Humic Acid Removal in Water via UV Activated Sodium Perborate Process

Abstract

Humic acid (HA) has complex molecular structure and is capable of adsorption, ion exchange, and chelation with organic and inorganic pollutants in water bodies, worsening water quality and jeopardizing human health and ecological environment. How to effectively remove HA from water is one of the research focuses of this paper. In this study, the UV-activated sodium perborate (SPB) synergistic system (UV/SPB) was established to eliminate HA in water. The effects of initial HA concentration, SPB dose, and initial pH value on the HA elimination were determined, and the main mechanisms of the synergy and HA degradation were explored. The outcomes show that the HA elimination ratio by the sole UV and only SPB system were only 0.5% and 1.5%, respectively. The HA removal of UV/SPB reached 88.8%, which can remove HA more effectively than other systems. Free radical masking experiment proved that hydroxyl radical produced by SPB activation is the main active substance for HA removal. The results of UV-vis absorption spectrum, absorbance ratio, specific UV absorbance, and excitation–emission matrix spectroscopy verified that the UV/SPB system can effectively decompose and mineralize HA.

1. Introduction

As a non-uniform macromolecular polymer, humic acid (HA) is the major ingredient of natural organic matter (NOM), which is the product of polymerization of diverse biologic remains for ages [1]. HA has a complex molecular structure and contains many organic functional groups, such as hydroxyl, carboxyl, carbonyl, methoxy, and quinone groups, which are capable of adsorption, ion exchange, and chelation with organic and inorganic pollutants in water bodies, worsening water quality and jeopardizing human health and ecological environment [2]. Finding a proper way to effectively remove HA from water is one of the focuses in environmental research.

The main methods for controlling HA in water are physical and chemical oxidation ways. Physical methods to remove HA include coagulation [3], flocculation [4], and adsorption [5], but these methods only transfer HA to solid phase, and subsequent solid waste treatment is still required. Chemical oxidation is of great interest because of its rapid decomposition and mineralization for HA [6]. Photocatalysis [7,8,9], Fenton oxidation [10], and electrochemical oxidation [11] are the common chemical oxidation methods for organic wastewater treatment; however, they universally have some shortcomings such as harsh reaction conditions and complex operation.

Sodium perborate (SPB, NaBO₃) is a widely used in situ oxidizing agent. Distinct from the sodium percarbonate, SPB is not an adduct from inorganic salt and H₂O₂ but contains cyclic perborate ions (B₂O₈H₄²⁻) consisting of two peroxide chains without BO₃⁻ ions [12]. SPB produces H₂O₂ stably after SPB is dissolved in water, so it is a good substitute for H₂O₂ [13]. Compared with liquid H₂O₂, the solid SPB is safe and easy to store and transport. Most importantly, the activation of SPB can form hydroxyl radicals (·OH) over a wide range. Ultraviolet (UV) radiation [14] and transition metal ions [15] are the main approaches for SPB activation. Researchers have used UV-activated perborate for organic pollutant removal [13]. Besides, the perborate has been employed as the oxidant in the homogeneous photo-Fenton and heterogeneous Fenton-like reactions for dye and phenol degradation [12,16], respectively. Among them, the UV activation method is easy operation and safe without secondary pollution, so it could efficaciously excite H₂O₂ to decompose organics from wastewater [17,18].

Although the UV-activated peroxide and even UV-activated SPB method have been applied for removing organics in water, the elimination of HA through the UV activated SPB is rarely reported. Therefore, the importance of this study was establishing a UV/SPB synergistic system (UV/SPB) for the reduction of HA in water. With the convenient and efficient spectrophotometric method [19], the effects of initial HA concentration, SPB dose, and initial pH value on HA decontamination were studied. The main active substances generated in the synergistic system for HA removal were determined by free radical masking experiment, and the degradation mechanism was systematically explored by UV spectra, total organic carbon, and three-dimensional excitation-emission matrix spectroscopy (3D-EEM).

