Effect of humic acid on the stabilization of cadmium in soil by coprecipitating with ferrihydrite

Enhanced degradation of enoxacin using ferrihydrite-catalyzed heterogeneous photo-Fenton process

The ferrihydrite-catalyzed heterogeneous photo-Fenton reaction shows great potential for environmental remediation of fluoroquinolone (FQs) antibiotics. The degradation of enoxacin, a model of FQ antibiotics, was studied by a batch experiment and theoretical calculation. The results revealed that the degradation efficiency of enoxacin reached 89.7% at pH 3. The hydroxyl radical (∙OH) had a significant impact on the degradation process, with a cumulative concentration of 43.9 μmol L-1 at pH 3. Photogenerated holes and electrons participated in the generation of ∙OH. Eleven degradation products of enoxacin were identified, with the main degradation pathways being defluorination, quinolone ring and piperazine ring cleavage and oxidation. These findings indicate that the ferrihydrite-catalyzed photo-Fenton process is a valid way for treating water contaminated with FQ antibiotics.

Fe(II)-catalyzed phase transformation of Cd(II)-bearing ferrihydrite-kaolinite associations under anoxic conditions: New insights to role of kaolinite and fate of Cd(II)

Cadmium-bearing ferrihydrite-kaolinite associations (Cd-associations) are commonly found in cadmium-contaminated paddy soils in tropical and subtropical regions. In the presence of anaerobic conditions caused by flooding, the creation of Fe(II) can facilitate the transformation of ferrihydrite into secondary Fe (hydr)oxides, resulting in the redistribution of Cd. However, the role of kaolinite in iron oxides transformation and changes in Cd chemical species have largely not been determined. In this study, Cd-associations were prepared for reaction with Fe(II) under anoxic conditions. The results obtained from powder XRD and EXAFS indicated that the presence of kaolinite association noticeably hastened the transformation of ferrihydrite into crystalline goethite. Specific surface area and electrochemical analyses revealed that smaller particle sizes and higher reactivity of ferrihydrite within Cd-associations collaboratively contribute to the acceleration. Chemical analyses demonstrated a significant negative correlation between ferrihydrite-Fe and aqueous-Cd, and a significant positive correlation between crystalline-Fe and residual-Cd. HRTEM analyses indicated that a portion of the Cd was incorporated into the crystal lattices of lepidocrocite and goethite, with the majority of Cd being sequestered within goethite lattice. These findings provide new insights into the roles of clay minerals in the geochemical cycling of Fe and Cd in paddy soils under anoxic conditions.

Analog soil organo-ferrihydrite composites as suitable amendments for cadmium and arsenic stabilization in co-contaminated soils

Remediation of CdAs co-contaminated soils has long been considered a difficult problem to solve, as Cd and As have distinctly different metallic characters. Amending contaminated soils with traditional single passivation materials may not always work well in the stabilization of both Cd and As. Here, we reported that analog soil organo-ferrihydrite composites made with either living or non-living organics (bacterial cells or humic acid) could achieve stabilization of both Cd and As in contaminated soils. BCR and Wenzel sequential extractions showed that organo-ferrihydrite, particularly at 1 wt% loading, shifted liable Cd and As to more stable phases. Organo-ferrihydrite amendments significantly (p < 0.05) increased soil urease, alkaline phosphatase and catalase enzyme activities. With organo-ferrihydrite amendments, the bioavailable fraction of Cd decreased to 35.3 % compared with the control (65.1 %), while the bioavailable As declined from 29.4 % to 12.4%. Soil pH, microbial community abundance and diversity were almost unaffected by organo-ferrihydrite. Ferrihydrite and organo fractions both contributed to direct Cd-binding, while the organo fraction probably maintained the Fe-bound As via lowering ferrihydrite phase transformation. Compared to pure ferrihydrite, organo-ferrihydrite composites performed better not only in reducing liable Cd and As, but also in maintaining soil quality and ecosystem functions. This study demonstrates the applications of organo-ferrihydrite composites in eco-friendly remediation of CdAs contaminated soils, and provides a new direction in selecting appropriate soil amendments.

