Removal Of Silica Research Articles

Iminium-Bridged Resorcinol-Silane Networks and Their Pyrolyzed Derivatives as Electrode Materials for the Electrocatalytic Oxygen Reduction Reaction and Supercapacitors.

We report the synthesis of iminium-bridged resorcinol-silane (I-R-S) gel networks through the reaction of resorcinol with 3-aminopropyl triethoxysilane (APTES) in the presence of acetone under ambient conditions. Ambient pressure drying leads to monolithic gels with minimal shrinkage and cracks, eliminating the use of conventional supercritical drying. The gels were found to have bulk and skeletal densities of 0.90 and 1.23 g/cm3, respectively. Interestingly, it was found that acetone plays the dual role of solvent (solubilizing the precursors, APTES, and resorcinol) and reactant by reacting with amine present in APTES to form imine functionalities. Furthermore, the imine was found to react with resorcinol to form I-R-S gel networks. The I-R-S gel was found to be hydrophobic with a contact angle of between 99 and 103°. Upon pyrolysis at higher temperatures (600, 800, and 1000 °C), the I-R-S gels transformed to silica containing N-doped carbon as confirmed by X-ray photoelectron spectroscopy (XPS). Further removal of silica by hydrofluoric acid (HF) treatment lead to porous N-doped carbon which was found to effectively electrocatalyze the oxygen reduction reaction (ORR) by mixed 3e- pathway. Note that the sample pyrolyzed at 600 °C followed by treatment with HF displayed an ∼2e- pathway with a higher peroxide yield of 91%. Due to the high specific surface area and increased microporosity, the pyrolyzed samples treated with HF were found to exhibit a specific capacitance of 213 F/g with capacity retention of 90% for up to 2100 cycles.

(Digital Presentation) Application of Polarity Reversal and Performance Analysis of Continuous Electrocoagulation

An important challenge for continuous treatment by electrocoagulation (EC) is the passivation and fouling of electrodes which increases during the longer operation (Ingelsson et al., 2020). The application of polarity reversal (PR) and treatment performance of EC was investigated using continuous flow reactor for simultaneous removal of silica and hardness (calcium and magnesium) from produced water. Polarity reversal times (PRT) from 30 s to 10 min were studied at a fixed charge loading of 2000 C L- 1 (i.e., the amount of charge passed per unit volume of produced water treated) using Fe and Al electrodes. Periodic PR was found to reduce the fouling and de-passivate the electrodes by changing the surface chemistry at the electrode-electrolyte interface (Ingelsson et al., 2020; Yasri et al., 2022). During 90 minutes of continuous treatment the chronopotentiometric data indicated that a PRT of 10 min was more effective in reducing the cell voltage for both Fe and Al electrodes [Fig. 1 (a & b)]. With a longer PRT of 10 min, there is more time for the acid boundary layer to form on the anode, which mitigates metal precipitation at the anode. In contrast the higher cell voltage that persists in direct current EC (DC-EC) could be due to the precipitation of Ca and Mg minerals in the alkaline solution on the cathode surface, leading to cathode passivation (Chow et al., 2021). After the first polarity reversal, both electrodes have been anodic, removing passivation layers from the surface and reducing the cell voltage. On reversing the polarity, the cathode becomes the anode, and the acidic pH on the formed at the electrode interface will facilitate the dissolution of Ca and Mg precipitates on the electrode surface, reducing the cell voltage significantly. For Fe-EC, the contaminant removal performance increased with PRT, and the highest removal was observed with DC-EC (Fig. 1c). The increasing removal performance with PRT for Fe-EC is consistent with the increase in faradaic efficiency observed of 55%, 68%, 85%, and 99.8% with 30 sec, 2 min and 10 min PRT, and DC respectively. The lower faradaic efficiencies for Fe-EC with short PRTs are likely due to redox reactions of iron species at the electrode surface (Chow et al. 2021).The contaminant removal at a charge loading of 2000 C L–1 using DC-EC with Al electrodes was similar to that for PR-EC with PRTs of 2 to 10 min (Fig. 1d). Slightly lower Si and hardness removal was observed for Al-EC at the shortest PRT of 30 s. For Al-EC, the faradaic efficiency was also observed to increase with PRT, with values of 150%, 220% and 287% obtained at 30 sec, 2 min and 10 min PRT respectively. The super-faradaic (>100%) efficiencies can be explained by dissolution of the Al from the cathode under the local alkaline conditions that develop on the electrode surface. With a short PRT, there is less time for the alkaline pH to develop at the surface and hence the faradaic efficiency was reduced.

