Adsorption of Free Chlorine on Tetravalent Lead Corrosion Product (PbO 2 )

Abstract

Lead dioxide (PbO₂) is a new form of lead corrosion product discovered in the drinking water distribution systems. It is formed via the chlorination of lead-containing plumbing materials. In this study, we investigated the adsorption of free chlorine (HOCl/OCl⁻) on PbO₂ to explore the loss of free chlorine in the bulk solution and the reactivity of adsorbed free chlorine in the presence of PbO₂. Our results indicated that adsorption reached equilibrium in approximately 6 h and ionic strength did not significantly affect adsorption. Langmuir adsorption isotherm provided good fittings of the experimental data. Maximum adsorption capacity of PbO₂ was dependent on the solution pH value, with the maximum occurring at pH 8, which can be explained by electrostatic interactions between free chlorine and PbO₂ surfaces. Both carbonate and phosphate buffers inhibited free chlorine adsorption, and phosphate was a stronger inhibitor. Adsorbed free chlorine was found to maintain its oxidation ability. Our results indicated that the presence of PbO₂ may induce a loss of free chlorine in the bulk solution because of adsorption. Nevertheless, adsorbed free chlorine remains active and may exhibit disinfection capability. This is the first study to show that free chlorine can act as an “adsorbate” in the distribution system and this reaction should be considered in the redox transformation of corrosion products.

Introduction

Lead-containing plumbing materials (LCPMs) including lead service line and leaded brass and solder were commonly used in the drinking water distribution system in the past because of their malleable nature and resistance to corrosion (Gregory, 1993). Although pure lead pipes had been banned in the 1980s, because of the enormous cost and difficulties in complete replacement, LCPMs are still in use to convey drinking water to consumer's taps in many countries (Hayes et al., 2010).

A series of electrochemical reactions in drinking water can lead to the formation of lead corrosion products on the surface of LCPMs. Pb(II) solids including cerussite (PbCO₃) and hydrocerussite (Pb₃(CO₃)₂(OH)₂) were considered to be the solid phases controlling the plumbosolvency and lead release in the drinking water distribution system (Schock, 1980, 1989; Schock and Gardels, 1983; Davidson et al., 2004). However, tetravalent lead corrosion product (PbO₂) was recently discovered in some distribution systems wherein free chlorine was used as the disinfectant (Schock et al., 1996, 2001; Renner, 2004). Laboratory studies have verified that PbO₂ can be formed via the chlorination of Pb(II) solids (Edwards and Dudi, 2004; Lytle and Schock, 2005; Liu and Korshin, 2008; Zhang and Lin, 2011). Because of its low solubility, PbO₂ is considered a good passivating layer preventing lead release from underneath materials (Lytle and Schock, 2005).

In a recent study on the chlorination of hydrocerussite and cerussite by Liu and Korshin (2008), it was found that free chlorine consumption exhibited a lag phase followed by a rapid loss accompanying by the formation of PbO₂. Although Liu and Korshin (2008) proposed an autocatalytic oxidation mechanism for PbO₂ formation to explain this unusually rapid loss of free chlorine, it is noted that the amount of PbO₂ formed was not determined to verify the reaction stoichiometry. Considering the common interactions between soluble species and solid phases, adsorption of free chlorine on the PbO₂ surfaces, which has not been studied in the past, may also contribute to the loss of free chlorine in the bulk solution phase. If such adsorption is significant, it must be considered when investigating the kinetics and mechanism of PbO₂ formation from the chlorination of Pb(II) solids. In addition, the loss of free chlorine may be problematic as free chlorine is commonly used to provide disinfectant residual to prevent microbial regrowth in the drinking water distribution system (AWWA, 1999).

The main objective of this study was to characterize the adsorption of free chlorine on PbO₂ to understand the fate and reactivity of free chlorine when PbO₂ is present. Both adsorption kinetics and adsorption isotherms were studied. In addition, the effects of solution pH, ionic strength, and buffer type and concentration on free chlorine adsorption were investigated.