Humic acid and fulvic acid facilitate the formation of vivianite and the transformation of cadmium via microbially-mediated iron reduction

The effects of humic acids (HA) and fulvic acids (FA) on the fate of Cd in anaerobic environment upon microbial reduction of Cd-bearing ferrihydrite (Fh) with Geobacter metallireducens were investigated. The results showed that HA and FA could promote the reductive dissolution of Fh and the formation of vivianite. After incubation of 38 d, vivianite accounted for 47.19%, 59.22%, and 48.53% of total Fe in biological control batch (BCK), HA and FA batches (C/Fe molar ratio of 1.0), respectively, by Mössbauer spectroscopy analysis. In terms of Cd, HA and FA could promote the release of adsorbed Cd during the initial bioreduction process, but reassuringly, after 38 d the dissolved Cd with HA and FA addition batches were 0.58-0.91 and 0.99-1.08 times of the BCK, respectively. The proportions of residual Cd in HA batches were higher than FA and BCK batches, indicating that HA was better than FA in immobilizing Cd. This might be because the quinone groups in HA could act as electron shuttle. This study showed that HA facilitated the transformation of vivianite better than FA, and Cd can be stabilized by resorption or co-precipitation with vivianite, providing a theoretical support for the translocation of Cd in sediment-water interface.

Humic acid coupled with coal gasification slag for enhancing the remediation of Cd-contaminated soil under alternated light/dark cycle

In this study, the synthesis of a coal gasification slag-humic acid (SA) hybrid was purposed for the remediation of cadmium (Cd)-contaminated soil. In order to investigate the effect of SA on the Cd-contaminated soil and plant growth, a series of experiments were carried out under different illumination condition. The results showed that the SA has some the photocatalytic activity, and adding 10 wt% of SA to the soil could obviously improve the soil fertility and decrease the mobility of Cd in the soil under alternated light/dark cycle (12L/12D); the content of the residual fraction in the SA-amended soil reached 69.5%, and the Cd decreasing rates for the leaf, stem, and root of Artemisia ordosica were near 100%, 91.3%, and 75.3%, respectively. Characterizations of amendments suggested that the synergistic effect of precipitation and surface complexation played a major role in the remediation of Cd-contaminated soil.

Combined effects of ferrihydrite coating and ionic type on the transport of compost-derived dissolved organic matter in saturated porous media

Field application of manure compost introduces a large quantity of dissolved organic matter (DOM), which can affect the migration of DOM-associated contaminants. In this study, the transport of humic acid (HA) and compost-derived dissolved organic matter (CDOM) in two porous media under various conditions, including ionic types, ionic strength, and influent concentrations, were investigated by column experiments and modeling analysis. Increasing Na⁺ concentration did not affect the transport of CDOM and HA in quartz sands, but inhibited CDOM transport in ferrihydrite (Fh)-coated sands. The retention recoveries of CDOM in Fh-coated sands were not changed with increasing NaCl concentration, suggesting that the adsorption of CDOM on Fh-coated sands caused by increasing NaCl concentration was a reversible process. Ca²⁺ could reduce the mobility of CDOM and HA through bridge bonding and electrostatic interaction. CDOM had a higher mobility than HA in quartz sands under CaCl₂ conditions because the aggregation stability of CDOM was stronger than that of HA. The ferrihydrite coating increased the roughness of sand surface, resulting in lower mobility of CDOM in the Fh-coated sands than in quartz sands. A part of CDOM adsorbed onto Fh-coated sand was strongly bound through ligand exchange-surface complexation. The pore volume of CDOM required to saturate adsorption sites onto the Fh-coated sand depends on the influent CDOM concentration. The influent CDOM with higher concentration required less pore volume to achieve adsorption equilibrium. Modeling analysis suggested that the types of deposition sites provided by Fh-coated sand are mainly irreversible sites. Our findings demonstrated that CDOM transport in the sand columns may change the porous medium's physicochemical properties and alter the hydrochemistry conditions. Therefore, these factors mentioned above should not be ignored when evaluating the environmental risks of CDOM.

The effect of water flooding at grain-filling stage on the uptake of heavy metal(loid)s by rice in slightly alkaline paddy soil

Continuous flooding has been widely used in paddy field to decrease the accumulation of heavy metal(loid)s by rice due to their decreased solubility and bioavailability of most heavy metal(loid)s under water flooding condition. A field experiment with six drainage treatments during grain-filling stage was performed to investigate the influence of different water flooding conditions on availability of cadmium (Cd), lead (Pb), chromium (Cr), arsenic (As), and mercury (Hg) and their accumulation by rice in alkaline paddy soil. The results showed at the end of experiment, the availability of Cd and Pb in soil with continuous flooding for above 25 days after full heading significantly decreased compared with draining immediately after full heading of rice. But some increase of the As availability in soil with different water regimes was observed at 15 days after beginning of experiment where the As availability was below its detection limit. Meanwhile, the concentrations of Cd and Cr both in rice grains and straws were decreased evidently with prolonged flooding at rice filling stage. The concentrations of Pb and Hg in grain were all below the detection limit, but increased in straw with draining 15 days later compared with draining immediately after full heading of rice. However, water regimes during grain-filling stage had little effect on As uptake by rice in alkaline paddy soil. Moreover, this study also discovered that organic matter may played a critical role in controlling availability of heavy metal(loid)s in alkaline soil. This work demonstrated that at alkaline paddy soil, maintaining water flooding until 5 days before harvest in rice grain-filling stage was an effective method to improve rice safety without decreasing yields in paddy field polluted with Cd, Cr, and As, but careful consideration is required for Pb and Hg.