PH-sensitive and targeted core-shell and yolk-shell microcarriers for in vitro drug delivery

Recently, biocompatible and high-performance drug carriers have revolutionized drug delivery systems. This study aims to design and develop a slow, targeted delivery system of curcumin (C) and gemcitabine (G) as two types of anticancer drugs. Three-layer shells containing two types of biodegradable polymers i.e. chitosan (CHI) and poly (methacrylic acid-coethyleneglycol dimethacrylate) (ME) were coated on Fe3O4 (MG) and/or MG@SiO2 cores. Silica removal in the next stage produced core-shell and yolk-shell structures, as confirmed via transmission electron microscopy (TEM). The loading and releasing of drugs in phosphate buffer saline (PBS) with pH = 5.5 and pH = 7.4 were examined via UV–Vis spectroscopy. In both types of microparticles, acceptable loading (LE%) and encapsulation (EE%) efficiencies were obtained. Nevertheless, the LE% values of yolk-shell microparticles (H-MG@CHI.ME.CHI) were significantly higher than those of the core-shell microparticles (MG@CHI.ME.CHI). This higher loading capacity arises from the presence of cavities in the yolk-shell structure. Both microcapsules showed a release rate of over 80% for both drugs at acidic pH, while the release of drugs was relatively lower in the neutral environment. Further, the cytotoxic effect of microparticles, determined through the MTT method, was consistent with the results of drug release.

Removal of lignin and silica from rice straw for enhanced accessibility of holocellulose for the production of high-value chemicals

The intricate nature and rigidity of rice straw, particularly the presence of lignin and silica, hinders the catalytic valorization, consequently decreasing the yield of target products. This study reports the concurrent removal of lignin and silica from rice straw to obtain enriched holocellulose, then transforming it to furfural (FUR) and levulinic acid (LA). Interestingly, rice straw in the form of powder displays an improved removal of lignin (51.0%) and silica (92.0%) during ammonia treatment. Encouragingly, adding organic solvents, such as THF, to the aqueous system during the pretreatment of rice straw improves the lignin removal to 60.0%. Upon improving lignin removal to 60%, the obtained holocellulose enriched solid residue yields 71.0% FUR along with 52.0% LA, which is 8 and 4-fold higher than what is obtained with parent rice straw, signifying the importance and the prerequisite of lignin and silica removal from rice straw.

Silica extraction followed by biogas generation from rice straw: Investigating the impact of pretreatment on purity of silica, biogas yield and microbial diversity along with insights on techno-economic analysis

Valorisation of agriculture waste namely rice straw (RS) for the recovery of value-added products such as high purity silica and biogas has gained tremendous research interest. Presence of silica and lignin limits the utilization of these wastes as potential feedstock for biological treatment. Therefore, this study investigated the impact of alkali (NaOH) pre-treatment at varied concentration of 2%, 4%, 6%, 8%, and 10% (w/v)) on lignin and silica removal at 90 °C so that the silica can be recovered as a value-added product and the lignin free biomass can be utilized as feedstock to estimate the biogas generation at mesophilic temperature (35 ± 2 °C) for 45 days. The impact of pre-treatment was studied and supported by the SEM, FTIR for morphological, and compositional variations while the AD is supported by 16 s rRNA metagenomic sequencing. Results revealed that increasing the concentration of NaOH up to 8% improved silica extraction with a purity of 97.34% with simultaneous lignin removal by 50 – 70%. Pretreatment with 2% NaOH was more effective in facilitating the accessibility of cellulose and hemicellulose to microbes during AD. In summary, this study concludes 2% NaOH treatment is ideal for both silica and biogas recovery.