Experimental Protocols

Chemicals

PbO₂ was prepared by oxidizing 1 mM Pb(NO₃)₂ solution by 1.1 mM of HOCl according to the procedures described by Lin and Valentine (2008). HOCl solutions were prepared by diluting an ∼4% NaOCl stock solution (Sigma-Aldrich). Analytical-grade KI (Merck) was used as the source of iodide. NaHCO₃ (Nacalai Tesque) and Na₃PO₄·12H₂O (Merck) were used to prepare pH buffer. Analytical-grade KNO₃ (Sigma-Aldrich) was used to adjust ionic strength. The solution pH was adjusted using 1 N HCl and NaOH. All solutions were prepared by ultrapure water obtained from a Millipore DirectQ system.

Potentiometric titration

The potentiometric titration was used to determine the point of zero charge (pH_pzc) of the synthesized PbO₂ particles. The solution used was prepurged with nitrogen gas to remove carbonate ions. During the course of the experiment, the solution was continuously stirred to ensure a homogeneous suspension. The titration procedure was adopted from that used by Lu et al. (1996). Briefly, 0.1 N HCl or NaOH was added to a 100 mL PbO₂ suspension at 25°C in the presence of a gentle stream of nitrogen gas, and the equilibrium pH was recorded after each addition of HCl and NaOH. The PbO₂ loading employed was 1 g/L.

Adsorption experiments

All adsorption experiments were conducted using 25-mL gas-tight polyethylene vessels at a constant ambient temperature of 25°C. After filling the experimental solution containing the desired loading of PbO₂ and free chlorine, the vessels were sealed and covered by aluminum foil to prevent light-induced free chlorine decay. The vessels were placed on a shaker rotating at 200 rpm to ensure a complete mixing during the course of the experiment. The adsorption kinetics was investigated using solutions with pH values ranging from 6 to 10 buffered by either carbonate or phosphate. The variation of pH value before and after each experiment was found to be within 0.2 pH units. The residual free chlorine concentration in the aqueous phase was measured as a function of time using filtered samples obtained by passing the solutions through a syringe filter equipped with a 0.2 μm pore size PTFE membrane (Titan). To account for the natural decay of free chlorine, a blank sample without the addition of PbO₂ was prepared. The adsorbed free chlorine was determined by the difference in residual free chlorine between the blank and the PbO₂-containing sample. For isotherm experiments, the adsorbed free chlorine concentration at equilibrium was determined using different initial free chlorine concentrations. The effects of ionic strength (0.01–0.05 M KNO₃), pH (6–10), and buffer type and concentration (0.5–8.0 mM carbonate [C_T] and phosphate) on free chlorine adsorption were also investigated. To study the reactivity of adsorbed free chlorine, free chlorine concentrations in unfiltered samples after reaching equilibrium were measured in selected experiments and compared against those measured in blank samples.

Analytical methods

Free chlorine concentration in the stock solution and experimental solution was measured using the DPD-FAS method and iodometric method, respectively, following the procedures described in the Standard Methods (APHA, AWWA, and WEF, 2005). Images acquired by a JEOL 6700F field emission scanning electron microscopy (FE-SEM) were used to determine the morphological feature of the synthesized PbO₂. The X-ray diffraction (XRD) pattern was acquired by a Siemens XRD D5005. The specific surface area of PbO₂ was measured by the seven-point N₂-BET using a NOVA 4200e surface area analyzer. The precalibrated pH meter (F-51, Horiba) was used to measure solution pH.

