Guoxin Huang

Remediation of nitrate–nitrogen contaminated groundwater using a pilot-scale two-layer heterotrophic–autotrophic denitrification permeable reactive barrier with spongy iron/pine bark

A novel two-layer heterotrophic-autotrophic denitrification (HAD) permeable reactive barrier (PRB) was proposed for remediating nitrate-nitrogen contaminated groundwater in an oxygen rich environment, which has a packing structure of an upstream pine bark layer and a downstream spongy iron and river sand mixture layer. The HAD PRB involves biological deoxygenation, heterotrophic denitrification, hydrogenotrophic denitrification, and anaerobic Fe corrosion. Column and batch experiments were performed to: (1) investigate the NO3(-)-N removal and inorganic geochemistry; (2) explore the nitrogen transformation and removal mechanisms; (3) identify the hydrogenotrophic denitrification capacity; and (4) evaluate the HAD performance by comparison with other approaches. The results showed that the HAD PRB could maintain constant high NO3(-)-N removal efficiency (>91%) before 38 pore volumes (PVs) of operation (corresponding to 504d), form little or even negative NO2(-)-N during the 45 PVs, and produce low NH4(+)-N after 10 PVs. Aerobic heterotrophic bacteria played a dominant role in oxygen depletion via aerobic respiration, providing more CO2 for hydrogenotrophic denitrification. The HAD PRB significantly relied on heterotrophic denitrification. Hydrogenotrophic denitrification removed 10-20% of the initial NO3(-)-N. Effluent total organic carbon decreased from 403.44mgL(-1) at PV 1 to 9.34mgL(-1) at PV 45. Packing structure had a noticeable effect on its denitrification.

Ammonium removal from groundwater using a zeolite permeable reactive barrier: a pilot-scale demonstration

In situ remediation of ammonium-contaminated groundwater is possible through a zeolite permeable reactive barrier (PRB); however, zeolites finite sorption capacity limits the long-term field application of PRBs. In this paper, a pilot-scale PRB was designed to achieve sustainable use of zeolite in removing ammonium (NH(4)(+)-N) through sequential nitrification, adsorption, and denitrification. An oxygen-releasing compound was added to ensure aerobic conditions in the upper layers of the PRB where NH(4)(+)-N was microbially oxidized to nitrate. Any remaining NH(4)(+)-N was removed abiotically in the zeolite layer. Under lower redox conditions, nitrate formed during nitrification was removed by denitrifying bacteria colonizing the zeolite. During the long-term operation (328 days), more than 90% of NH(4)(+)-N was consistently removed, and approximately 40% of the influent NH(4)(+)-N was oxidized to nitrate. As much as 60% of the nitrate formed in the PRB was reduced in the zeolite layer after 300 days of operation. Removal of NH(4)(+)-N from groundwater using a zeolite PRB through bacterial nitrification and abiotic adsorption is a promising approach. The zeolite PRB has the advantage of achieving sustainable use of zeolite and immediate NH(4)(+)-N removal.

Study on Heterotrophic-Autotrophic Denitrification Permeable Reactive Barriers (HAD PRBs) for In Situ Groundwater Remediation

High concentration of nitrate in drinking water is thought to be related to methemoglobinemia, cancers and even death. Due to the increasing anthropogenic activities, nitrate in groundwater is increasing in many areas of the world. Nitrate contamination is caused by nitrogenous fertilizers, livestock manures, agricultural irrigation, etc. This study overviewed the latest developments in nitrate in situ remediation and summarized advantages and disadvantages of each remediation approach. Currently physical adsorption (PA), biological denitrification and chemical reduction (CR) are the three approaches receiving considerable attention. Nitrate adsorbents in PA will ultimately get to the state of saturation due to adsorbed nitrate and its competing anions. BD is divided into heterotrophic denitrification (HD) and autotrophic denitrification (AD). A large number of liquid, solid and gas organic carbons in HD have been evaluated. For AD, hydrogenotrophic denitrification can be sustained by zero-valent iron (ZVI) which produces cathodic hydrogen. Low solubility of reduced sulfur species, sulfate production and biomass yield limit the applicability of sulfur autotrophic denitrification. The main disadvantage of ZVI-based CR is the release of ammonium under acidic conditions. More recently, a heterotrophic-autotrophic denitrification (HAD) approach has shown encouraging results. PA, cellulose-based HD, ZVI-based CR and AD, and their combined approaches can be applied by means of permeable reactive barrier (PRB). BD PRBs and ZVI PRBs have been successfully applied.

