Investigating sorption characteristics of copper ions onto insolubilized humic acid by using a continuously monitored solid phase extraction technique

Effects of humic acid on Pb2+ adsorption onto polystyrene microplastics from spectroscopic analysis and site energy distribution analysis

Microplastics (MPs), act as vectors of heavy metal pollutants in the environment, is of practical significance to study the adsorption process and mechanism on heavy metals. In this study, polystyrene microplastics (PSMPs) were used as model MPs to study the adsorption of Pb²⁺ on PSMPs and the effects of humic acid (HA) on the adsorption process. The results showed that HA promoted the adsorption of Pb²⁺ on PSMPs, and the higher the concentration of HA, the greater the adsorption of Pb²⁺. With the increase of pH value and decrease of ionic strength, the adsorption capacity of PSMPs for Pb²⁺ increased. The scanning electron microscope equipped with the energy dispersive spectroscope (SEM–EDS), fourier transform-infrared spectra (FT-IR) and X-ray photoelectron spectroscopy (XPS) analysis showed that Pb²⁺ could be adsorbed directly onto PSMPs and also indirectly by HA. The higher K_SV values in the PSMPs-HA-Pb²⁺ system than PSMPs-HA system by fluorescence analysis of HA suggested that HA acted as a bridging role in the adsorption of Pb²⁺ on PSMPs. The site energy distribution analysis further revealed that HA increased the average site energy μ(E^*) and its standard deviation σ_e^* of PSMPs by introducing more adsorption sites, thus enhanced the adsorption affinity of PSMPs. This study provided more thoughts and insights into the adsorption behavior and mechanism of MPs for Pb²⁺ in aquatic environments.

Pb (II) bioavailability to algae (Chlorella pyrenoidosa) in relation to its complexation with humic acids of different molecular weight

Humic acid (HA) has a major influence on the environmental fate of metal ions due to its heterogeneity in chemical compositions, structure and functional groups. In this study, we investigated the effect of humic acid (HA) with different molecular weight (Mw) on the bioavailability of Pb for a representative algae-Chlorella pyrenoidosa. The results showed that HA with larger Mw had stronger inhibitory effects on the bioavailability of Pb to algae, and the biosorption capacity of Pb decreased with increasing Mw, which is in accordance with the variations of complexation capacities of Pb for HA fraction. In addition, we found that HA with Mw lower than 10 kDa could increase the biosorption capacity of Pb. The considerable differences among the Mw fractions on Pb biosorption were mainly attributed to their properties and corresponding complexation capacities. Phenolic groups were responsible for the variations of binding capacities among different Mw fractions, and it could also better explain the bioaccumulation of Pb to the membranes of algae. By using NICA-Donnan model, we found that over 60% of Pb ions were bound by HAs through specific binding, and the formation of Pb-HAs complex were non-bioavailable to algae, which was proved by the considerably decreasing percentage of internalized Pb. This study provided further insight into the bioavailability of Pb to algae as influenced by the complexation of HA with metal ion such as Pb.

The role of major functional groups: Multi-evidence from the binding experiments of heavy metals on natural fulvic acids extracted from lake sediments

Fulvic acid (FA) plays a key role in governing the environmental geochemistry behavior of heavy metals. In this work, the roles of major functional groups were investigated based on binding experiments of heavy metals on natural FA extracted from lake sediments. The results showed that the adsorption capacities were ranked as Cu²⁺ > Pb²⁺ >Cd²⁺. The differences of peak area at 3412, 1713, 617and 2430 cm^-1 pre- and post-binding reactions in FTIR spectra suggested that phenolic, carboxyl and nitrogen-containing groups were the major functional groups providing sites binding heavy metals. Moreover, the results of bi-Langmuir model and the ionic strength effects jointly indicated that electrostatic attraction was the key mechanism during the adsorption process. The fitted results of Ligand-binding model suggested that the major functional groups in FA were classified into two types binding sites: weak (i.e. phenolic and carboxyl groups) and strong binding sites (i.e. nitrogen-containing groups). Additionally, there might be p-benzoquinone-like formed in FA which were then reduced to hydroquinone during the adsorption process, corresponding to the changing of peak area at 1614 and 830 cm^-1 in FTIR spectra, the occurrence of Peak C in Fluorescence excitation-emission matrix (EEM) spectra and the ratios of H/C (<1) and O/C (≈1). The organic matter in sediments from Lake Wuliangsuhai presented similar characteristics with terrestrial plants due to the lake characterized by Phragmites australis and Potamogeton pectinatus L. being the dominant species, which shared large proportions of woody tissues as well as waxy hydrocarbons resembling that of terrestrial plants. This work is useful to insight the environmental effects of FA on heavy metals in environment.