Results and Discussion

Characteristics and point of zero charge (pH_pzc) of synthesized PbO₂ particles

Figure 1 shows the SEM image and XRD pattern of the synthesized PbO₂ particles. The SEM image indicated that the synthesized PbO₂ were aggregates consisting of small particles and the XRD pattern suggested that the synthesized PbO₂ was plattnerite (β-PbO₂). The surface area of the synthesized PbO₂ was 17 m²/g. The potentiometric titration curves in the presence of 0.1 M and 0.01 M KNO₃ are shown in Fig. 2, in which the y-axis represents the adsorbed hydrogen ions concentration on the PbO₂ surfaces determined using the following equation: \documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{bm} \usepackage{mathrsfs} \usepackage{pifont} \usepackage{stmaryrd} \usepackage{textcomp} \usepackage{portland, xspace} \usepackage{amsmath, amsxtra} \pagestyle{empty} \DeclareMathSizes {10} {9} {7} {6} \begin{document} \begin{align*} [{\rm H}^ + _{\rm ads}] = {\rm C}_{\rm a} - {\rm C}_{\rm b} + [{\rm OH}] - [{\rm H}^ +] \tag{1} \end{align*} \end{document}

FIG. 1.

(a) SEM image and (b) XRD pattern of the synthesized PbO₂ particles.

FIG. 2.

Potentiometric titration curves of PbO₂ in the presence of 0.01 and 0.1 M KNO_3.

where C_a and C_b (M) represent the concentration of added acid and base, respectively.

The pH_pzc of the synthesized PbO₂, which occurs at \documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{bm} \usepackage{mathrsfs} \usepackage{pifont} \usepackage{stmaryrd} \usepackage{textcomp} \usepackage{portland, xspace} \usepackage{amsmath, amsxtra} \pagestyle{empty} \DeclareMathSizes {10} {9} {7} {6} \begin{document} $$[{\rm H}^ + _{\rm ads}] = 0$$ \end{document} , was determined to be pH 7.4, regardless of the different ionic strengths employed. The pH_pzc determined in our study was similar to those (pH 7.4 and 8.3) reported in the literature (Kosmulski, 2001). Liu et al. (2009) observed a negative zeta potential of PbO₂ at pH values lower than 6.4, suggesting that the pH_pzc of their PbO₂ was smaller than pH 6.4. The smaller pH_pzc might be due to the different polymorphs of PbO₂, that is, scrutinyite (α-PbO₂), used by Liu et al. (2009).

Adsorption kinetics

The adsorption of free chlorine as a function of time using solutions with pH values ranging from 6 to 10 is presented in Fig. 3a. The adsorption equilibrium was reached in about 6 h. The amounts of adsorbed free chlorine at 6 h as a function of pH are shown in Fig. 3b. Approximately 60% of the total adsorbed free chlorine was adsorbed in the first hour for all pH values. The rate of adsorption and the total adsorbed free chlorine were the highest at pH 8 followed by those at pH 7, 9, 6, and 10, which could be explained by the electrostatic interaction between free chlorine and PbO₂ surfaces as discussed later. The pseudo-first-order and pseudo-second-order kinetics shown in Equations (2) and (3), respectively, were employed to simulate the adsorption kinetics (Ho and McKay, 1999; Ho, 2004). \documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{bm} \usepackage{mathrsfs} \usepackage{pifont} \usepackage{stmaryrd} \usepackage{textcomp} \usepackage{portland, xspace} \usepackage{amsmath, amsxtra} \pagestyle{empty} \DeclareMathSizes {10} {9} {7} {6} \begin{document} \begin{align*} \log (q_e - q_t) = \log q_e - \frac {k_1} {2.303} t \tag {2} \end{align*} \end{document} \documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{bm} \usepackage{mathrsfs} \usepackage{pifont} \usepackage{stmaryrd} \usepackage{textcomp} \usepackage{portland, xspace} \usepackage{amsmath, amsxtra} \pagestyle{empty} \DeclareMathSizes {10} {9} {7} {6} \begin{document} \begin{align*} {t \over q_t} = {1 \over k_2 q_e^2} + {1 \over q_e}t \tag{3} \end{align*} \end{document}

FIG. 3.

