lower pH (Srivastava et al. 2009; Campos et al. 2020). The pH increase weakens the protonation of H⁺ resulting in less competition between H⁺ ions and heavy metal ions for active adsorption sites. As a result, the removal of heavy metal ions increases with an increase in pH. The point of zero charge (pH_pzc) of PAC-APAM WTRs used in this study was 9.1. Since all pH values in the studies were lower than pH_pzc, the surface of the PAC-APAM WTRs particles was positively charged. With solution pH increase, the adsorbent surface gradually decreased in positive charge (Zhu et al. 2016) with negatively charged active sites increasing (Srivastava et al. 2009). This results in a pH increase that lowers the electrostatic repulsion between heavy metal ions in aqueous solution and the surface of adsorbent particle, which consequently promotes the adsorption of heavy metal ions (Srivastava et al. 2009). Previous studies that compared the spectra of Fourier transform infrared spectroscopy before and after the adsorption of heavy metal ions by PAC-APAM WTRs found that O-H groups, carboxyl groups, and Fe(Al)-O functional groups participated in the adsorption reactions by complicated pathways (Duan & Fedler 2021c). Those chemical adsorption reactions are possibly positively correlated with solution pH. The related detailed specific mechanisms require further studies.

The adsorption removal of Cu²⁺, Pb²⁺, Cd²⁺, and Zn²⁺ at pH 6 in mono-component systems under same adsorption condition was 64.10%, 56.30%, 36.60%, and 17.90%, respectively. The adsorption removal range of Cu²⁺, Pb²⁺, Cd²⁺, and Zn²⁺ at pH 6 in multi-component adsorption systems was 38.10% to 60.95%, 31.95% to 51.00%, 28.8% to 34.3%, and 6.45% to 14.30%, respectively. For all metals tested, the mono-component system removed more of the metals than the multi-metal component system, indicating that competitive adsorption occurred in the multi-component systems.

The order of heavy metal ion competitive adsorption at pH 6 was analyzed by using the t-test in binary systems and one-way ANOVA test in ternary and quaternary systems. Results showed that in the binary systems, the removal percentage of Cu²⁺ was significantly higher than Pb²⁺ (P-value = 0.001), Cd²⁺ (P-value = 5.2×10⁻⁵), and Zn²⁺ (P-value = 1.5×10⁻⁶), the removal percentage of Pb²⁺ was significantly higher than Cd²⁺ (P-value = 6.5×10⁻⁵) and Zn²⁺ (P-value = 1.7×10⁻⁶), and the removal percentage of Cd²⁺ was significantly higher than Zn²⁺ (P-value = 1.9×10⁻⁶). Overall, in ternary and quaternary systems, the adsorption affinity followed the same order as in binary systems: Cu²⁺>Pb²⁺>Cd²⁺>Zn²⁺. However, the removal percentage difference between Pb²⁺ and Cd²⁺ was not significant in the Cu²⁺-Pb²⁺-Cd²⁺ system (P-value = 0.086; the removal: 33.10% and 29.20%, respectively) and in the quaternary system (P-value = 0.135; the removal: 31.95% and 28.80%, respectively). In the other cases, the adsorption removal percentage displayed a statistically significant difference between different heavy metal ions (all P-values < 0.05).

In spite of competitive adsorption existing, temperature significantly increased the removal of Cu²⁺ and Pb²⁺ (all values < 0.05). However, competitive adsorption changed the temperature effects on the adsorption removal of Cd²⁺ and Zn²⁺. Positive temperature effects on Cd²⁺ or Zn²⁺ adsorption turned out to be weaker with more coexisting competitive heavy metal ion species. The increase in the adsorption removal of both ions with temperature increased, but was not statistically significant (all P-values >0.05) in ternary and quaternary systems. Even in binary systems, the temperature effect was not significant (P-value = 0.57) for Cd²⁺ between 30 °C and 40 °C (P-value = 0.57) in the Cd²⁺ and Zn²⁺ system. For Zn²⁺, the temperature effect was not significant between 20 °C and 30 °C (P-value = 0.44) in the Cu²⁺ and Zn²⁺ system, between 20 °C and 30 °C (P-value = 0.08) and between 30 °C and 40 °C (P-value = 0.29) in the Pb²⁺ and Zn²⁺ system, and between 30 °C and 40 °C (P-value = 0.28) in the Cd²⁺ and Zn²⁺ system. The results implied that, although the adsorption removal of the heavy metal ions by PAC-APAM WTRs increased with temperature increase, the temperature effects in the competitive adsorption systems became non-significant for the metal ions with relatively lower adsorption removal in mono-component systems.