Promoting the removal of fine particles by surfactants in a novel cyclone with heterogeneous-condensation agglomeration: A combined experimental and molecular dynamics study

Using heterogeneous-condensation agglomeration to enhance removal of fine particles by conventional equipment is an economical and efficient method. Recently proposed Cloud-Air-Purifying (CAP) technology, which combines heterogeneous-condensation agglomeration with a centrifugal field, has significantly improved the removal efficiency of fine particles in gas cyclones. In this work, surfactants were used to further enhance the removal of fine particles. The effect of three surfactants on promoting the removal of fine particles was investigated experimentally. The results indicated that non-ionic surfactant octylphenol polyoxyethylene ether (10) (OP-10) had the best effect on promoting the removal of methylated silica (SiO2-CH3) particles, because it improved heterogeneous condensation of fine particles and collision agglomeration between particles by improving the surface wettability, resulting in the formation of larger aggregates. Furthermore, the interaction energy, mean square displacement and hydrogen bond interactions in different simulation systems were calculated by molecular dynamics simulation to study the mechanism of surfactants on improving the surface wettability. The simulation results demonstrated the hydrophobic tail chains of OP-10 molecules spontaneously adsorbed on the nonpolar SiO2(001)–CH3 surface through the Van der Waals interaction, which changed the surface polarity and reduced the solid–liquid interaction energy. Meanwhile, hydrogen bonds were formed between polar head groups and water molecules, which enhanced the interaction between water molecules and the surface. Moreover, the mobility of water molecules is enhanced by surfactant molecules. The spatial distribution of the head group and tail chain of surfactant is an important factor affecting the surface wettability. This work is of great importance for enhancing the removal of fine particles in the dedusting system with heterogeneous-condensation agglomeration technology.

A novel approach for the synthesis of vaterite and calcite from incense sticks ash waste and their potential for remediation of dyes from aqueous solution

The dumping of incense sticks ash due to no known application of it makes it an emerging solid waste. The presence of toxic heavy metals may categorize them as hazardous materials but the presence of high Calcium and silica has drawn the attention of the value-added minerals recovery. Here, calcium oxides were recovered from the ISA, by minimizing the solid waste. Incense sticks ash contains nearly 49–55% which acts as a potential candidate as a raw material for calcium carbonate. The synthesis of CCMPs, from ISA, involves pre-treatment that includes: the extraction of ferrous particles, and the removal of aluminum and silica. The synthesized CCMPs were characterized by sophisticated instruments for detailed information where UV–Vis spectroscopy (UV–Vis), exhibited absorption peaks for calcium carbonate at 210 nm, while dynamic light scattering (DLS) was shown that the average particle size is 1280 nm. Fourier transform infrared (FTIR) and x-ray diffraction (XRD) confirmed the microcrystalline nature of the particles, where FTIR showed bands in the 1800-700 cm−1 range. Both FESEM and TEM revealed the rhombohedral shape of the CCMPs whose size varied from 0.5 μm to 6 μm and purity was 94–96%. Finally, calcium carbonate particles efficiency was analyzed for the remediation of methyl red dye from the aqueous solution and about 48% dye was removed within 1 h only.

Comparison of various delignification/desilication pre-treatments and indigenous fungal cellulase for improved hydrolysis of paddy straw.