Investigating the potential of synthetic humic-like acid to remove metal ions from contaminated water

Humic acid can effectively bind metals and is a promising adsorbent for remediation technologies. Our studies initially focussed on Cu²⁺ as a common aqueous contaminant. Previous studies indicate that carboxylic groups dominate Cu²⁺ binding to humic acid. We prepared a synthetic humic-like acid (SHLA) with a high COOH content using catechol (0.25 M) and glycine (0.25 M) with a MnO₂ catalyst (2.5% w/v) at pH = 8 and 25 °C and investigated the adsorption behaviour of Cu²⁺ onto it. The SHLA exhibited a range of adsorption efficiencies (27%-99%) for Cu²⁺ depending on reaction conditions. A pseudo-second-order kinetic model provided the best fit to the experimental data (R² = 0.9995-0.9999, p ≤ 0.0001), indicating that chemisorption was most likely the rate-limiting step for adsorption. The equilibrium adsorption data showed good fits to both the Langmuir (R² = 0.9928-0.9982, p ≤ 0.0001) and Freundlich (R² = 0.9497-0.9667, p ≤ 0.0001) models. The maximum adsorption capacity (q_m) of SHLA increased from 46.44 mg/g to 58.78 mg/g with increasing temperature from 25 °C to 45 °C. Thermodynamic parameters (ΔG⁰ = 2.50-3.69 kJ/mol; ΔS⁰ = 0.06 kJ/(mol·K); ΔH⁰ = 15.23 kJ/mol) and values of R_L (0.0142-0.3711) and n (3.264-3.527) show that the adsorption of Cu²⁺ onto SHLA was favourable, spontaneous and endothermic in nature. Over six adsorption/desorption cycles using 0.5 M HCl for the desorption phase, there was a 10% decrease of the adsorption capacity. A final experiment using a multi-metal solution indicated adsorption efficiencies of up to 84.3-98.3% for Cu, 86.6-98.8% for Pb, 30.4-82.9% for Cr, 13.8-77.4% for Ni, 9.2-62.3% for Cd, 8.6-51.9% for Zn and 4.6-42.1% for Co. Overall, SHLA shows great potential as an adsorbent to remove metals from water and wastewater.

Relating Cd2+ binding by humic acids to molecular weight: A modeling and spectroscopic study

Molecular weight (Mw) is a fundamental property of humic acids (HAs), which considerably affect the mobility and speciation of heavy metals in the environment. In this study, soil humic acid (HA) extracted from Jinyun Mountain, Chongqing was ultra-filtered into four fractions according to the molecular weight, and their properties were characterized. Complexation of cadmium was investigated by titration experiments. For the first time, Langmuir and non-ideal competitive adsorption-Donna (NICA-Donnan) models combined with fluorescence excitation-emission matrix (EEM) quenching were employed to elucidate the binding characteristics of individual Mw fractions of HA. The results showed that the concentration of acidic functional groups decreased with increasing Mw, especially the phenolic groups. The humification degree and aliphaticity increased with increasing Mw as indicated by elemental composition analysis and FT-IR spectra. The binding capacity of Cd²⁺ to Mw fractions of HA followed the order UF1 (<5kDa)>UF2 (5-10kDa)>UF4 (>30kDa)>UF3 (10-30kDa). Moreover, the distribution of cadmium speciation indicated that the phenolic groups were responsible for the variations in binding of Cd²⁺ among different Mw fractions. The results of fluorescence quenching illustrated that the binding capacity of Cd²⁺ to Mw fractions was controlled by the content of functional groups, while the binding affinity was largely influenced by structural factors. The results provide a better understanding of the roles that different HA Mw fractions play in heavy metal binding, which has important implications in the control of heavy metal migration and bio-toxicity.