In this study, Q_mix is defined as the maximum adsorption capacity of a heavy metal ionic species onto the PAC-APAM WTRs in a multi-component system, while Q_mono is the maximum adsorption capacity of the same ion in a mono-component system. The ratio of Q_mix to Q_mono is widely used to evaluate the effects of competitive adsorption of heavy metal ions (Neris et al. 2019). Both Q_mix and Q_mono were obtained by curve fitting data into the Langmuir model in the corresponding systems (Table 1). Results showed that all ratios of Q_mix to Q_mono were smaller than one (Table 1) indicating that the presence of other heavy metal ions decreased the adsorption of a heavy metal ionic species and had destructive effects (Neris et al. 2019). The reduction in adsorption was different with the difference of the coexisting ion or ions (Table 1). For example, Q_mix/Q_mono of Cu²⁺ was 0.702 and 0.723 in the Cu²⁺ and Pb²⁺ system and Cu²⁺ and Cd²⁺ system, while it was 0.958 in the Cu²⁺ and Zn²⁺ system. It showed that the adsorption reduction of Zn²⁺ on Cu²⁺ in the binary system was less than Pb²⁺ and Cd²⁺.

As mentioned above, the competitive adsorption in different multi-component systems was primarily monolayer adsorption. Therefore, the ML model was employed to further evaluate the unequal competition (Neris et al. 2019). The η values of Cu²⁺ were always smaller than the other metals no matter which competitive systems was used (Table 3), meaning that Cu²⁺ had greater affinity with the active sites on PAC-APAM WTRs. The higher η values of Zn²⁺ in various adsorption systems indicated that the affinity of Zn²⁺ with the active sites on the adsorbent was weaker than others. The higher η values of Pb²⁺ than Cd²⁺ showed the greater affinity of Pb²⁺ with active sites than Cd²⁺.

Both ionic radius and Pauling's electronegativity of metal ions are often used to explain and even predict the competitive adsorption behaviors of various heavy metal ions (Neris et al. 2019). Generally, the metal ion with a higher ionic radius or higher electronegativity has higher affinity with active sites than others (Neris et al. 2019). The affinity order of the four heavy metal ionic species with active sites on PAC-APAM WTRs in this study should have been Pb²⁺ (1.19 Å)>Cd²⁺ (0.97 Å)>Zn²⁺ (0.83 Å)>Cu²⁺ (0.73 Å) (Neris et al. 2019) when only the ionic radius is considered. The order would be Pb²⁺ (2.33)>Cu²⁺ (1.95)>Cd²⁺ (1.69)>Zn²⁺ (1.63) (Neris et al. 2019) if only electronegativity is considered. However, the affinity order found in this study was Cu²⁺>Pb²⁺>Cd²⁺>Zn²⁺. This difference in order can be attributed to the comprehensive effects of the physicochemical characteristics of metal ions in the aqueous solutions (Neris et al. 2019), interactions of different metal species, and the complicated chemical processes simultaneously caused by various functional groups existing in PAC-APAM WTRs including O-H groups, carboxyl groups, and Fe(Al)-O groups (Duan & Fedler 2021c).

The kinetic studies were performed by curve-fitting data into pseudo-first-order and pseudo-second-order models and the resulting model parameters are provided in Table 4. The results showed that the adsorption of the four tested heavy metal ionic species onto PAC-APAM WTRs in mono-component systems and multi-component systems could be regarded as following a pseudo-second-order process. This is based on the comparison of the coefficients of determination (R²) of the two kinetic models (Table 4) for the same metal ion in the same adsorption system. Higher R² means better consistency between the experimental values and the model predicted values of q_t (Liu & Lian 2019).

The results showed that the initial average adsorption rate from zero to 0.25 h of a heavy metal ionic species (R_0.25) in the mono-component systems (Figure 4(a)) was positively correlated with the maximum adsorption capacity (Table 1). The slopes in Figure 4(a) illustrated that the order of the R_0.25 was Cu²⁺>Pb²⁺>Cd²⁺>Zn²⁺ in the mono-component systems.