The efficient removal of lignin and silica from paddy straw is essential for its volarization into biofuels and other value-added products. In this work, different chemical pre-treatments viz. acid/alkali, organosolv and deep eutectic solvents were carried out to assess the extent of delignification and desilication of paddy straw. Maximum lignin and silica removal of 96.08 and 95.51% was observed with two step acid (0.5% sulphuric acid) followed by alkali (4% sodium hydroxide) pre-treatment with significantly low total lignin (2.30%) and silica content (0.80%) of the treated straw residue. The treated straw residue contained significantly high holocellulose (91.65%), cellulose (75.01%) and hemicellulose content (16.64%). Among the four indigenous fungal isolates, Penicillium mallochii (JS17) cellulase showed better accessibility for the treated straw residue with maximum release of 504.18mgg-1 of reducing sugars and saccharification efficiency of 56.90%. The two-step acid/alkali pre-treatment of paddy straw was highly effective for removing lignin and silica from paddy straw, thereby, resulting in enhanced enzymatic accessibility of the substrate and more efficient hydrolysis of cellulose into fermentable sugars.

Silica removal at sewage treatment plants causes new silica deficiency

The dissolved silicate (DSi) concentration in coastal waters has decreased due to anthropogenic activities. Many studies have indicated that dam construction is a main reason for this reduction. However, recently, dam construction alone has not been sufficient to explain the DSi reduction in some coastal waters. In this study, we focused on silica removal at sewage treatment plants (STPs). DSi and particulate silica (PSi) concentrations were measured in STP influent and effluent waters from September 2020 to September 2021. Dissolution experiments on PSi were also conducted to estimate the fraction of soluble PSi in the STP influent. DSi and PSi were removed by 29.5% and 96.9%, respectively, at the STP. In addition, the soluble PSi in the STP influent accounted for 20.3% of the PSi removed. Therefore, in addition to the DSi removal in STPs, removal of soluble PSi can also cause potential DSi depletion in downstream and coastal waters. In addition to the effect of dams, the silica supply delivered to coastal waters may be further reduced in the future due to the progress of sewage treatment development in coastal areas.

Contaminants Removal from Wastewater of LFP Batteries Recycling Process Using Adsorption Methods

Wastewater from the LFP battery cathode recycling process still contains metals such as lithium, calcium, sodium, and silica. Adsorption method was used to remove metal ions in the artificial waste of LFP batteries. This experiment was carried out by adding 2, 4, 6, 8, and 10 g of activated carbon, respectively, into 100 mL of an artificial waste solution then put into water-bath shaker at room temperature. For kinetic data, the filtrate was taken every 1 hour, while for equilibrium data the filtrate was taken every 24 hours. The concentrations of the initial solution and the filtrate were analyzed by ICP-OES. The experimental results showed the maximum percentage removal of lithium, calcium, sodium, and silica was at a dose of 100 g L-1 of activated carbon, which were 99.90%, 97.51%, 86.10%, and 79.08%, respectively. It was found that the Freundlich equilibrium model was more suitable for the adsorption isotherm indicated by the correlation coefficients value for lithium, calcium, sodium, and silicon were 0.9869, 0.8850, 0.8974, and 0.9576, respectively. This adsorption followed the Pseudo second order kinetic model derived from correlation coefficient values of chemical kinetic studies.

Effective removal of dissolved silica from white carbon black wastewater by iron electrode electrocoagulation: Process optimization and simulation

The chemical coagulation method treats White Carbon Black (WCB) wastewater with high cost and low silica removal rate. Meanwhile, aluminum electrode is mostly used in the treatment of SiO2-containing wastewater by electrocoagulation (EC), however, the price of aluminum is high despite the effective removal. This work is dedicated to the application of cheap iron electrode to achieve effective removal of dissolved silica from WCB wastewater with low energy consumption. Through system optimization design, the current density (i), plate spacing (l), plate-to-water ratio (n) and electrode passivation (a) were synergistically regulated to improve the SiO2 removal rate of iron. In addition, the electricity, concentration, velocity and temperature field distributions of the silica removal system were investigated by a multiphysical field coupled simulation model. An optimal silica removal rate of 97.4% was obtained with the theoretical iron consumption of 0.251 g Fe/L, adsorption capacity of 660 mg SiO2/(g Fe L) and electricity consumption of 0.312 Wh/L. The simulations found that EC facilitates mass and heat transfer in the cell, while the anode with square holes is more favorable to enhance the iron corrosion rate. Eventually, a new, efficient and simple approach was proposed compared to chemical coagulation for silica removal from WCB wastewater.