2. Experimental

2.1. Chemicals

HA was purchased from Aladdin Industrial Co., Shanghai, China. The average molecular weight of HA was 2485 Da. Sodium perborate (NaBO₃, SPB) and sodium sulfate (Na₂SO₄) were also purchased from Aladdin Industrial Co., Shanghai, China. Sodium hydroxide (NaOH) and sulfuric acid (H₂SO₄) were purchased from Sinopharm Chemical Reagent Co., Shanghai, China. Sodium carbonate (Na₂CO₃) and sodium dihydrogen phosphate (NaH₂PO₄) were bought from Tianjin Kemiou Chemical Reagent Co., Ltd., Tianjin, China. Sodium nitrate (NaNO₃) was manufactured by Tianjin Hengxing Chemical Reagent Manufacturing Co., Ltd., Tianjin, China. Sodium bicarbonate (NaHCO₃) was supplied by Tianjin Kaitong Chemical Reagent Co., Ltd., Tianjin, China. Sodium chloride (NaCl) was obtained from Tianjin Fengchuan Chemical Reagent Co., Ltd., Tianjin, China. Tertiary butanol (TBA) was purchased from Tianjin Damao Chemical Reagent Factory, Tianjin, China. All chemicals used were of analytical grade without further purification.

2.2. Experimental Procedures

The HA removal experiments were conducted in 25 °C. A beaker served as the reactor was kept on the magnetic stirrer, and the UV lamp (power 16 W, wavelength 254 nm, Philips Co., Shanghai, China) was placed above the beaker with the distance 3.5 cm. The UV fluence in this study was computed to be 35.2 mJ cm⁻² for 60 min. A certain volume of HA solution was diluted to 100 mL before the beginning of experiment. Then the SPB was introduced to the HA simulated wastewater, and the UV irradiation was begun to initiate the reaction. At specified time intervals, 2.5 mL aliquots were withdrawn for measuring the absorbance. All the experiments were performed at least two times. To confirm the generation of ·OH, TBA was used as the scavengers.

2.3. Method and Analyses

The absorbance of solution was detected through UV-vis spectrophotometer (SP-725 Shanghai Spectral Instrument Co., Ltd., Shanghai, China) with external standard method at a wavelength of 254 nm to determine the HA removal efficiency [20]. The equation was as follows:

$$\mathrm{HA} \mathrm{removal} \mathrm{efficiency}=\frac{{\mathrm{C}}_0{-\mathrm{C}}_{\mathrm{t}}}{{\mathrm{C}}_0} \times 100\%$$

(1)

where C₀ was the beginning HA amount, and C_t was the HA amount at the treatment time t.

The HA molecular structure variation was determined by a series of specific UV-vis adsorption spectra. The values of A₂₅₃, A₂₀₃, A₂₅₄, A₄₃₆, A₂₅₀, A₃₆₅, A₄₆₅, and A₆₆₅ were determined by spectrophotometer at the wavelengths of 203 nm, 250 nm, 253 nm, 254 nm, 365 nm, 436 nm, 465 nm, and 665 nm, respectively [21]. In addition, continuous changes in absorbance of the solution at 200–800 nm were also measured to characterize the changes of HA molecular structure.

Total organic carbon (TOC) was measured by TOC analyzer (VCPH, Shimadzu, Japan). Specific UV absorbance (SUVA_x) was calculated with A_x and TOC [22].

$${\mathrm{SUVA}}_{\mathrm{x} }=\frac{{\mathrm{A}}_{\mathrm{x}}}{\mathrm{TOC}} \times 100\%$$

(2)

where A_x was the sample absorbance at x nm.

The 3D-EEM spectrum (FL4500, Hitachi, Tokyo, Japan) was used to explore the HA decomposition mechanism. The ranges emission and excitation wavelengths were from 280 to 550 nm and 200 to 400 nm, separately, and their corresponding slits were 10 and 5 nm, respectively.

3. Results and Discussion

3.1. Study on HA Removal by UV/SPB System

3.1.1. Comparison of HA Removal Performance during Different Systems

The removal of HA was firstly investigated in three processes, which were UV, SPB, and UV/SPB, as seen in Figure 1. The experimental conditions were as follows: HA concentration of 10 mg·L⁻¹, SPB dose of 1 mmol·L⁻¹, and initial pH 3. The HA was hardly removed, and the removal ratio was 0.5% after 60 min by the only UV process. The elimination of HA by the sole SPB process was also insignificant, and its decolorization ratio was 1.5% at 60 min. The decontamination efficiency by the UV/SPB process was 88.8%, which increased more significantly than the other two processes. Besides, with the same molecular weight of H₂O₂, the HA removal by the UV/H₂O₂ was conducted, and its elimination ratio only achieved 40.2% after 60 min. SPB is often used for in situ chemical oxidation, and SPB has the same function as H₂O₂ when SPB was dissolved in water (Equation (3)) [12]. In the sole SPB system, although H₂O₂ was generated in the system, it cannot be activated to produce ·OH, so HA was scarcely removed. In the UV/SPB system, H₂O₂ released from SPB can form ·OH after being irradiated by UV (Equation (4)) [14], which could oxidize and destroy functional groups in HA molecular structure.