Laboratory column study for evaluating a multimedia permeable reactive barrier for the remediation of ammonium contaminated groundwater

In order to remediate ammonium contaminated groundwater, an innovative multimedia permeable reactive barrier (M-PRB) was proposed, which consisted of sequential columns combining oxygen releasing compound (ORC), zeolite, spongy iron and pine bark in the laboratory scale. Results showed that both ammonium and nitrate could be reduced to levels below the regulatory discharge limits through ion exchange and microbial degradation (nitrification and denitrification) in different compartments of the M-PRB system. The concentration of dissolved oxygen (DO) increased from 2 to above 20 mg/L after the simulated groundwater flowed through the oxygen releasing column packed with ORC, demonstrating that ORC could supply sufficient oxygen for subsequent microbial nitrification. Ammonium was efficiently removed from about 10 to below 0.5 mg N/L in the aerobic reaction column which was filled with biological zeolite. After 54 operating days, more than 70% ammonium could be removed by microbial nitrification in the aerobic reaction column, indicating that the combined use of ion exchange and nitrification by biological zeolite could ensure high and sustainable ammonium removal efficiency. To avoid the second pollution of nitrate produced by the former nitrification, spongy iron and pine bark were used to remove oxygen and supply organic carbon for heterotrophic denitrification in the oxygen removal column and anaerobic reaction column separately. The concentration of nitrate decreased from 14 to below 5 mg N/L through spongy iron-based chemical reduction and microbial denitrification.

Long-term effect of nitrate on Cr(VI) removal by Fe 0 : column studies

Lab-scale parallel continuous-flow column experiments were performed to assess the long-term effect of nitrate (NO3−) on hexavalent chromium (Cr(VI)) removal by scrap iron (Fe0). The first column (L1) was fed with the Cr(VI) solution and the second column (L2) was loaded with the Cr(VI) + NO3− solution. Raman spectroscopy and scanning electron microscopy energy-dispersive X-ray analyses (SEM-EDS) were conducted to investigate the changes of the iron oxides on Fe0. The results showed that the process of Cr(VI) removal by Fe0 was divided into three different stages in the presence of NO3−: inhibition period (<198 pore volumes (PVs)); promotion period (198∼1025 PVs); and complete passivation period (1025∼1300 PVs). During the 462∼1025 PVs, Cr(VI) removal capacity in L2 was about 2.5 times higher than that in L1, and the longevity of L2 than L1 was 275PVs longer. NO3− exhibited the most dominant effect on the Cr(VI) removal by Fe0 in the last two stages. New magnetite (Fe3O4) produced by the redox reaction of NO3− and Fe0 was discovered on the surface of the Fe0 obtained from L2. The new generated Fe3O4 could directly reduce the Cr(VI) and could also act as an inhibitor for the formation of passive film on the Fe0 surface as well as an electron mediator that facilitated electron transport from Fe0 to adsorbed Cr(VI).

Heterotrophic-Autotrophic Denitrification Permeable Reactive Barriers

Two potential heterotrophic-autotrophic denitrification permeable reactive barriers (HAD PRBs) were evaluated to remediate groundwater in situ. The first HAD PRB (Column 1) was packed with a mixture of spongy iron, pine bark and sand between 5 and 145 cm from bottom. The second HAD PRB (Column 2) was filled with a spongy iron and sand mixture layer between 5 and 35 cm from bottom, and a pine bark layer between 35 and 145 cm from bottom. The results showed that during operation over the 45 pore volumes, the influent NO3-N concentration of ≤100 mg/L was mostly denitrified in Columns 1 and 2 at the flow rates of ≤0.30 m/d. The high NO3-N percent removals (97–100 %) for both columns were achieved at hydraulic retention times ranging from 8.75 to 17.51 d. Most of the influent NO3-N was removed in the first 25 cm at the low (23 mg/L) and middle (46 mg/L) NO3-N concentrations and in the first 65 cm at the high concentration (104 mg/L) by Columns 1 and 2. Packing structure had a negligible effect on the performance of the two columns. Both HAD PRBs were highly feasible and effective in in situ groundwater remediation.