Binding characteristics of Cu2+ to natural humic acid fractions sequentially extracted from the lake sediments

Humic acids (HAs) determine the distribution, toxicity, bioavailability, and ultimate fate of heavy metals in the environment. In this work, ten HA fractions (F1-F10) were used as adsorbent, which were sequentially extracted from natural sediments of Lake Wuliangsuhai, to investigate the binding characteristics of Cu²⁺ to HA. On the basis of the characterization results, differences were found between the ten extracted HA fractions responding to their elemental compositions and acidic functional groups. The characterization results reveal that the responses of ten extracted HA fractions to their elemental compositions and acidic functional groups were different. The O/C and (O + N)/C ratio of F1-F8 approximately ranged from 0.66 to 0.53 and from 0.72 to 0.61, respectively; the measured results showed that the contents of phenolic groups and carboxyl groups decreased from 4.46 to 2.60 mmol/g and 1.60 to 0.58 mmol/g, respectively. The binding characteristics of Cu²⁺ to the ten HA fractions were well modeled by the bi-Langmuir model; the binding behavior of Cu²⁺ to all the ten HA fractions were strongly impacted by pH and ionic strength. The FTIR and SEM-EDX image of HA fractions (pre- and post-adsorption) revealed that carboxyl and phenolic groups were responsible for the Cu²⁺ sorption on the ten sequentially extracted HA fractions process, which is the same with the analysis of the ligand binding and bi-Langmuir models Accordingly, the adsorption capacity of the former HA fractions on Cu²⁺ were higher than the latter ones, which may be attributed to the difference of carboxyl and phenolic group contents between the former and latter extracted HA fractions. Additionally, the functional groups with N and S should not be neglected. This work is hopeful to understand the environmental effect of humic substances, environmental geochemical behavior, and bioavailability of heavy metals in lakes.

Insolubilization of Chestnut Shell Pigment for Cu(II) Adsorption from Water

Chestnut shell pigment (CSP) is melanin from an agricultural waste. It has potential as an adsorbent for wastewater treatment but cannot be used in its original state because of its solubility in water. We developed a new method to convert CSP to insolubilized chestnut shell pigment (ICSP) by heating, and the Cu(II) adsorption performance of ICSP was evaluated. The conversion was characterized, and the thermal treatment caused dehydration and loss of carboxyl groups and aliphatic structures in CSP. The kinetic adsorption behavior obeyed the pseudo-second-order rate law, and the equilibrium adsorption data were well described with both the Langmuir and the Freundlich isotherms. ICSP can be used as a renewable, readily-available, easily-producible, environmentally-friendly, inexpensive and effective adsorbent to remove heavy-metal from aquatic environments.

Sorption of Cu(2+) on humic acids sequentially extracted from a sediment

In addition to the diverse properties of humic acids (HAs) extracted from different soils or sediments, chemical compositions, functional groups and structures of HAs extracted from a single soil or sediment could also be diverse and thus significantly affect sorption of heavy metals, which is a key process controlling the transfer, transformation and fate of heavy metals in the environment. In this study, we sequentially extracted four HA fractions from a single sediment and conducted the sorption experiments of Cu(2+) on these HA fractions. Our results showed that aromaticity and acidic group content of HA fraction decreased with increasing extraction. Earlier extracted HA fraction had higher sorption capacity and affinity for Cu(2+). There were two fractions of adsorbed Cu(2+) on HAs, i.e., ion exchanged fraction and surface bonded fraction, which can be captured mechanically by the bi-Langmuir model with good isotherm fitting. The ion exchanged fraction had larger sorption capacity but lower sorption affinity, compared with the surface bonded fraction. The dissociated carboxyl groups of HAs were responsible for both fractions of Cu(2+) sorption, due to the more Cu(2+) sorption on the earlier extracted HA fraction with more carboxyl groups and at higher pH. The intensive competition between H(+) and the exchangeable Cu(2+) could result in the decrease of ion exchanged capacity and affinity for Cu(2+) on HAs.

Preparation and characterization of humic acid-carbon hybrid materials as adsorbents for organic micro-pollutants

The present work involves the preparation of novel adsorbent materials by the insolubilization and hybridization of humic acid (HA) with carbon. The prepared materials were characterized by N2 adsorption, elemental analysis, Fourier transform infrared spectroscopy, scanning electron microscopy, transmission electron microscopy, solid-state (13)C cross polarization magic angle spinning nuclear magnetic resonance, and low-field nuclear magnetic resonance (NMR) relaxometry on wetted samples. The water solubility of these materials and the lack of effect of oxidants were also confirmed. With this background, the adsorption capacities toward phenol, 2,4,6-tricholrophenol, and atrazine were evaluated, using these as model compounds for organic micropollutants of concern in water. Experimental results show that the prepared materials are mesoporous and have a higher surface area than humic acid and even than the porous carbon in the case of carbon coating. They retain the basic features of the starting materials with lowered functional group content. Moreover, there are interesting new features. NMR relaxometry shows that equilibration of water uptake is very fast, making use in water simple. They have higher adsorption capacities than the pure materials, and they can be applied under a wide range of environmental conditions.