$${\mathrm{NaBO}}_3{+\mathrm{H}}_2\mathrm{O} \to { \mathrm{NaBO}}_2+{\mathrm{H}}_2{\mathrm{O}}_2$$

(3)

$${\mathrm{H}}_2{\mathrm{O}}_2+hv \to 2·\mathrm{OH}$$

(4)

3.1.2. Effect of HA Concentration

The influence of HA concentration on the removal of HA by the UV/SPB system is shown in Figure 2a. The experimental conditions were SPB amount of 1 mmol L⁻¹ and initial pH 3. The removal ratio decreased as the initial concentration of HA was increased. As the HA concentration was raised from 5 to 15 mg L⁻¹, the removal ration diminished from 89.8% to 70.8% after 60 min. It proved that the reactive substances formed during the UV/SPB process were persistently consumed because the active species generated in the system were insufficient to oxidize more and more contaminants in the solution, and the competition among HA molecules and reactive species became increasingly fierce with the gradual rise of HA concentration. Moreover, the HA amount enhancement would absorb more UV radiation [23], thereby inhibiting the activation of H₂O₂ and subsequently formation of ·OH radicals, which led to the decrease in HA removal.

3.1.3. Influence of SPB Dose

The SPB is the source of active radicals which is significant for the synergetic process. The influence of SPB dosage on the HA elimination was investigated (Figure 2b). The experimental conditions were HA concentration of 10 mg L⁻¹ and initial pH 3. The HA removal ratio enhanced from 53.0% to 88.8% as the SPB dosage was raised from 0.25 to 1.0 mmol L⁻¹ after 60 min. With the increase of SPB concentration, the amount of active substances produced in the system was enhanced, which promoted the HA removal. However, excessive SPB would scavenge the ·OH and generate peroxy hydroxyl radical HO₂∙ (Equation (5)) [24]. The redox potential of HO₂∙ is lower than that of ·OH. Therefore, the superfluous SPB (2.0 mmol L⁻¹) resulted in the decline of HA removal.

$$·{\mathrm{OH}+\mathrm{H}}_2{\mathrm{O}}_2 \to { \mathrm{H}}_2{\mathrm{O}+\mathrm{HO}}_2^{·}$$

(5)

3.1.4. Influence of Initial pH

Figure 2c shows the effect of different beginning pH on the HA removal during the UV/SPB treatment. The experimental conditions were HA concentration of 10 mg L⁻¹ and SPB amount of 1 mmol L⁻¹. The HA removal decreased from 88.8% to 58.4% after 60 min while the pH value was elevated from 3 to 11. The HA molecule is neutral under strong acidic conditions, so its photochemical activity is stronger than those during neutral and basic circumstances. The redox potential E_{·OH, H2O} is also affected by pH. With the increase of pH from 3 to 11, the redox of E_{·OH, H2O} is declined from 2.62 V (pH = 3) to 2.15 V (pH = 11) [25]. Under the alkaline condition, the ·OH would be transformed to O·⁻ (E = 1.78 V) via reaction (Equation (6)) [26], which had a lower oxidation capacity than that of ·OH. As the pH increased to 11, the predominant form of H₂O₂ is changed to HO₂⁻, which has a higher rate constant with ·OH (k = 7.5 × 10⁹ M⁻¹ s⁻¹, Equation (7)) than the reaction of H₂O₂ and ·OH (k = 2.7 × 10⁷ M⁻¹ s⁻¹, Equation (8)) [27], leading to the consumption of ·OH.