(a) Adsorption of free chlorine by PbO₂ as a function of time at different pH values. The lines represent the pseudo-first-order kinetics modeling. (b) The amount of adsorbed free chlorine at 6 h as a function of pH values. Experimental conditions: PbO₂=120 mg/L; initial free chlorine=4.2 mg/L as Cl₂; C_T=4 mM; temperature=25°C.

where q_e and q_t (mg as Cl₂/g) represent the amount of free chlorine adsorbed at equilibrium and at time t (h), respectively; k₁ (h⁻¹) and k₂ (g/[mg as Cl₂·h]) represent the pseudo-first-order and pseudo-second-order rate constants, respectively.

Table 1 shows the results of the least-square fittings of experimental data to Equations (2) and (3). The R² for the pseudo-first-order kinetics (0.88–0.96) were higher than those for the pseudo-second-order kinetics (0.75–0.92). In addition, the values of q_e obtained using the pseudo-first-order model were more consistent with the experimentally determined values (q_e,_exp). These results suggested that the pseudo-first-order model provided a better simulation for the kinetics of free chlorine adsorption on the PbO₂ surfaces. The use of the pseudo-first-order kinetics to model the adsorption of free chlorine as a function of time is demonstrated in Fig. 3a.

Table 1.

Fitting Results of Pseudo-First-Order and Pseudo-Second-Order Kinetics

	Pseudo-first-order kinetics			Pseudo-second-order kinetics
pH	q_e (mg as Cl₂/g)	k₁ (h⁻¹)	R²	q_e (mg as Cl₂/g)	k₂ (g/[mg as Cl₂·h])	R²	q_e,exp (mg as Cl₂/g)
6	15.4(±2.4)	1.31(±0.53)	0.91	11.9(±1.1)	0.53(±0.16)	0.88	16.5
7	18.6(±2.9)	1.56(±0.67)	0.90	14.9(±1.6)	0.49(±0.17)	0.82	20.0
8	19.5(±3.7)	1.86(±1.04)	0.88	15.9(±1.4)	0.66(±0.22)	0.76	21.0
9	16.4(±2.4)	1.35(±0.53)	0.91	12.2(±1.5)	0.63(±0.25)	0.75	17.5
10	12.6(±1.4)	1.19(±0.33)	0.96	9.8(±1.1)	0.44(±0.13)	0.92	13.4

Values in parentheses are 95% confidence level. q_e,exp represents the amount of adsorbed free chlorine measured at equilibrium. Experimental conditions: PbO₂=120 mg/L; pH 6–10; initial free chlorine=4.2 mg/L as Cl₂; C_T=4 mM; temperature=25°C.

Effects of pH on the adsorption isotherm

The isotherm study was conducted by determining the aqueous and adsorbed free chlorine concentrations when equilibrium was reached using different initial free chlorine concentrations. Figure 4 shows the isotherm data obtained for pH 6–10. The Langmuir adsorption isotherm and Freundlich adsorption isotherm shown in Equations (4) and (5), respectively, were used to fit the experimental data. \documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{bm} \usepackage{mathrsfs} \usepackage{pifont} \usepackage{stmaryrd} \usepackage{textcomp} \usepackage{portland, xspace} \usepackage{amsmath, amsxtra} \pagestyle{empty} \DeclareMathSizes {10} {9} {7} {6} \begin{document} \begin{align*} \frac {1} {C_ {is}} = \frac {1} {C_ {is, \max} \cdot K_ {iL}} \cdot \frac {1} {C_ {iw}} + \frac {1} {C_ {is, \max}} \tag {4} \end{align*} \end{document} \documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{bm} \usepackage{mathrsfs} \usepackage{pifont} \usepackage{stmaryrd} \usepackage{textcomp} \usepackage{portland, xspace} \usepackage{amsmath, amsxtra} \pagestyle{empty} \DeclareMathSizes {10} {9} {7} {6} \begin{document} \begin{align*} C_{is} = K_F \cdot C_{iw}^n \tag{5} \end{align*} \end{document}

FIG. 4.