Adsorption of uranium(VI) and thorium(IV) by insolubilized humic acid from Ajloun soil - Jordan

Humic acid from Ajloun soil has been extracted and insolubilized. The insolubilized humic acid (NaIHA) was characterized by Fourier transform infrared spectroscopy, elemental analysis, thermal gravimetric analysis, X-ray diffraction and differential scanning calorimetry. Adsorption of U(VI) and Th(IV) by NaIHA was studied using batch technique at different temperatures (25.0, 35.0 and 45.0 °C) and at different pH values (1.00, 2.00 and 3.00). It was found that NaIHA has higher uptake for Th(IV) than U(VI), and that the metal ion uptake by NaIHA increased with pH and reached a maximum at pH = 3. The kinetic studies were done, and showed that the equilibrium time for each metal ion occurs at 6 h to achieve maximum uptake level. Adsorption data were evaluated according to the Pseudo second-order reaction kinetic. The metal ions uptake properties by the NaIHA fit Langmuir, Freundlich and Dubinin-Radushkevich adsorption isotherms. Thermodynamic functions, ΔG°, ΔH° and ΔS° were determined for each metal ion. The positive values of ΔG° indicate that adsorption process is not highly favorable, while ΔH° values indicated that this process is endothermic. On the other hand, the process has positive entropy which means that the adsorption process increases the disorder of the system and it is entropy driven. Column experiments were used for the determination of metal ion loading capacity and desorption studies. The uptake capacities in column technique of U(VI) and Th(IV) ions are 2.63 and 4.85 mg metal ion/g NaIHA respectively. Recovery of U(VI) and Th(IV) ions was carried out by treatment of loaded insolubilized humic acid with 0.1 M and 1.0 M HNO3, the best recovery for U(VI) and Th(IV) ions were obtained when 1.0 M HNO3 was used.

Synthesis, characterization, and systematic studies of a novel aluminum selective chelating resin

A procedure is detailed for the selective analysis of trace aluminum by flame atomic absorption spectrophotometer coupled with off-line column separation and preconcentration. Chelating resin was synthesized by covalent functionalization of Amberlite XAD-16 by 2-(2-hydroxyphenyl) benzoxazole. The modified resin was characterized using FT-IR spectroscopy, energy dispersive x-ray analysis, elemental analysis, thermogravimetric analysis/differential thermal analysis, and minimum energy run. The optimum sorption was at pH 9 ± 0.1 with corresponding t(1/2) of only 7 min. Many competitive anions and cations studied did not interfere at all in the selective determination of Al(III), at the optimized conditions. The resin shows maximum sorption capacity of 21.58 mg g(-1) and can be regenerated up to 75 cycles without any discernible capacity loss. The Langmuir isotherm model provides the better correlation of the experimental data (r(2) = 0.999) in comparison to Freundlich isotherm model, while the Scatchard analysis revealed homogeneous binding sites in the chelating resin. Analytical figures of merit were evaluated by accuracy (standard reference materials and recovery experiment), precision (RSD <5%), and detection limit (2.8 μg L(-1)). The applicability was demonstrated by analysis of trace aluminum in biological, environmental, and food samples.

Adsorption kinetics and desorption of Cu(II) and Zn(II) from aqueous solution onto humic acid

The adsorption kinetics of Cu(II) and Zn(II) from aqueous solution onto humic acid (HA) were investigated under different conditions. The results showed that HA was particularly effective for adsorbing heavy metals and that the effective pH range was above 6. The adsorption force was not single and dependent on solution pH. The experimental data were found to comply with the pseudo-second-order kinetic model with adsorption capacity, q(e cal) more close to the measured results. Increment of initial adsorbate concentration (1-4 mg/L) and temperature (283-303 K) are both favorable to adsorption process. Based on the desorption experiments, it is further indicated that the main adsorption force was complexation effect between organic ligands of HA surface and metal ions and the percentage of desorption is 50% for Cu and 30% for Zn, respectively.