$${\mathrm{OH}}^{-}+·\mathrm{OH}\to {\mathrm{H}}_2{\mathrm{O}+\mathrm{O}}^{-}$$

(6)

$${\mathrm{HO}}_2^{-}+·\mathrm{OH} \to { \mathrm{OH}}^{-}{+\mathrm{HO}}_2^{·} \mathrm{k}=7.5 \times {10}^9 {\mathrm{M}}^{-1} {\mathrm{s}}^{-1}$$

(7)

$$·{\mathrm{OH}+\mathrm{H}}_2{\mathrm{O}}_2 \to { \mathrm{H}}_2{\mathrm{O}+\mathrm{HO}}_2^{·} \mathrm{k}=2.7 \times {10}^7 {\mathrm{M}}^{-1}$$

(8)

3.1.5. HA Removal in Different Water Bodies

Figure 2d indicates the HA removal in various water bodies by the UV/SPB system. After 60 min reaction, the removal ratios of HA during deionized water, tap water, and lake water were 88.8%, 59.6%, and 47.5%, separately. It showed that the lake water and tap water inhibited the removal of HA. The reasons were as follows. Firstly, the other NOM in lake would compete with ·OH generated by UV/SPB. Secondly, the presence of various anions in the lake water and tap water that could inhibit the activity of the oxidative species, leading to a lower efficiency of HA elimination.

3.1.6. Effect of Common Anions in Water

Figure 3 demonstrates the effect of common anions such as CO₃²⁻, HCO₃⁻, Cl⁻, NO₃⁻, SO₄²⁻, and H₂PO₄⁻ for the HA elimination by the UV/SPB. In Figure 3a, when the CO₃²⁻ amount was increased from 1 to 10 mmol L⁻¹, the removal efficiency decreased from 63.7% to 44.9%. The reason was that ·OH yielded in the system was consumed by CO₃²⁻ to produce CO₃·⁻ with weak oxidation capacity (Equation (9)) [28], leading to the decrease of HA removal.

$$\mathrm{OH}·{+\mathrm{CO}}_3^{2-}\to {\mathrm{CO}}_3^{·-}{+\mathrm{OH}}^{-} \mathrm{k}=4.2 \times {10}^8 {\mathrm{M}}^{-1}·{\mathrm{s}}^{-1}$$

(9)

As shown in Figure 3b, with the increase of HCO₃⁻ concentration (1 to 10 mmol L⁻¹), the HA elimination efficiency decreased gradually (74.2% to 53.5%) at 60 min. HCO₃⁻ also consumed ·OH produced in the system (Equation (10)). Besides, the solution pH would be increased after the HCO₃⁻ addition [29].

$$·{\mathrm{OH}+\mathrm{HCO}}_3^{-} \to { \mathrm{H}}_2{\mathrm{O}+\mathrm{CO}}_3^{·-} \mathrm{k}=4.2 \times {10}^8 {\mathrm{M}}^{-1}·{\mathrm{s}}^{-1}$$

(10)

In Figure 3c, as the Cl⁻ was enhanced from 1 to 30 mmol L⁻¹, the removal of HA decreased from 84.1% to 79.9%. Excessive Cl⁻ would consume ·OH and form chlorine species (Equations (11) and (12)). The reduction in oxidation capacity led to a slight decrease in HA removal [30].

$$\mathrm{OH}·{+\mathrm{Cl}}^{-} \to { \mathrm{ClOH}}^{·-} \mathrm{k}=4.3 \times {10}^9 {\mathrm{M}}^{-1}·{\mathrm{s}}^{-1}$$

(11)

$${\mathrm{Cl}}^{-}{+\mathrm{Cl}}^{-} \to { \mathrm{Cl}}_2^{·-} \mathrm{k}=8 \times {10}^9 {\mathrm{M}}^{-1}·{\mathrm{s}}^{-1}$$

(12)

The more greater the addition of NO₃⁻, the lower HA removal ratio was observed in Figure 3d. While 20 mmol L⁻¹ NO₃⁻ was added, the HA removal was decreased to 33.4%. UV activated NO₃⁻ can yield free radical NO₂· (E₀ = 0.867 V), which has lower oxidation ability and would be annihilated during the UV/SPB (Equations (13)–(15)) [31]. In addition, NO₃⁻ could also directly consume ·OH (Equation (16)) [32].