Methods of silica removal from pyrometallurgical processing wastes of ilmenite concentrate

This article presents a study on the processing of waste dust from electrical smelting of ilmenite concentrates with the removal of silica from them by alkaline and fluoride methods. The study of the smelting dust leaching by caustic soda solutions included investigation of the effect of sodium hydroxide concentration, process time, temperature, S:L ratio. The optimum conditions of concentrate electric smelting dust leaching - temperature 80-90 °С, duration 90-120 minutes, S:L ratio = 1:5, sodium hydroxide solution concentration 110-115 g/dm3 were determined. The optimum conditions for fluorination of electric melting dust were determined, at which the sublimation degree of silicon fluoride was 84.2 %. Studies have been performed to decomposite obtained silicon-containing sublime in the presence of ammonia agent. The optimum pyrolysis modes that provide the separation of fluoride and silicon oxide - temperature 530-560 °C and duration of 60-80 min have been determined based on the results of thermal analysis and studies on the process duration effect. The silicon oxide content in the obtained product was 96.3%.

Permeability heterogeneity of sandstone intrusion fluid‐escape systems, Panoche Hills, California: Implications for sedimentary basins globally

AbstractNatural surface gas seeps provide a significant input of greenhouse gas emissions into the Earth’s atmosphere and hydrosphere. The gas flux is controlled by the properties of underlying fluid‐escape conduits, which are present within sedimentary basins globally. These conduits permit pressure‐driven fluid flow, hydraulically connecting deeper strata with the Earth’s surface; however they can only be fully resolved at sub‐seismic scale. Here, a novel ‘minus cement and matrix permeability’ method using three‐dimensional X‐ray micro‐computed tomography imaging enables the improved petrophysical linkage of outcrop and sub‐surface data. The methodology is applied to the largest known outcrop of an inactive fluid‐escape system, the Panoche Giant Intrusion Complex in Central California, where samples were collected along transects of the 600 to 800 m stratigraphic depth range to constrain porosity and permeability spatial heterogeneity. The presence of silica cement and clay matrix within the intergranular pores of sand intrusions are the primary control of porosity (17 to 27%) and permeability (≤1 to ca 500 mD) spatial heterogeneity within the outcrop analogue system. Following the digital removal of clay matrix and silica (opal‐CT and quartz) cement derived from the mudstone host strata, the sand intrusions have porosity−permeability ranges of ca 30 to 40% and 103 to 104 mD. These calculations are closely comparable to active sub‐surface systems in sedimentary basins. Field observations revealed that, at decreasing depth, the connected sand intrusion network reduces in thickness and becomes carbonate cemented, terminating at carbonate mounds formed from methane escape at the seafloor. A new conceptual model integrates the pore‐scale calculations and field‐scale observations to highlight the key processes that control sand intrusion permeability, spatially and temporally. The study demonstrates the control of matrix and cement addition on the physical properties of fluid‐escape conduits, which has significance for hydrocarbon reservoir characterization and modelling, as well as subsurface CO2 and energy storage containment assessment.

