Base-activated Persulfate Research Articles

Silver recovery from silicon solar cells waste by hydrometallurgical and electrochemical technique

In this study, hydrometallurgical and electrochemical methods were combined to achieve an innovative strategy for the effective recovery of the finest silver metal from silicon solar waste. The waste was thoroughly characterized by X-Ray diffraction, Scanning Electron Microscopy, X-Ray Absorption Spectroscopy, and Inductively Coupled Plasma Optical Emission Spectroscopy. A chemometric approach based on experimental design was used to find the best conditions for the leaching process based on a combined base-activated persulfate and ammonia system while a novel method known as electrodeposition-redox replacement was used to recover it. A remarkable pure silver recovery of 98.7±1.4 % was achieved. Additionally, Scanning Electron Microscopy analysis confirmed the enrichment of Ag particles on the electrode. Overall, these promising results showed how flexible the electrodeposition-redox replacement approach is in producing a range of valuable functional materials from intricate hydrometallurgical solutions including multiple metal impurities.

Base-activated persulfate strategy for ceramic membrane cleaning after treatment of natural surface water

The natural organic matters (NOM) have been proved to form the hydraulically irreversible membrane fouling, which limit the application of the membrane in water treatment processes. To solve the membrane fouling caused by NOM, a novel one-step chemical cleaning method for the ceramic membrane derived from the base-activated persulfate oxidation process was developed in this study. When the SiC ceramic membranes was used to filtrate practical river water, the permeate flux dropped 65% as ΔTMP reached 30 kPa. After 2 h immersion of the fouled membrane in the cleaning solution (2 mM PMS and 8 mM NaOH), the flux recovery rate nearly reached 100%. Direct backwash of the membrane by the above cleaning solution further reduced the cleaning time to 25 mins, while achieving similar cleaning effect. Then the cleaned membranes were immersed in NaClO solution to release all the residual foulants, but the fluorescence excitation-emission matrix spectra (EEM) and total organic carbon (TOC) of the solution were found insignificant, which demonstrated the base-activated persulfate strategy had effectively removed the irreversible membrane fouling. Free radical quenching experiments were carried out and·O2- was identified as the dominant ROS responsible for the removal of the membrane foulants. The permeate flux and filtration efficiency of the SiC membrane were stable after 5 consecutive fouling-cleaning cycles, and the membrane structure was not damaged. In summary, the base-activated persulfate strategy is an efficient and cleaner method for SiC membrane cleaning, which is potential to be applied in practical membrane water treatment process.

Monitoring of in-situ chemical oxidation for remediation of diesel-contaminated soil with electrical resistivity tomography

In-situ chemical oxidation (ISCO) with persulfate, an electrically conductive oxidant, provides a powerful signal for noninvasive geophysical techniques to characterize the remediation process of hydrocarbon contaminants. In this study, remediation with ISCO is conducted in laboratory sandboxes to evaluate the ability of electrical resistivity tomography (ERT) for monitoring the base-activated persulfate remediation process of diesel-contaminated soil. It was found that the resistivity of contaminated sand significantly decreased from 846 Ω·m to below 10 Ω·m after persulfate injection, and all measured chemical parameters showed a noticeable increase. Natural degradation and contamination plume migration were not evident in a reference sandbox without treatment. The area with a resistivity ratio < 0.95 based on imaging before and after injection indicated downward migration of the oxidation plume due to density-driven flow. A comparison between remediation and reference sandboxes showed that the observed resistivity decrease can be due to both contaminant degradation as well as the oxidation plume itself in the contaminated source zone. In contrast, the resistivity decrease in the area with low contamination concentration is attributed to the oxidation plume alone. The derived relationships between resistivity and contaminant indicators further emphasize that the contribution of contaminant consumption to resistivity change in the source area is 25.6%, while it is <16% in the low or non-contaminated area. Although this study showed that resistivity is not solely affected by the chemical transformation of diesel components, it can be combined with sampling data to allow an assessment of the effectiveness of ISCO treatment and to identify target areas for subsequent treatment.