Effects of pH value on adsorption isotherm. Experimental conditions: PbO₂=80 mg/L; initial free chlorine=0.8–6 mg/L as Cl₂; C_T=4 mM; temperature=25°C. The lines represent modeling results using the Langmuir adsorption isotherm.

where C_is represents the amount of free chlorine adsorbed on PbO₂ (mg as Cl₂/g), C_iw represents the aqueous free chlorine concentration (mg as Cl₂/L), C_is,max represents the adsorption maximum for free chlorine (mg as Cl₂/g), K_iL represents the Langmuir adsorption constant (L/mg as Cl₂), K_F represents the Freundlich adsorption constant ([mg as Cl₂]¹⁻ⁿ·Lⁿ/g), and n represents Freundlich exponent.

The least-square fittings of experimental data to Equations (4) and (5) are summarized in Table 2. The R² for the Langmuir adsorption isotherm (0.93–0.99) were slightly higher than those for the Freundlich adsorption isotherm (0.86–0.93), indicating that the former provided a slightly better description of the experimental data. The use of the Langmuir isotherm to model the experimental data is also shown in Fig. 4. The adsorption maximum showed the following sequence: pH 8>pH 7>pH 9>pH 6>pH 10, which was consistent with that for the adsorption rate determined in the kinetic study. The pK_a of HOCl is 7.5 (Stumm and Morgan, 1996), which is very close to the pH_pzc of the PbO₂ used in this study (pH 7.4). Thus, the surface charge of PbO₂ and the net charge of free chlorine species (HOCl and OCl⁻) shift almost synchronously as a function of pH, that is, when the solution pH becomes more acidic than pH 7.4, the PbO₂ surfaces become more positively charged and HOCl becomes the dominant species of free chlorine; when the solution pH becomes more basic than pH 7.5, the PbO₂ surfaces are more negatively charged and OCl⁻ becomes the dominant form of free chlorine. Considering the electrostatic interaction between free chlorine and PbO₂ surfaces, it is expected that the maximum adsorption would occur at a pH between 7.4 and 7.5, wherein a minimum electrostatic repulsion exists. A decrease in the adsorption maximum would be expected when the pH is deviated either more acidic or basic from this optimal pH value. Our experimental results (Fig. 3b) were consistent with this trend, suggesting that electrostatic interaction between free chlorine and the PbO₂ surfaces played a crucial role in the adsorption process.

Table 2.

Langmuir and Freundlich Adsorption Model Fittings of Isotherm Data Determined at pH 6–10

	Langmuir isotherm				Freundlich isotherm
	C_is,max
pH	mg as Cl₂/g	μmol/m²	K_il L/mg as Cl₂	R²	K_F (mg as Cl₂)¹⁻ⁿ·Lⁿ/g	n	R²
6	18.5	15.3	3.03	0.98	12.3	0.36	0.86
7	23.3	19.3	4.42	0.99	15.5	0.37	0.93
8	27.1	22.5	3.08	0.94	18.6	0.33	0.89
9	18.8	15.6	3.93	0.99	12.6	0.38	0.92
10	14.5	12.0	3.31	0.93	10.0	0.37	0.92

