Study of different methods for enhancing the nitrate removal efficiency of a zero-valent metal process

Efficient Removal of Nitrate in Neutral Solution Using Zero-Valent Al Activated by Soaking

Nitrate is a contaminant widely found in surface water, and a high concentration of nitrate can pose a serious threat to human health. Zero-valent iron is widely used to reduce nitrate in aqueous solution, but an acidic condition is required. Zero-valent aluminum has a much lower redox potential (E₀(Al³⁺/Al⁰) = -1.662 V) than zero-valent iron (E₀(Fe²⁺/Fe⁰) = -0.44 V), making it a better choice for reduction of nitrate. However, a passive oxide film covering on Al surfaces inhibits its electron transfer. In this work, metal Al powder was activated by a soaking procedure in deionized water. It was found that nitrate in neutral solution can be efficiently and completely reduced by soaked Al, even if the concentration of nitrate-N was up to 100 mg L^-1. Using an optimal soaking time, the soaked Al can remove >90% of nitrate in aqueous solution within ∼2 h at 50 °C. Furthermore, the nitrate reduction efficiency increased with increasing reaction temperature and dosage of Al powder. After reaction, only ∼50% of pristine N content was left in the form of ammonia ions (NH₄⁺) in aqueous solution. Mechanism analyses showed that after soaking, Al particle surfaces were covered by a layer of loose and fine Al(OH)₃ grains, which can shorten the induction time for the beginning of the reaction between inner Al and outside ions or molecules. This is the reason why soaked Al has a high efficiency for nitrate removal. The present results indicate that soaking is an effective way to activate Al to remove nitrate in water.

Ultrasound process-enhanced removal of the toxic disinfection by-product bromate from water by aluminum: A comparative study

As bromate removal and reduction can be also achieved using metals, aluminum (Al) appears as the most promising one for reduction of bromate because Al is abundant element and exhibits a high reduction power. Reactions between bromate and Al shall be even enhanced through ultrasound (US) process because US can facilitate mass transfer on liquid/solid interfaces and clean surfaces via generating microscale turbulence to facilitate reactions. Therefore, the aim of this study is for the first time to investigate the effect of US on bromate removal by Al metal. Specifically, Al particle would be treated by HCl to afford HCl-treated Al (HCTAL), which is capable of removing bromate and even reducing it to bromide. Such a mechanism is also validated by density function theory calculation through determining adsorption energy as -152.8 kJ/mole, and oxygen atoms of bromate would be extracted and reacted with Al atoms, releasing bromide ion. US not only facilitated bromate removal by further increasing removal capacity under the acidic condition but also suppressed the inhibitive effect from basicity at relatively high pH. The spent HCTAL could still remove bromate and convert it to bromide after regeneration. These features indicate that US considerably enhances bromate removal by Al. PRACTITIONER POINTS: Bromate removed by Al is elucidated by DFT calculation with E_absorption  = -152.8 kJ/mole. Oxygen atoms of bromate are extracted and reacted with Al atoms, releasing bromide ion. A higher power of ultrasound would substantially enhance bromate removal efficiency. Ultrasound also suppresses the inhibitive effect from basicity at relatively high pH. With ultrasound, the interference of co-existing anions on bromate removal is lessened.

Nitrate Removal by Zero-Valent Metals: A Comprehensive Review

Nitrate is a widespread water contaminant that can pose environmental and health risks. Various conventional techniques can be applied for the removal of nitrate from water and wastewater, such as biological denitrification, ion exchange, nanofiltration, and reverse osmosis. Compared to traditional methods, the chemical denitrification through zero-valent metals offers various advantages, such as lower costs, simplicity of management, and high efficiencies. The most utilized material for chemical denitrification is zero-valent iron (ZVI). Aluminium (ZVA), magnesium (ZVM), copper (ZVC), and zinc (ZVZ) are alternative zero-valent metals that are studied for the removal of nitrate from water as well as from aqueous solutions. To the best of our knowledge, a comprehensive work on the use of the various zero-valent materials that are employed for the removal of nitrate is still missing. Therefore, in the present review, the most recent papers concerning the use of zero-valent materials for chemical denitrification were analysed. The studies that dealt with zero-valent iron were discussed by considering microscopic (mZVI) and nanoscopic (nZVI) forms. For each Fe0 form, the effects of the initial pH, the presence or absence of dissolved oxygen, the initial nitrate concentration, the temperature, and the dissolved ions on the nitrate removal process were separately evaluated. Finally, the different materials that were employed as support for the nanoparticles were examined. For the other zero-valent metals tested, a detailed description of the works present in the literature was carried out. A comparison of the various features that are related to each considered material was also made.