Investigation of fluoride and silica removal from semiconductor wastewaters with a clean coagulation-ultrafiltration process

Fluoride and silica contamination in water is a worldwide issue due to the wastewater discharge from semiconductor industry. Coagulation-ultrafiltration (UF) process is commonly used to treat semiconductor wastewaters, but it requires excessive amounts of coagulant/flocculant. In this study, a clean coagulation-UF process using chemical mechanical polishing (CMP) wastewater as a coagulant to treat fluoride-containing wastewater was firstly proposed. The fluoride-containing wastewater, CMP wastewater, mixed fluoride-containing wastewater and CMP wastewater, and mixed fluoride-containing wastewater and polyaluminium chloride/polyacrylamide were compared to investigate turbidity removal efficiency, fluoride removal efficiency and membrane fouling resistance. The negatively charged SiO2 particles in CMP wastewater were found to be adsorbed on the positively charged CaF2 particles surface through electrostatic interaction. Results indicated that CMP wastewater provided a superior turbidity removal efficiency and a minimal membrane fouling resistance compared to conventional coagulant. After UF filtration, the fluoride concentration and turbidity were 2.09 mg L−1 and 0 NTU, respectively, which met the median fluoride effluent standard. The extended Derjaguin-Landau-Verwey-Overbeek theory showed that increasing molar ratio of SiO2 to CaF2 improved the interfacial free energy between membrane and suspended particles, thus mitigating membrane fouling. This clean design principle and strategy will broaden the sustainability of coagulation-UF process for wastewater treatment.

One-step catalytic pyrolysis of pre-treated rice straw to biofuel over waste-extracted Ni–H3PW12O40 activated nano-catalyst

ABSTRACT Alternative, renewable and carbon-neutral fuels are an urgent requirement. Pyrolysis has received the most attention in the conversion of biomass into fuel. The main disadvantages of chemical and physical properties of pyrolysis crude bio-oil include its high oxygen content and acidity. Catalytic fast pyrolysis can overcome this problem. Rice straw is an abundant biomass residual that can be of environmental negative impact and is not of proper use. Therefore, this research work introduces the use of this residual in the production of fuel sources by one-step catalytic pyrolysis with the reusability of the employed catalyst. A waste extraction catalyst modified with acidic and nickel sites has been prepared and was utilised for the proposed target in this study. The removal of silica, lignin and minerals by treatment of the rice straw with KOH and then HNO3 in a green terminated process improved the catalytic biomass conversion from 83% to 96% at a catalyst dose of 5%, at 250°C. The pre-treatment of the rice straw enables the reusability of the catalyst in direct catalytic pyrolysis where the employed catalyst exhibits good stability in four consecutive runs. The used KOH and HNO3 were neutralised by mixing to produce KNO3 as a valuable fertiliser.

Electroprecipitation Mechanism Enabling Silica and Hardness Removal through Aluminum-Based Electrocoagulation

Electroprecipitation Mechanism Enabling Silica and Hardness Removal through Aluminum-Based Electrocoagulation

Hollow carbon spheres for diclofenac and venlafaxine adsorption

Silica@polymer spheres with a core@shell structure were synthesized and thermally annealed at 800 ºC to obtain silica@carbon spheres (SiO2@CSs). The silica core was removed by etching with NaOH, hollow carbon spheres (CSs) being obtained. The particle size of SiO2@CSs and CSs increased with the ethanol/water (E/W) volumetric ratio (2, 4.5, and 7) employed in the first synthesis step (i.e., the Stöber's method to obtain silica particles). Moreover, the average diameter of the materials prepared with E/W ratio of 2 was affected by the etching of the SiO2 core (from 168 to 109 nm), in contrast with those synthesized at higher E/W ratios of 4.5 and 7 (251–245 and 270–284 nm, respectively). The specific surface area (SBET) of the CSs ranged from 271 to 602 m2 g−1, which are more porous than SiO2@CSs (SBET in the range 115–144 m2 g−1). Adsorption kinetic and equilibrium studies were carried out with diclofenac and venlafaxine as model organic micropollutants (OMPs). Despite the silica removal was not effective for all the CSs (TGA residue ranging from 3 to 46 wt%), the kinetic studies and the ATR-FTIR spectra confirmed the positive effect of having a hollow core (i.e., removing the silica core). Equilibrium studies demonstrated that CSs prepared with an E/W ratio of 7 were the best performing material when considering both OMPs. Moreover, these CSs performed well to remove a set of 24 OMPs from wastewater effluents and thus, they are an interesting option for water or wastewater treatment.