Effect of ionic strength

Figure 5 shows the adsorption of free chlorine as a function of time in the presence of different ionic strengths, with KNO₃ ranging from 0.01 to 0.05 M. The adsorption kinetics and adsorption capacity were similar for all ionic strengths employed, although a slightly higher adsorption was found in the presence of 0.05 M KNO₃. The differences in the adsorbed free chlorine at equilibrium among the three ionic strengths employed were less than 15%, which may result from experimental errors. As physical sorption is known to be significantly affected by the ionic strength (Stumm and Morgan, 1996), our results suggested that free chlorine was most likely to be chemisorbed on the PbO₂ surfaces. The specific interactions between the free chlorine species (HOCl and OCl⁻) and the PbO₂ surfaces were proposed in the following equations: \documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{bm} \usepackage{mathrsfs} \usepackage{pifont} \usepackage{stmaryrd} \usepackage{textcomp} \usepackage{portland, xspace} \usepackage{amsmath, amsxtra} \pagestyle{empty} \DeclareMathSizes {10} {9} {7} {6} \begin{document} \begin{align*} \equiv {\rm Pb}^{\rm IV} {\rm OH} + {\rm H}^{+} + {\rm OCl}^{-} \rightarrow \equiv {\rm Pb}^{\rm IV} {\rm OCl} + {\rm H}_2 {\rm O} \tag{6} \end{align*} \end{document} \documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{bm} \usepackage{mathrsfs} \usepackage{pifont} \usepackage{stmaryrd} \usepackage{textcomp} \usepackage{portland, xspace} \usepackage{amsmath, amsxtra} \pagestyle{empty} \DeclareMathSizes {10} {9} {7} {6} \begin{document} \begin{align*} \equiv {\rm Pb}^{\rm IV} {\rm OH} + {\rm HOCl} \rightarrow \equiv {\rm Pb}^{\rm IV} {\rm HOCl}^{+} + {\rm OH}^{-} \tag{7} \end{align*} \end{document}

FIG. 5.

Effect of ionic strength on adsorption of free chlorine on PbO₂. Experimental conditions: PbO₂=120 mg/L; initial free chlorine=6.1 mg/L as Cl₂; pH=7; C_T=4 mM; temperature=25°C.

≡ Pb^IV OH can be protonated to be \documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{bm} \usepackage{mathrsfs} \usepackage{pifont} \usepackage{stmaryrd} \usepackage{textcomp} \usepackage{portland, xspace} \usepackage{amsmath, amsxtra} \pagestyle{empty} \DeclareMathSizes {10} {9} {7} {6} \begin{document} $$\equiv {\rm Pb}^{\rm IV}{\rm OH}_2^ +$$ \end{document} at pH<pH_pzc and deprotonated to be≡Pb^IV O⁻ at pH>pH_pzc.

Effects of buffer type and concentration

Figure 6 shows the adsorbed free chlorine concentration as a function of carbonate and phosphate buffer concentrations at pH 7. The adsorbed free chlorine decreased with the increasing concentration of both buffers, and phosphate showed a stronger inhibitory effect than carbonate. It should be noted that phosphate has been shown to inhibit the reductive dissolution of PbO₂ because of its adsorption on the PbO₂ surfaces (Shi and Stone, 2009a, 2009b). Based on the dissociation constants for carbonate (pK_a1=6.4 and pK_a2=10.3) and phosphate (pK_a1=2.1, pK_a2=7.2, and pK_a3=12.3) (Stmm and Morgan, 1996), the dominant species of carbonate and phosphate at pH 7 are \documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{bm} \usepackage{mathrsfs} \usepackage{pifont} \usepackage{stmaryrd} \usepackage{textcomp} \usepackage{portland, xspace} \usepackage{amsmath, amsxtra} \pagestyle{empty} \DeclareMathSizes {10} {9} {7} {6} \begin{document} $${\rm HCO}_3^ - / {\rm H}_2{\rm CO}_3$$ \end{document} and \documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{bm} \usepackage{mathrsfs} \usepackage{pifont} \usepackage{stmaryrd} \usepackage{textcomp} \usepackage{portland, xspace} \usepackage{amsmath, amsxtra} \pagestyle{empty} \DeclareMathSizes {10} {9} {7} {6} \begin{document} $${\rm H}_2{\rm PO}_4^ - / {\rm HPO}_4^{2 -}$$ \end{document} , respectively. These species can compete with free chlorine for the adsorption sites on the PbO₂ surfaces. Phosphate is known to be a stronger complexing agent than carbonate (Stmm and Morgan, 1996). In addition, the more negatively charged phosphate species may also contribute to its stronger affiliation to the positively charged PbO₂ surfaces at pH 7 and result in its stronger inhibition on free chlorine adsorption observed in our experiments.

FIG. 6.

Effects of carbonate and phosphate buffer concentration on adsorption of free chlorine on PbO₂ at equilibrium. Experimental conditions: PbO₂=120 mg/L; initial free chlorine=6.1 mg/L as Cl₂; pH=7; temperature=25°C.