Mechanistic insights into the environmental fate of tetracycline affected by ferrihydrite: Adsorption versus degradation

Tetracycline (TC), a widely used antibiotic, is frequently detected in soil environments. It has a strong tendency to form complexes with metals, including iron (oxyhydr)oxide. In this study, ferrihydrite (Fh), a representative iron oxyhydroxide of the iron plaques on the surface of plant roots, was chosen to study the contributions of iron oxyhydroxide on the environmental fate of TC in the rhizosphere environment. Fh adsorption isotherm of TC showed good fitting to the Freundlich model, and the Fh adsorption capacity of TC was found much larger than the other iron oxyhydroxide of high crystallinity. The adsorption mechanisms mainly included electrostatic interaction, H-bonding, and complexation. The results of FTIR and XPS spectra revealed that tricarbonylamide, dimethylamino, and the hydroxyl in the B ring of TC were mainly responsible for the complexation with Fh surface hydroxyl groups. Furthermore, it should be noted that the adsorbed TC on Fh could be degraded and the degradation kinetics of TC better fitted to the pseudo-second-order model. Fh could promote electron transfer from TC to Fe(III) on the Fh surface, which led to the degradation of TC and the formation of Fe(II) ions. The degradation pathways of TC mainly involved three reactions: hydroxylation, dealkylation, and deamination. This study provides mechanistic insights on TC-Fh interaction, which improves the understanding of TC fate in the rhizosphere environment.

Insights into naphthalene degradation in aqueous solution and soil slurry medium: Performance and mechanisms

The performance of naphthalene (NAP) degradation in peroxodisulfate (PDS) and peroxymonosulfate (PMS) oxidation systems by nano zero valent iron (nZVI) combined with citric acid (CA) activation was reported in aqueous solution and soil slurry medium. The results in aqueous solution tests indicated that 98.1% and 98.9% of NAP were individually degraded in PDS/nZVI/CA and PMS/nZVI/CA systems within 2 h when the dosages of PDS, PMS, nZVI and CA were 1.0 mM, 0.1 mM, 0.2 mM and 0.1 mM, respectively. The consequences of scavenging tests and electron paramagnetic resonance detection demonstrated that HO• and SO₄^-• were the key factors on NAP removal. The presence of surfactants could consume ROSs and inhibit NAP removal. In addition, GC-MS was applied for the determination of NAP degradation intermediates, and three possible NAP degradation pathways were proposed in PDS oxidation process and two pathways in PMS oxidation process, respectively. The results in soil slurry medium showed that the presence of CA could promote the dissolution of soil minerals and the desorption of NAP from soil medium. 93.5% and 96.8% degradation of NAP were obtained in PDS/nZVI/CA and PMS/nZVI/CA systems within 24 h. Besides, the existence of DOM in soil could promote Fe(II)/Fe(III) cycle and NAP degradation through electron transfer. Based on the NAP degradation performance in the actual groundwater and soil medium, the above findings could provide basis and strong support for the potential application of PDS/nZVI/CA and PMS/nZVI/CA systems in the remediation of NAP contaminated sites.

Effects of organic matter components and incubation on the cement-based stabilization/solidification characteristics of lead-contaminated soil

Stabilization/solidification (S/S) has been studied since 1950s and widely used for the treatment of potentially toxic elements (PTEs). The coexistence of organic matter (OM) and PTEs can cause a very complicated mechanism for cement-based S/S applications and bring challenges from both scientific and engineering perspectives. To fill in the knowledge gap, this paper investigates for the first time the effects on S/S characteristics of OM components and incubation, which are the two main factors that result in the inconsistency in the leaching characteristics from the available studies. OM samples with different components (humic acid (HA) and fulvic acid (FA)) and contents were mixed into lead-contaminated soil and incubated for different durations of up to 90 days. The experimental results show that the strength of stabilized soils increases with increasing incubation duration and the lead leaching concentration of stabilized soils is decreased by 60.7%-83.6% from zero to 90 days. The lead leaching concentration of the HA group, which is 144.0% higher with no incubation than the non-OM group, becomes 58.3% lower with 90 days of incubation. The leaching concentration of the FA group remains much higher than those of the other groups. Finally, a competing mechanism of HA-weakening cement hydration reactions and stabilizing lead with a critical incubation duration of 14 days-is proposed, together with a cooperating mechanism of FA-weakening cement hydration reactions and releasing lead.