Materials In Aquatic Environments Research Articles

Assessment of corrosion resistance and reliability of Cu/diamond composite materials in aquatic environment

Assessment of corrosion resistance and reliability of Cu/diamond composite materials in aquatic environment

Comparison studies: The aging character of mask melt blown layers, polypropylene, and polyethylene terephthalate and their adsorption behavior for tetracycline hydrochloride

Melt-blown layer as a crucial component in medical surgical masks could release microplastics (MPs) into the environment after discarding. This study investigated the aging characteristics of the mask melt-blown layer, polypropylene (PP) and polyethene terephthalate (PET) MPs, as well as compared the adsorption capacity of tetracycline hydrochloride (TCH) on the virgin and aged MPs to gain adsorption mechanism. The carbonyl index (CI) index and two-dimensional correlation spectroscopy (2D-COS) analysis showed that the melt blown layer had the lower degree of aging than PP. Compared with the virgin PP and PET, the main reason for the lowest adsorption capacity of TCH on the virgin melt blown layer was that its unique dense microchannel structure led to the strongest hydrophobicity. The virgin PP and PET which had different chemical structures exhibited similar adsorption capacity of TCH. The adsorption of TCH on the MPs was physisorption, which was both controlled by diffusion, partitioning interaction and pore filling. The increase in the adsorption capacity of TCH on the aging MPs was attributed to the increase of hydrophilicity, pore size and specific surface area. In addition, the influence of environmental factors on the adsorption of TCH were discussed. Allover, this research provided comprehensive insights into the enrichment behavior associated with mask melt-blown materials in aquatic environments, and offered valuable data to support the investigation of the desorption behavior between MPs and antibiotics on organisms.

Molecular investigation of the multi-phase photochemistry of Fe(III)-citrate in aqueous solution.

Iron (Fe) is ubiquitous in nature and found as FeII or FeIII in minerals or as dissolved ions Fe2+ or Fe3+ in aqueous systems. The interactions of soluble Fe have important implications for fresh water and marine biogeochemical cycles, which have impacts on global terrestrial and atmospheric environments. Upon dissolution of FeIII into natural aquatic systems, organic carboxylic acids efficiently chelate FeIII to form [FeIII-carboxylate]2+ complexes that undergo a wide range of photochemistry-induced radical reactions. The chemical composition and photochemical transformations of these mixtures are largely unknown, making it challenging to estimate their environmental impact. To investigate the photochemical process of FeIII-carboxylates at the molecular level, we conduct a comprehensive experimental study employing UV-visible spectroscopy, liquid chromatography coupled to photodiode array and high-resolution mass spectrometry detection, and oil immersion flow microscopy. In this study, aqueous solutions of FeIII-citrate were photolyzed under 365 nm light in an experimental setup with an apparent quantum yield of (φ) ∼0.02, followed by chemical analyses of reacted mixtures withdrawn at increment time intervals of the experiment. The apparent photochemical reaction kinetics of Fe3+-citrates (aq) were expressed as two generalized consecutive reactions of with the experimental rate constants of j1 ∼ 0.12 min-1 and j2 ∼ 0.05 min-1, respectively. Molecular characterization results indicate that R and I consist of both water-soluble organic and Fe-organic species, while P compounds are a mixture of water-soluble and colloidal materials. The latter were identified as Fe-carbonaceous colloids formed at long photolysis times. The carbonaceous content of these colloids was identified as unsaturated organic species with low oxygen content and carbon with a reduced oxidation state, indicative of their plausible radical recombination mechanism under oxygen-deprived conditions typical for the extensively photolyzed mixtures. Based on the molecular characterization results, we discuss the comprehensive reaction mechanism of FeIII-citrate photochemistry and report on the formation of previously unexplored colloidal reaction products, which may contribute to atmospheric and terrestrial light-absorbing materials in aquatic environments.