Potential of the base-activated persulfate for polymer-plugging removal in low temperature reservoirs

Conventional polymer-plugging removers perform badly in low temperature reservoirs and this has prompted researchers to look for more effective removers. In this work, base-activated sodium persulfate (PDS) was used to degrade polymer-plugging. The performance of the novel remover was first compared with that of conventional removers at 35 °C. Then, the degradation mechanism involved and factors of influence were investigated by carrying out radical scavenger and single-factor experiments, respectively. Finally, corrosion tests were performed and the ability of the new remover to remove plugging in porous media was investigated by carrying out flooding tests. The results show that the PDS/NaOH system is a much more potent remover at temperatures as low as 35 °C (giving a degradation efficiency of 100% compared to 10–70% using conventional removers). Furthermore, the radical scavenger experiments show that HO· is the predominant radical responsible for polymer gel degradation. The degradation efficiency is affected by the concentrations of the PDS and NaOH solutions, the temperature, and also by the salinity of the polymer gel. We also demonstrate that the polymer gel can be completely degraded when its salinity is 30,000 mg/L by optimizing the temperature and concentration of PDS and NaOH. The corrosion rate using PDS/NaOH was found to be 1.085 g m−2 h−1 which satisfies the requirements for entry well fluid. In addition, the static plugging degradation and dynamic plugging removal tests show that PDS/NaOH can degrade the plugging effectively and recover the permeability of the sandpack to over 65%. Our results suggest that the PDS/NaOH system is a highly promising plugging remover for use in low-temperature polymer flooding reservoirs and is a good replacement for the ones conventionally used.

In-situ chemical oxidation of chlorendic acid by persulfate: Elucidation of the roles of adsorption and oxidation on chlorendic acid removal

The oxidation of chlorendic acid (CA), a polychlorinated recalcitrant contaminant, by heat-, mineral-, and base-activated persulfate was investigated. In pH 3–12 homogeneous (i.e., solid-free) solutions, CA was oxidized by •OH and SO4•- radicals, resulting in a nearly stoichiometric production of Cl−. The rate constants for the reaction between these radicals and CA were measured at different temperatures by electron pulse radiolysis, and were found to be kOH = (8.71 ± 0.17) × 107 M−1s−1 and kSO4 = (6.57 ± 0.83) × 107 M−1s−1 at 24.5 °C for •OH and SO4•-, respectively. CA was oxidized at much slower rates in solutions containing iron oxyhydroxide or aquifer soils, partially due to the adsorption of CA on these solids. To gain further insight into the effect of solids during in-situ remediation of CA, the adsorption of CA onto iron (hydr)oxide, manganese dioxide, silica, alumina, and aquifer soils was investigated. The fraction of CA that was adsorbed on these materials increased as the solution pH decreased. Given that the solution pH can decrease dramatically in persulfate-based remedial systems, adsorption may reduce the ability of persulfate to oxidize CA. Overall, the results of this study provide important information about how persulfate can be used to remediate CA-contaminated sites. The results also indicate that the groundwater pH and geology of the subsurface can have a significant influence on the mobility of CA.

Persulfate oxidizing system for biomass pretreatment and process optimization

The functions of radicals in the pretreatment of lignocellulose remain a fascinating topic about which there are more questions than answers. The effect of persulfate-based radical species on lignin degradation and carbohydrate yield was investigated. Wheat straw was pretreated with three persulfate activation systems which generated OH•, SO42−•, and O2•−. Radical species were monitored using nitrobenzene, anisole, and hexachloroethane, respectively. Pretreatment variables affecting sugar yield were optimized by using the response surface method based on central composite design. Enzymatic hydrolysis and compositional analysis were performed to quantify changes in biomass composition, lignin removal, and glucan yield. Fourier transmission infrared spectroscopic and scanning electron microscopy were used to characterize the effect of pretreatment on biomass structure. The maximum sugar yields were 66.8%, 49.4%, and 41.7% in base, heat, and hydrogen peroxide activated persulfate, separately, while untreated material showed only 3.7% glucan yield. The radical analysis confirmed the existence of radicals in these three systems. The results from compositional analysis and Fourier transmission infrared spectroscopic provided proof that parts of lignin and hemicellulose were degraded after the treatment. Scanning electron microscopy indicted that radical pretreatment converted the smooth surface of wheat straw to that of scattered, uneven morphology. Base-activated persulfate, capable of producing sulfate radical, hydroxyl radical, and superoxide anion radical was demonstrated as an efficient radical system to treat biomass.