Treatment Technologies for Organics and Silica Removal in Steam-Assisted Gravity Drainage Produced Water: A Comprehensive Review

Produced water from steam-assisted gravity drainage (SAGD) operations is a complex mixture of inorganic and organic constituents. Silica, carbonate and bicarbonate, and hardness are three inorganic constituents of interest, whereas the organic contaminants are composed of free and emulsified oil (F&EO) and water-soluble organic (WSO) fractions. Although the current warm lime softening (WLS) and hot lime softening (HLS) in SAGD operations are able to remove particles, silica, and organics, improving the removal efficiency of organics and silica still faces technical and operational challenges given their complex chemistry in produced water. Much effort has been put toward developing effective and cost-efficient treatment processes to handle organics and silica. In this work, we have systematically reviewed the properties of silica and the characteristics of dissolved organics in SAGD produced water, as well as the working principle and removal mechanisms of several techniques, including membrane technology, adsorption, coagulation–flocculation, and hybrid processes. Different analytical techniques for characterizing dissolved organics have been assessed, and the dominant and representative organic species in produced water have been identified. The advantages and limitations, impact factors, application performance, and effectiveness of each technology have been discussed in terms of the removal efficiency for silica and organics. Some remaining challenges and perspectives for future research are also presented. This work provides an improved understanding of the characteristics of silica and organics in SAGD produced water, as well as different treatment technologies, with useful implications for developing effective, feasible, and cost-efficient treatment processes with the objective of enhancing the removal of silica and organics from produced water, thereby leading to economic and environmental performance improvements.

Investigation of electrode passivation during electrocoagulation treatment with aluminum electrodes for high silica content produced water.

One of the main challenges for the implementation of electrocoagulation (EC) in water treatment are fouling and passivation of the electrodes, especially for applications with high contaminant concentrations. For the first time, we investigated in this study the process of fouling mitigation by polarity reversal during the EC treatment of boiler blowdown water from oil-sands produced water, characterized by high silica concentrations (0.5-4 g L-1). This effluent is typically obtained from an evaporative desalination process in oil production industries. Potentiodynamic characterisation was used to study the impact of passivation on the anode dissolution. Although a charge loading of 4,800 C L-1 was found to remove about 98% of silica from a 1 L batch of 4 g L-1 Si solution, fouling reduced the performance significantly to about 40% in consecutive cycles of direct current EC (DC-EC) treatment. Periodic polarity reversal (PR) was found to reduce the amount of electrode fouling. Decreasing the polarity period from 60 to 10 s led to the formation of a soft powdery fouling layer that was easily removed from the electrodes. In contrast, with DC operation, a hard scale deposit was observed. The presence of organics in the field samples did not significantly affect the Si removal, and organics with high levels of oxygen and sulfate groups were preferentially removed. Detailed electrochemical and economic investigations suggest that the process operating at 85 °C achieves 95% silica removal (from an initial concentration of 481 mg L-1) with an electrical energy requirement of 0.52 kWh m-3, based on a charge loading of 1,200 C L-1, an inter-electrode gap of 1.8 cm and a current density of 16 mA cm-2.

Electrochemical grafting of organic films on silica.

Traditionally, self-assembled monolayers formed on silicon require the removal of the insulating and chemically inert silica layer that naturally forms on the surface of crystalline silicon. The removal of silica is thought to be necessary in order to expose the conducting Si-H surface, which is reactive towards molecules. Here we report the unexpected result of electrochemical formation of thin organic films on silica-terminated silicon with silica thickness up to 20 nm. The process is facilitated by the electrochemical generation of aryl radicals that react with silanol groups at the distal end of silica.