Bacterial Community in the Inoculum

Although our previous studies indicated the two heterotrophic-autotrophic denitrification permeable reactive barriers (HAD PRBs) contained heterotrophic and autotrophic denitrifying bacteria and aerobic heterotrophs, convincing molecular and biochemical evidence for their existence is lacking and the bacterial communities remain largely unknown. Using polymerase chain reaction (PCR) and 16S rRNA, the bacterial community composition in the inoculum introduced into the two HAD PRBs were assessed in this study. The extracted deoxyribonucleic acid (DNA) fragment of about 23 kb in length indicated integral genomic DNA was successfully achieved. The A 260/A 280 ratio of approximately 1.72 suggested the genomic DNA could be directly used for subsequent PCR amplification. The 27F/1492R primer pair was successfully able to obtain an approximately 1500-bp specific band. The inoculum contained aerobic heterotrophic bacteria (belonging to Adhaeribacter and Flavisolibacter), heterotrophic denitrifiers (belonging to Bacillus, Clostridium, Flavobacterium, Steroidobacter and Novosphingobium), hydrogenotrophic denitrifiers (belonging to Pseudomonas) and the other anaerobic bacteria (belonging to Anaerovorax, Azoarcus, Geobacter and Desulfobulbu). The diversity of bacteria from the inoculum was high, with at least 13 bacterial genera present.

Heterotrophic-Autotrophic Denitrification

A novel heterotrophic-autotrophic denitrification (HAD) approach supported by granulated spongy iron, pine bark and mixed bacteria was proposed for remediation of nitrate contaminated groundwater in an aerobic environment. The HAD involves biological deoxygenation, chemical reduction (CR) of nitrate and dissolved oxygen (DO), heterotrophic denitrification (HD) and autotrophic denitrification (AD). The experimental results showed 0.121 d, 0.142 d and 1.905 d were needed to completely remove DO by HAD, spongy iron and mixed bacteria respectively. Spongy iron played a dominant role in deoxygenation in the HAD. After 16 days, NO3-N removal was approximately 100, 6.2, 83.1, 4.5 % by HAD, CR, HD, AD, respectively. CR, HD and AD all contributed to the overall removal of NO3-N, but HD was the most important denitrification mechanism. There existed symbiotic, synergistic and promotive effects of CR, HD and AD within the HAD. The different environmental parameters (e.g. water temperature) showed different effects on HAD. HAD was capable of providing steady denitrification rate (1.233–1.397 mg/L/d) for 3.5 months. Pine bark could provide sufficient organic carbon, spongy iron could steadily remove DO, and microbial activity maintained relatively constant. HAD denitrification was zero order with a reaction rate constant (K) of 1.3220 mg/L/d.

Remediation of Nitrate‐contaminated Groundwater by a Mixture of Iron and Activated Carbon

Nitrate contamination in groundwater has become a major environmental and health problem worldwide. The aim of the present study is to remediate groundwater contaminated by nitrate and develop potential reactive materials to be used in PRBs (Permeable Reactive Barriers). A new approach was proposed for abiotic groundwater remediation by reactive materials of iron chips and granular activated carbon particles. Batch tests were conducted and remediation mechanisms were discussed. The results show that nitrate decreases from 86.31 to 33.79 mg⋅L−1 under the conditions of near neutral pH and reaction time of 1h. The combination of iron chips and activated carbon particles is cost‐effective and suitable for further use as denitrification media in PRBs. Nitrogen species don’t change significantly with the further increase in reaction time (>1 h). The iron‐activated carbon‐water‐nitrate system tends to be steady‐state. Small amounts of ammonium and nitrite (0.033–0.039 and 0.14–3.54 mg⋅L−1, respectively) appear a...