Highly efficient and selective leaching of silver from electronic scrap in the base-activated persulfate – ammonia system

A system composed of persulfate salt and ammonia in highly alkaline aqueous solution is developed and examined for leaching metallic silver from elements of the electronic waste materials (e-scrap). Strong base activates persulfate ions providing in situ generation of highly reactive oxygen molecules. The oxidized metal forms then well soluble complex ions with ammonia ligands. The kinetic studies of the leaching process were performed for pure metallic silver. They revealed that the efficiency of the process is affected by the type of the persulfate salt. By employing potassium persulfate one obtains significantly (more than 50% for silver plates and more than 100% for silver powder) increased efficiency of silver dissolution compared to the solution composed of either sodium or ammonium persulfates. In the range of persulfate concentrations between 0.02 and 0.23mol/L the apparent reaction order with respect to the persulfate concentration was similar for all persulfate salts and was estimated to be around 0.5. The room temperature (22±2°C) seems to be an optimal temperature for the leaching process. An increase in the temperature resulted in the significant drop in the silver dissolution rate due to the decreased solubility of oxygen. Based on these results a possible mechanism of dissolving silver is discussed and the optimal composition of the leaching solution is formulated. The obtained formulation of the leaching solution was applied for the extraction of silver coatings of Cu-based e-waste scrap and the obtained results revealed an important effect of copper in the mechanism of the leaching process. The regression analysis of the leaching curve indicated that each gram of base-activated potassium persulfate under the specified conditions may leach almost 100mg of silver coatings in a form of well soluble diamminesilver (I) complex. The silver complex can be relatively easy reduced to metallic silver. The method developed is relatively cheap, low toxic and does not produce harmful by-products.

Highly efficient persulfate oxidation process activated with Fe0 aggregate for decolorization of reactive azo dye Remazol Golden Yellow

The commercially available Fe0 aggregate has advantages of low-cost, fast-effective decontamination, reusability, and ease of operation. However, little study has been done on the performance of Fe0 aggregate as a catalyst in degradation of azo dyes, particularly used in persulfate (PS) oxidation process. This study investigated decolorization of a reactive azo dye, Remazol Golden Yellow (RGY, Reactive Orange 107), by persulfate oxidation activated with Fe0 aggregate. RGY decolorization was not effective in ultrasound-activated, heat-activated, and base-activated persulfate oxidation; however, a significant decolorization improvement was achieved by applying Fe0 aggregate to activated persulfate (PS/Fe0). Decolorization was strongly influenced by pH, Fe0, persulfate dosages and temperature. The suitable conditions for RGY decolorization were pH 6.0, PS 5×10–3M, and Fe0 0.5g/L. This condition yields 98% color removal of 100mg/L RGY solution within 20min treatment; the azo bonds of RGY were completely broken down. RGY decolorization followed the first-order kinetics. Activation energy of the PS/Fe0 system was 0.479kJ/mol, suggesting the temperature dependence of RGY decolorization is small. The presence of inorganic salt in RGY solution had an adverse effect on decolorization. The inhibitory effect of various inorganic salts on decolorization followed the sequence of Na2HPO4≫NaHCO3≫NaClO4>NaCl>NaNO3>NaClO4>no salt. The Fe0 aggregate was reusable and a satisfactory decolorization efficiency was achieved with the repeated use of Fe0 for five times. The PS/Fe0 process provides an efficiency and effective technology for RGY decolorization.

Oxidative degradation of levofloxacin in aqueous solution by S2O82−/Fe2+, S2O82−/H2O2 and S2O82−/OH− processes: A comparative study

The performance of levofloxacin (LFX) degradation in aqueous solutions in ferrous ion-activated persulfate (S2O82−/Fe2+), peroxide-activated persulfate (S2O82−/H2O2), and base-activated persulfate (S2O82−/OH−) systems was evaluated and compared. The LFX degradation by all studied activated persulfate systems was divided into two oxidation periods: a fast degradation of target compound within the first minute and subsequent gradual oxidation within the remaining reaction time. Notably, without consideration of the first minute, the rest of the LFX degradation in the S2O82−/Fe2+ and S2O82−/H2O2 system followed the pseudo-first-order kinetic model. Among the studied activation techniques, the Fe2+-activated persulfate system demonstrated the highest efficacy in LFX degradation, mineralization and persulfate utilization, followed by the peroxide-activated persulfate oxidation. Generally, all studied sulfate radical-based advanced oxidation technologies proved to be promising tool for the treatment of LFX contaminated water/wastewater and, especially, groundwater.