$${\mathrm{NO}}_3^{-}+hv \to { \mathrm{NO}}_2^{·}{+\mathrm{O}}^{·-}$$

(13)

$${\mathrm{NO}}_3^{-}+hv \to { \mathrm{NO}}_2^{-}+\mathrm{O}$$

(14)

$${2\mathrm{NO}}_2^{·}{+\mathrm{H}}_2\mathrm{O}\to {\mathrm{NO}}_3^{-}{+\mathrm{NO}}_2^{-}{+2\mathrm{H}}^{+}$$

(15)

$$·{\mathrm{OH}+\mathrm{NO}}_3^{-} \to { \mathrm{NO}}_3^{·}{+\mathrm{OH}}^{-} \mathrm{k}=4.0 \times {10}^5 {\mathrm{M}}^{-1}·{\mathrm{s}}^{-1}$$

(16)

Figure 3e shows that the increase in SO₄²⁻ concentration did not affect the HA removal effect. When the SO₄²⁻ dosage enhanced to 20 mmol L⁻¹, the HA removal efficiency could reach 88.0%. According to the literature, SO₄²⁻ does not react with active species generated in the system [33,34], so it does not affect the HA removal remarkably.

As presented in Figure 3f, the HA elimination changed insignificantly with the addition of H₂PO₄⁻. When the H₂PO₄⁻ amount enhanced from 10 to 30 mmol L⁻¹, the HA elimination efficiencies were diminished from 86.3% to 82.4%. Although H₂PO₄⁻ can react with ·OH· to form hydrogen phosphate radical (HPO₄⁻) (Equation (17)) [35], the reaction rate is too low, which could not obviously impact the HA removal.

$$·{\mathrm{OH}+\mathrm{H}}_2{\mathrm{PO}}_4^{-} \to { \mathrm{HPO}}_4^{·-}{+ \mathrm{H}}_2\mathrm{O} \mathrm{k} = 2 \times {10}^4 {\mathrm{M}}^{-1}·{\mathrm{s}}^{-1}$$

(17)

3.2. Mechanism of HA Removal by UV/SPB

3.2.1. Scavenging Test

TBA could scavenge ·OH during oxidation process (k_{TBA, ·OH} = 3.8–7.6 × 10⁸ M⁻¹·s⁻¹) [36]. Figure 4 demonstrates the influence of TBA addition on the HA removal in the UV/SPB. The experimental conditions were HA concentration of 10 mg L⁻¹, SPB dose of 1 mmol L⁻¹, and initial pH 3. As can be seen from the figure, adding TBA obviously restrained the HA elimination, which diminished from 16.5% to 11.5% with the addition of TBA enhanced from 0.05 to 0.5 mol L⁻¹. It testified that the main oxidizing species in the synergistic system could be ·OH. The elimination of HA was not completely inhibited after adding TBA, and this could be due to the fact that HO₂∙ produced by decomposition of H₂O₂ could also generate other reactive radicals, such as superoxide anion radicals (O₂^·−) and single oxygen (¹O₂) (Equations (18)–(22)) [37,38], which also have a certain oxidation capacity and cannot be entirely suppressed by TBA.

$${\mathrm{H}}_2{\mathrm{O}}_2\to 2·\mathrm{OH}$$

(18)

$$·{\mathrm{OH} + \mathrm{H}}_2{\mathrm{O}}_2 \to { \mathrm{HO}}_2^{·}+{\mathrm{H}}_2\mathrm{O}$$

(19)

$${\mathrm{HO}}_2^{·} \to { \mathrm{O}}_2^{·-}{+\mathrm{H}}^{+}$$

(20)

$$·{\mathrm{OH} + \mathrm{O}}_2^{·-} \to {{ }^1\mathrm{O}}_2{+\mathrm{OH}}^{-}$$

(21)

$${2\mathrm{O}}_2^{·-}{ + 2\mathrm{H}}^{+} \to {{ }^1\mathrm{O}}_2{+\mathrm{H}}_2{\mathrm{O}}_2$$

(22)

3.2.2. HA Decomposition Mechanism

The absorbance ratios (A₂₅₃/A₂₀₃, A₂₅₀/A₃₆₅, A₂₅₄/A₄₃₆ and A₄₆₅/A₆₆₅) could reflect the transformation of HA molecular structure [39]. The evolution of these ratios in the UV/SPB system is shown in Figure 5a. With the increase in reaction time, the value of A₂₅₃/A₂₀₃ decreased from 0.98 to 0.44, indicating that the stability of functional groups (such as carboxyl and carbonyl groups) in HA aromatic structure gradually diminished. The increase of A₂₅₀/A₃₆₅ value from 2.42 to 3.20 suggested the decrease in the HA molecular weight. The A₂₅₄/A₄₃₆ increased from 4.63 to 5.60, demonstrating that the HA chromophore was destroyed. The value of A₄₆₅/A₆₆₅ decreased from 3.5 to 1.0 and proved the destruction of aromaticity in HA.