Biobased materials as potential precursors for disinfection by-products in water

This study investigated the fate of biobased materials in aquatic environments, focusing on their degradability and chemical characteristics of the breakdown products. Two commercial biobased materials, flax biocomposite (biopolymer or BPM) and biodegradable tissue paper (biopaper or BPP), were included in solubility and disinfection by-product (trihalomethanes, THMs) formation experiments. Deionized (DI), tap, river, and lake waters were used in this study. The results indicate that BPP had high disintegration and high dissolution of organics while BPM had low dissolution of organics. After incubation for seven days at an initial concentration of 20 g/L for BPM and 10 g/L for BPP in DI water, the total solids produced from BPM was 4.3 mg/L and 136 mg/L for BPP. Soluble organics represented by dissolved organic carbon (DOC) from both biobased materials were highly biodegradable up to 85%. Our results show that soluble organics from both materials reacted with chlorine to form THMs, mainly as chloroform. Even though the total THMs provided by BPP was higher, the reactivity of soluble organics from BPM with chlorine to form THMs (40 µg THMs formed/mg DOC) was more than twice that of soluble organics from BPP (18 µg THMs formed/mg DOC). Disinfection by-products potential and biodegradable DOC of biobased materials were revealed for the first time. Our work demonstrated that biobased materials, if not appropriately managed, could have adverse effects on drinking water quality.

Degradation of ester linkages in rice straw components by Sphingobium species recovered from the sea bottom using a non-secretory tannase-family α/β hydrolase.

Microbial decomposition of allochthonous plant components imported into the aquatic environment is one of the vital steps of the carbon cycle on earth. To expand the knowledge of the biodegradation of complex plant materials in aquatic environments, we recovered a sunken wood from the bottom of Otsuchi Bay, situated in northeastern Japan in 2012. We isolated Sphingobium with high ferulic acid esterase activity. The strain, designated as OW59, grew on various aromatic compounds and sugars, occurring naturally in terrestrial plants. A genomic study of the strain suggested its role in degrading hemicelluloses. We identified a gene encoding a non-secretory tannase-family α/β hydrolase, which exhibited ferulic acid esterase activity. This enzyme shares the consensus catalytic triad (Ser-His-Asp) within the tannase family block X in the ESTHER database. The molecules, which had the same calculated elemental compositions, were produced consistently in both the enzymatic and microbial degradation of rice straw crude extracts. The non-secretory tannase-family α/β hydrolase activity may confer an important phenotypic feature on the strain to accelerate plant biomass degradation. Our study provides insights into the underlying biodegradation process of terrestrial plant polymers in aquatic environments.

The toxicity of graphene oxide affected by algal physiological characteristics: A comparative study in cyanobacterial, green algae, diatom

Though the main toxic mechanisms of graphene oxide (GO) to algae have been accepted as the shading effect, oxidative stress and mechanical damage, the effect of algal characteristics on these three mechanisms of GO toxicity have seldom been taken into consideration. In this study, we investigated GO toxicity to green algae (Chlorella vulgaris, Scenedesmus obliquus, Chlamydomonas reinhardtii), cyanobacteria (Microcystis aeruginosa) and diatoms (Cyclotella sp.). The aim was to assess how the physiological characteristics of algae affect the toxicity of GO. Results showed that 10 mg/L of GO significantly inhibited the growth of all tested algal types, while S. obliquus and C. reinhardtii were found to be the most susceptible and tolerant species, respectively. Then, scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were used to observe the physiological characteristics of the assessed algae. The presence of locomotive organelles, along with smaller and more spherical cells, was more likely to alleviate the shading effect. Variations in cell wall composition led to different extents of mechanical damage as shown by Cyclotella sp. silica frustules and S. obliquus autosporine division being prone to damage. Meanwhile, growth inhibition and cell division were significantly correlated with the oxidative stress and membrane permeability, suggesting the latter two indicators can effectively signal GO toxicity to algae. The findings of this study provide novel insights into the toxicity of graphene materials in aquatic environments.

Effects of reduced graphene oxide on humic acid-mediated transformation and environmental risks of silver ions

The reduction of Ag+ mediated by natural organic matters has been demonstrated to be an important process of Ag+ transformation and would influence the risks of Ag+ and Ag-containing materials in aquatic environment. Considering the large production of carbon nanomaterials (CNMs) and their inevitable release into the environment, the effects of CNMs on Ag transformation are of considerable interest. This study demonstrated that the humic acid-mediated reduction of Ag+ to free Ag nanoparticles (AgNPs) in aqueous phase was suppressed by coexisting reduced graphene oxide (rGO). A large amount of Ag+ was reduced on rGO surface, resulting in the generation of AgNPs-rGO composites. rGO at concentrations of 1–2 orders of magnitude lower than those of Ag+ would exhibit significant effects. The X-ray absorption fine structure spectroscopy study showed that Ag+ was first adsorbed on rGO surface cooperatively with humic acid and then rapidly reduced to AgNPs. The hydroxylic-OH on rGO could participate in the AgNPs formation and was oxidized to carbonyl during the reduction of Ag+. Additionally, the formed AgNPs-rGO had a relatively lower environmental risk compared to AgNPs or rGO alone. Overall, these results improve our understanding of the interaction between CNMs and Ag+ in aquatic systems.