Review of zero-valent aluminium based water and wastewater treatment methods

Zero-valent metals (ZVM) are widely used to remove heavy metals, contaminants, toxicity, etc. from water and wastewater. Zero-valent aluminium (ZVAl) has large surface area and high surface reactivity. It has enormous flexibility for the in-situ application. ZVAl can be applied as either a single or a bimetallic system as well as advanced oxidation processes (AOPs). It is observed that ZVAl is capable of generating hydroxyl and sulfate radicals in water medium, which remove non-biodegradable pollutants from aqueous solution. ZVAl-based processes can remove non-biodegradable organic contaminants from water medium within a short duration. ZVAl is also used as a reducing agent. It is efficient to reduce toxic hexavalent chromium to less toxic trivalent chromium. ZVAl, in various combinations in bimetallic system (Fe/Al, Pd/Al, Cu/Al), is able to remove various contaminants from aqueous medium. Overall, it can be concluded that ZVAl-based methods for water and wastewater treatment are promising environmental technologies.

Valorization of aluminum scrap via an acid-washing treatment for reductive removal of toxic bromate from water

Aluminum scrap (AS) is adopted for the first time as a readily available aluminum source to prepare zero-valent aluminum (ZVAl) for removing bromate from water via a reductive reaction. Since aluminum is easily oxidized to aluminum oxide (Al₂O₃) on exposure to air, an acid-washing pretreatment on AS is developed to remove the layer of Al₂O₃. HCl is found as the most effective acid to pretreat AS and the HCl-pretreated or acid-washed AS (AWAS) is able to remove bromate from water and convert it to bromide. Factors, such as temperature, pH, co-existing anions, and particle size, which influence the bromate removal using AWAS are also investigated. The mechanism of bromate removal by AWAS can be attributed to both reduction and adsorption. The elevated temperature also significantly improves bromate removal capacity of AWAS as well as the reaction kinetics. The bromate removal capacity of AWAS is substantially improved under acidic conditions. However, the basic conditions and co-existing anions suppress or interfere with the interaction between bromate and AWAS, leading to much lower removal capacities. The recyclability of AWAS is also evaluated and the acid-washing regeneration is necessary to restore its capacity. However, the mass of AWAS can gradually decrease due to multi-cycle acid-washing regeneration. Through this study, the valorization of AS via acid-washing is demonstrated and optimization of acid-washing parameters is presented. Our findings reveal that the acid-washing is a useful technique to utilize AS as an inexpensive and efficient material for removing bromate from water.

Ultrasound-assisted activation of zero-valent magnesium for nitrate denitrification: identification of reaction by-products and pathways

Zero-valent magnesium (Mg(0)) was activated by ultrasound (US) in an aim to promote its potential use in water treatment without pH control. In this context, nitrate reduction was studied at batch conditions using various doses of magnesium powder and ultrasound power. While neither ultrasound nor zero-valent magnesium alone was effective for reducing nitrate in water, their combination removed up to 90% of 50 mg/L NO3-N within 60 min. The rate of nitrate reduction by US/Mg(0) enhanced with increasing ultrasonic power and magnesium dose. Nitrogen gas (N2) and nitrite (NO2(-)) were detected as the major reduction by-products, while magnesium hydroxide Mg(OH)2 and hydroxide ions (OH(-)) were identified as the main oxidation products. The results from SEM-EDS measurements revealed that the surface oxide level decreased significantly when the samples of Mg(0) particles were exposed to ultrasonic treatment. The surface passivation of magnesium particles was successfully minimized by mechanical forces of ultrasound, which in turn paved the way to sustain the catalyst activity toward nitrate reduction.