Oxidative degradation of p-chloroaniline by copper oxidate activated persulfate

Oxidation of p-chloroaniline (PCA) by persulfate (PS) performed with a novel supported copper oxidate catalyst in an aqueous solution at ambient temperature (i.e. 20°C) was investigated in this study. This study focused mainly on determining the proportions of heterogeneous catalysis in the copper oxidate/PS combined system. There existed a more remarkable effect on the degradation of PCA in the copper oxidate/PS combined system than in the Cu2+/PS or only PS system. The effects of copper oxidate dosage, persulfate concentration, and initial solution pH on the oxidation of PCA were also evaluated. Higher copper oxidate dosage and persulfate concentration resulted in higher PCA degrading rates, the optimal initial pH was determined as 7.0. Moreover, the change in the degradation of PCA by pH was also investigated in terms of the contribution of dissolved copper ion in leaching solution. We inferred that homogeneous catalysis was of increasing importance and the copper ion dissolved from the copper oxidate was regarded as the key factor activating the persulfate under acidic conditions (pH 3.0), heterogeneous catalysis played the main role in the oxidation of PCA at pH 5–7. However, both heterogeneous catalysis and base-activated persulfate contributed to the degradation of PCA under alkaline conditions (pH 11). In addition, the radical mechanism was studied and three radical scavengers (phenol, methanol (MA) and Tert-butanol (TBA)) were used to determine the kind of major active areas taking part in the PCA degradation at pH 7.

Effect of Basicity on Persulfate Reactivity

The generation of reactive species in activated persulfate systems under different conditions of basicity was investigated using three probe compounds. Anisole was used to detect both sulfate radical and hydroxyl radical. Nitrobenzene was used to detect hydroxyl radical, and hexachloroethane was used as a reductant probe. Minimal probe compound degradation occurred in persulfate reactions conducted at pH≤10, demonstrating that a low flux of reactants is generated at acidic, neutral, and slightly basic pH regimes. In persulfate reactions at pH 12, sulfate and hydroxyl radical were generated but minimal reductants were produced. Scavenging studies showed that the dominant reactive species at basic pH was hydroxyl radical. The generation of reductants increased at high molar ratios of base to persulfate; however, hydroxyl radical generation rates increased only when molar ratios of base to persulfate were >3∶1. The results of this research demonstrate that the hydroxyl radical is the dominant reactive oxygen species in base-activated persulfate formulations and that overall reactivity increases with increasing base:persulfate ratios.

Mechanism of Base Activation of Persulfate

Base is the most commonly used activator of persulfate for the treatment of contaminated groundwater by in situ chemical oxidation (ISCO). A mechanism for the base activation of persulfate is proposed involving the base-catalyzed hydrolysis of persulfate to hydroperoxide anion and sulfate followed by the reduction of another persulfate molecule by hydroperoxide. Reduction by hydroperoxide decomposes persulfate into sulfate radical and sulfate anion, and hydroperoxide is oxidized to superoxide. The base-catalyzed hydrolysis of persulfate was supported by kinetic analyses of persulfate decomposition at various base:persulfate molar ratios and an increased rate of persulfate decomposition in D(2)O vs H(2)O. Stoichiometric analyses confirmed that hydroperoxide reacts with persulfate in a 1:1 molar ratio. Addition of hydroperoxide to basic persulfate systems resulted in rapid decomposition of the hydroperoxide and persulfate and decomposition of the superoxide probe hexachloroethane. The presence of superoxide was confirmed with scavenging by Cu(II). Electron spin resonance spectroscopy confirmed the generation of sulfate radical, hydroxyl radical, and superoxide. The results of this research are consistent with the widespread reactivity reported for base-activated persulfate when it is used for ISCO.

Effect of sorption on contaminant oxidation in activated persulfate systems

Sorption of hydrophobic contaminants to soils and subsurface solids is a significant limitation for the in situ chemical oxidation (ISCO) remediation of contaminated sites. A recently developed ISCO reagent, activated persulfate, was investigated for its potential to provide enhanced treatment of sorbed contaminants, a phenomenon that has previously been observed in catalyzed H2O2 propagations (CHP; modified Fenton's reagent). Perchloroethylene (PCE) and hexachlorocyclopentadiene (HCCP) were sorbed to diatomaceous earth and treated with CHP, iron (II)–citrate-activated persulfate, and base-activated persulfate. Gas-purge desorption was used to measure the maximum natural rate of desorption of the compounds. Enhanced treatment of both sorbed probe compounds was observed in CHP reactions, with PCE and HCCP destruction occurring more rapidly than their respective rates of gas-purge desorption. However, probe compound loss in both formulations of activated persulfate was equal or less than the rates of gas-purge desorption, indicating that enhanced treatment of the sorbed contaminants did not occur in the persulfate reactions. Activated persulfate treatment is likely ineffective for hydrophobic compounds that are sorbed to soils and subsurface solids, and is probably more effective for compounds that are not limited by desorption.