The destructive effects of the coexisting heavy metal ion or ions on the adsorption of a heavy metal ionic species caused the decrease of the R_0.25 (see slopes in Figure 4(b)–4(d)) in competitive systems compared with the R_0.25 in the mono-component systems. The decrease of the R_0.25 for each metal ion was caused by the specific coexisting metal ionic species and the number of the coexisting metal ionic species (see slopes in Figure 4(b)–4(d)). One-way ANOVA tests at P-value < 0.05 level showed that the R_0.25 of the four types of metal ions in a mono-component system was significantly higher than that in other systems (all P-values < 0.05) and the R_0.25 in quaternary system was significantly lower than that in other systems (all P-values < 0.05).

In recent years, more acid rain occurred. This acid rain results in stormwater runoff pH decreases, which results in an increase in the mobility and bioavailability of toxic heavy metals in natural systems. Previous studies along with this study showed that PAC-APAM WTRs are effective candidates that can be used as the media within stormwater bioretention systems to reduce or even eliminated the soluble mobile heavy metal ions by adsorption reactions. This study also shows that PAC-APAM WTRs had stronger ability to remove these heavy metal ions by precipitation mechanism under basic conditions than that under acidic conditions. Therefore, using PAC-APAM WTRs in stormwater bioretention systems to control heavy metal pollution in stormwater runoff not only brings environmental benefits but also provides an alternative approach to solving the disposal problem of municipal water treatment-produced wastes.

Although previous studies have shown that PAC-APAM WTRs are cost effective and a low risk adsorbent to be reused in stormwater bioretention systems or for soil remediation for removing soluble heavy metal ions (Duan & Fedler 2021a, 2021c), the knowledge regarding the competitive adsorption removal of soluble heavy metal ions in a multi-metal system is critical to understand their mobility and transport once PAC-APAM WTRs are used in stormwater bioretention systems as a layered filling media. Since Cu²⁺ and Pb²⁺ ions have a stronger affinity for the active sites on PAC-APAM WTRs in the multi-component systems, they would be relatively easily removed by adsorption in the PAC-APAM WTRs layer of a stormwater bioretention system with a larger molar amount at pH ≤ 6, hence the leaching of Cu²⁺ and Pb²⁺ would be less than the other two heavy metal ionic species. Conversely, Zn²⁺ with a relatively weaker affinity to adsorption caused by the existence of other heavy metal ions would be more mobile and more readily transport through the stormwater bioretention systems compared to other targeted heavy metal ions in stormwater runoff. This becomes more apparent with the temperature increase because higher temperature can increase the removal of heavy metal ions in various competitive adsorption systems, but the increase of Zn²⁺ removal by adsorption was not significant as Cu²⁺ and Pb²⁺. Additionally, the knowledge obtained in this study combined with the knowledge of solute transport is also crucial to predict the movement and distribution and to assess the potential risks of heavy metal ions in stormwater runoff through the stormwater bioretention systems using PAC-APAM WTRs as a media layer.

This study indicated that PAC-APAM WTRs were an effective adsorbent for the simultaneous adsorption removal of Cu²⁺, Pb²⁺, Cd²⁺, and Zn²⁺ in mono-component and multi-component systems. The effects of solution pH, temperature, initial concentration, and time on the adsorption removal of four heavy metal ionic species were evaluated in both mono-component and multi-component systems. The adsorption removal increased with the increase in pH and temperature. The maximum adsorption capacity of each metal species decreased to different extents due to the different destructive effects of the coexisting metal ions in the multi-component systems. The Langmuir model fitted the experimental data better than the Freundlich model, implying that adsorption with or without competition was a monolayer adsorption process. The competitive adsorption order from greatest to least was Cu²⁺>Pb²⁺>Cd²⁺>Zn²⁺ in the multi-component systems. The pseudo-second-order model predicted the kinetic process better in the mono-component and multi-component systems. This implies that the kinetic process could be diffusion limited. The initial average adsorption rate (R_0.25) in the multi-component systems was negatively influenced by competitive adsorption that caused a decrease in R_0.25. The R_0.25 of each heavy metal ionic species was highest in the mono-component system and lowest in the quaternary system. This work implied that Zn²⁺ in stormwater runoff could be more readily leached out of the bioretention system under the same acidic conditions once PAC-APAM WTRs were used as a media layer to treat frequently occurring soluble heavy metals in stormwater runoff.

This work was supported by Fundamental Research Program of Shanxi Province, China (Grant No. 202103021224082).

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.