Mixed sulfur-iron particles packed reactor for simultaneous advanced removal of nitrogen and phosphorus from secondary effluent

A mixed sulfur-iron particles packed reactor (SFe reactor) was developed to simultaneously remove total nitrogen (TN) and total phosphorus (TP) of the secondary effluent from municipal wastewater treatment plants. Low effluent TN (<1.5 mg/L) and TP (<0.3 mg/L) concentrations were simultaneously obtained, and high TN removal rate [1.03 g N/(L·d)] and TP removal rate [0.29 g P/(L·d)] were achieved at the hydraulic retention time (HRT) of 0.13 h. Kinetic models describing denitrification were experimentally obtained, which predicted a higher denitrification rate [1.98 g N/(L·d)] of SFe reactor than that [1.58 g N/(L·d)] of sulfur alone packed reactor due to the mutual enhancement between sulfur-based autotrophic denitrification and iron-based chemical denitrification. A high TP removal obtained in SFe reactor was attributed to chemical precipitation of iron particles. Microbial community analysis based on 16S rRNA revealed that autotrophic denitrifying bacteria Thiobacillus and Sulfuricella were the dominant genus, indicating that autotrophic denitrification played important role in nitrate removal. These results indicate that sulfur and iron particles can be packed together in a single reactor to effectively remove nitrate and phosphorus.

Effect of common ions on nitrate removal by zero-valent iron from alkaline soil

Zero-valent iron (Fe(0))-based permeable reactive barrier (PRB) technology has been proved to be effective for soil and groundwater nitrate remediation under acidic or near neutral conditions. But few studies have been reported about it and the effects of coexistent ions under alkaline conditions. In this study, nitrate reduction by Fe(0) was evaluated via batch tests in the presence of alkaline soil and common cation (Fe(2+), Fe(3+) and Cu(2+)) and anion (citrate, oxalate, acetate, SO(4)(2-), PO(4)(3-), Cl(-) and HCO(3)(-)). The results showed that cation significantly enhanced nitrate reduction with an order of Fe(3+)>Fe(2+)>Cu(2+) due to providing Fe(2+) directly or indirectly. Most anions enhanced nitrate reduction, but PO(4)(3-) behaved inhibition. The promotion decreased in the order of citrate>acetate>SO(4)(2-)>Cl(-)≈HCO(3)(-)≈oxalate≫PO(4)(3-). Ammonium was the major final product from nitrate reduction by Fe(0), while a little nitrite accumulated in the beginning of reaction. The nitrogen recovery in liquid and gas phase was only 56-78% after reaction due to ammonium adsorption onto soil. The solution pH and electric conductivity (EC) varied depending on the specific ion added. The results implied that PRB based Fe(0) is a potential approach for in situ remediation of soil and groundwater nitrate contamination in the alkaline conditions.

Reductive denitrification using zero-valent iron and bimetallic iron

A study of reductive denitrification of nitrate was conducted. Microscale zero-valent iron (ZVI) and palladium-coated iron (Pd/Fe) were used in the reduction of nitrate with variable pH. The solution pH was controlled by an auto controlling system instead of chemical buffers. Higher reduction rates were achieved with lower pH and lower pH gave the pseudo-first-order kinetics while it was close to the zero-order reaction when the pH of the solution was becoming high and nitrate concentration was higher. As it took several hours to convert intermediates to ammonia completely, the assumption, under which mass loss calculated from the measured ammonia concentration right after the reaction was the mass of nitrogen evolved, could lead to overestimation of the nitrogen selectivity. The current study confirmed that the palladium coating on the iron could increase the nitrogen selectivity, and the Pd/Fe system could also achieve the advantages of coupling of electron source and catalyst with regard to the engineering aspects.