Reactivity of adsorbed free chlorine

The reactivity of adsorbed free chlorine was investigated at pH 7, 8, 9, and 10 by comparing the free chlorine concentration determined using unfiltered samples against those measured in blank samples without PbO₂. The ratio of free chlorine concentration measured in the unfiltered sample and that in blank sample was defined as the recovery of free chlorine in the presence of PbO₂. It should be noted that although PbO₂ is a strong oxidant, it did not interfere with free chlorine measurement by the iodometric method at neutral or basic solution. It was previously reported that no more than 1.5% of PbO₂ was reduced by iodide in 30 s at pH 7 and 8 in the absence of free chlorine (Zhang et al., 2010).

The results of free chlorine recovery experiments are shown in Table 3. The recoveries ranged from 90% to 105%, suggesting that the adsorbed free chlorine maintained its strong oxidation ability. Additional desorption experiments conducted by adjusting the final pH to 4 showed that 70% of the adsorbed free chlorine was released from the PbO₂ surfaces in 20 min. Our results indicated that PbO₂ may induce a loss of free chlorine in the bulk solution because of adsorption. This is most likely to occur in households where lead service lines are used and chlorinated drinking water sits in these pipes stagnantly overnight. Nevertheless, the adsorbed free chlorine can provide disinfection capability that might be beneficial for preventing microorganism regrowth at this solid–water interface.

Table 3.

Recovery Test of Adsorbed Free Chlorine at Equilibrium

	Free chorine concentration (mg/L as Cl₂)
pH	Filtered sample	Unfiltered sample	Blank sample	Recovery (%)
7	1.5	3.9	4.2	93
8	1.3	4.2	4.0	105
9	2.0	3.9	4.1	95
10	2.3	3.6	4.0	90

Experimental conditions: PbO₂=120 mg/L; initial free chlorine=4.2 mg/L as Cl₂; adsorption period=6 h; C_T=4 mM; temperature=25°C.

Summary

The adsorption of free chlorine on PbO₂ was studied at difference pH values, ionic strengths, and buffer types and concentrations. The kinetic study showed that the adsorption equilibrium can be reached in about 6 h and the pseudo-first-order kinetics can be used to describe the adsorption rate. At equilibrium, the adsorption can be successfully described by the Langmuir adsorption isotherm. For the pH range investigated in this study (pH 6–10), it was found that the highest adsorption occurred at pH 8, which can be well explained by considering the electrostatic interaction between free chlorine species and PbO₂ surfaces. Ionic strength did not significantly affect the adsorption process. Both carbonate and phosphate buffers showed inhibitory effects on the free chlorine adsorption and phosphate was found to be a stronger inhibitor. The adsorbed free chlorine can maintain its oxidizing ability as evidenced by >90% recoveries of free chlorine in unfiltered samples. Our results suggested that the presence of PbO₂ may decrease the free chlorine concentration in the bulk solution because of adsorption. However, the adsorbed free chlorine, which was conventionally thought as the wall loss of disinfectant, maintained its strong oxidation ability and may provide disinfection capability.

Footnotes

Acknowledgment

This work was supported by the National University of Singapore (R-288-000-054-133).

Author Disclosure Statement

No competing financial interests exist.

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Adsorption of Free Chlorine on Tetravalent Lead Corrosion Product (PbO 2 )

Abstract

Abstract

Introduction

Experimental Protocols

Chemicals

Potentiometric titration

Adsorption experiments

Analytical methods

Results and Discussion

Characteristics and point of zero charge (pHpzc) of synthesized PbO2 particles

Adsorption kinetics

Effects of pH on the adsorption isotherm

Effect of ionic strength

Effects of buffer type and concentration

Reactivity of adsorbed free chlorine

Summary

Footnotes

Acknowledgment

Author Disclosure Statement

References

Characteristics and point of zero charge (pH_pzc) of synthesized PbO₂ particles