The UV-visible absorption spectrum can also manifest the changes of HA molecular structure. Figure 5b shows that the variation of the absorption spectrum of HA in the UV/SPB system over time. The absorption peak of HA at 200–250 nm weakened over time, indicating that ·OH generated in the UV/SPB system destroyed the chromophore groups and double bond structure of HA, and the unsaturated ketone was also oxidized. Moreover, the absorption peak moved to the direction of short wavelength, which was the phenomenon of blue shift. This demonstrated that the substitution reaction of a carbon atom occurred at the carbonylic group in the HA chromophore [40]. HA and fulvic acid are composed of linearly thickened aromatic rings and unsaturated carbons. When the carbon atom of chromophore, such as carbonyl, is substituted by some substituent groups, the absorption peak would shift to the short wavelength [41].

Specific ultraviolet absorbances (SUVA₂₅₄, SUVA₂₈₀, SUVA₃₆₅, and SUVA₄₃₆) were generally selected to characterize the degradation and mineralization for NOM. SUVA₂₅₄ reflects the molecular weight, SUVA₂₈₀ corresponds to the integrity of aromatic structure, SUVA₃₆₅ indicates the molecular volume, and SUVA₄₃₆ represents the chromophore situation in NOM [39]. It can be seen from Figure 5c that the decrease in the SUVA₂₅₄ and SUVA₂₈₀ values over time demonstrated that the molecular weight of organic compounds diminished, and the original aromatic structure was destructed after 60 min treatment in the UV/SPB. The decline of SUVA₃₆₅ indicated that the volume of organic molecules reduced as the reaction progressed. The decreasing value of SUVA₄₃₆ proved that the functional groups and chromophores were destructed by various reactive species. In addition, TOC in the system was lessened from 7.139 to 2.440 mg L⁻¹, and the mineralization efficiency reached 65.8%, demonstrating that most of HA had been mineralized into H₂O and CO₂. The results of UV spectrum and TOC proved that the UV/SPB synergistic treatment can effectively decompose the complex molecular structure of HA.

3D-EEM was used to further explore the degradation mechanism of HA in the UV/SPB system, and the results are shown in Figure 6. The scanning spectrum was divided into five regions because of the complexity of spectral response and scanning sample. The range of I and II can represent aromatic proteins in organic compounds which related to the structure of aromatic ring amino acids in NOM [42]. III region indicates fulminate-like substances related to hydroxyl and carboxyl groups in the humus structure. The range of region IV corresponds to the small molecular structure of organic matter [35]. The V region indicates to humic-like fluorescence [42,43]. It is clear that the fluorescence intensity of the five regions all diminished and gradually disappeared from 0 min (Figure 6a) to 15 min (Figure 6b) and 60 min (Figure 6c), which further proved that the HA molecular structure was decomposed and mineralized in this collaborative system.

4. Conclusions

In this study, the UV/SPB synergistic system was established to remove HA from water, and the experimental results demonstrated that the UV/SPB process can effectively decompose HA. Under the experimental conditions of 10 mg L⁻¹ HA concentration, 1 mmol L⁻¹ SPB dosage, and beginning pH 3, the HA removal efficiency reached 88.8% after 60 min treatment. The HA removals in lake water and tap water were significantly lower than that in deionized water. The anion effect experiments confirmed that except for Cl⁻, SO₄²⁻, and H₂PO₄⁻, the CO₃²⁻, HCO₃⁻, and NO₃⁻ inhibited the removal of HA to different degrees. The ·OH generated by SPB activation was suggested to be the main active substance for HA removal by masking experiments. The results of UV-vis spectrum, absorbance ratio, specific ultraviolet absorbance, and 3D-EEM jointly proved that the synergistic system could effectively degrade and mineralize HA in water.