Boron Doped Diamond as a Low Biofouling Material in Aquatic Environments: Assessment of Pseudomonas aeruginosa Biofilm Formation.

Boron doped diamond (BDD), given the robustness of the material, is becoming an electrode of choice for applications which require long-term electrochemical monitoring of analytes in aqueous environments. However, despite the extensive work in this area, there are no studies which directly assess the biofilm formation (biofouling) capabilities of the material, which is an essential consideration because biofouling often causes deterioration in the sensor performance. Pseudomonas aeruginosa is one of the most prevalent bacterial pathogens linked to water-related diseases, with a strong capacity for forming biofilms on surfaces that are exposed to aquatic environments. In this study, we comparatively evaluate the biofouling capabilities of oxygen-terminated (O-)BDD against materials commonly employed as either the packaging or sensing element in water quality sensors, with an aim to identify factors which control biofilm formation on BDD. We assess the monospecies biofilm formation of P. aeruginosa in two different growth media, Luria-Bertani, a high nutrient source and drinking water, a low nutrient source, at two different temperatures (20 and 37 °C). Multispecies biofilm formation is also investigated. The performance of O-BDD, when tested against all other materials, promotes the lowest extent of P. aeruginosa monospecies biofilm formation, even with corrections made for total surface area (roughness). Importantly, O-BDD shows the lowest water contact angle of all materials tested, that is, greatest hydrophilicity, strongly suggesting that for these bacterial species, the factors controlling the hydrophilicity of the surface are important in reducing bacterial adhesion. This was further proven by keeping the surface topography fixed and changing surface termination to hydrogen (H-), to produce a strongly hydrophobic surface. A noticeable increase in biofilm formation was found. Doping with boron also results in changes in hydrophobicity/hydrophilicity compared to the undoped counterpart, which in turn affects the bacterial growth. For practical electrochemical sensing applications in aquatic environments, this study highlights the extremely beneficial effects of employing smooth, O-terminated (hydrophilic) BDD electrodes.

Stabilization of Ag-Au Bimetallic Nanocrystals in Aquatic Environments Mediated by Dissolved Organic Matter: A Mechanistic Perspective.

Gold and silver nanoparticles can be stabilized endogenously within aquatic environments from dissolved ionic species as a result of mineralization induced by dissolved organic matter. However, the ability of fulvic and humic acids to stabilize bimetallic nanoparticles is entirely unexplored. Elucidating the formation of such particles is imperative given their potential ecological toxicity. Herein, we demonstrate the nucleation, growth, and stabilization of bimetallic Ag-Au nanocrystals from the interactions of Ag+ and Au3+ with Suwannee River fulvic and humic acids. The mechanisms underpinning the stabilization of Ag-Au alloy NPs at different pH (6.0-9.0) values are studied by UV-vis spectrophotometry, X-ray photoelectron spectroscopy (XPS), high-resolution transmission electron microscopy (HRTEM), and selected area electron diffraction (SAED). Complexation of free Ag+ and Au3+ ions with the Lewis basic groups (carbonyls, carboxyls, and thiols) of FA and HA, followed by electron-transfer from redox-active moieties present in dissolved organic matter initiates the nucleation of the NPs. Alloy formation and interdiffusion of Au and Ag atoms are further facilitated by a galvanic replacement reaction between AuCl4- and Ag. Charge-transfer from Au to Ag stabilizes the formed bimetallic NPs. A more pronounced agglomeration of the Ag-Au NPs is observed when HA is used compared to FA as the reducing agent. The bimetallic NPs are stable for greater than four months, which suggests the possible persistence and dispersion of these materials in aquatic environments. The mechanistic ideas have broad generalizability to reductive mineralization processes mediated by dissolved organic matter.

Simplified model of spectral absorption by non-algal particles and dissolved organic materials in aquatic environments.

Absorption by non-algal particles (NAP, ad) and colored dissolved organic matter (CDOM, ag) are frequently modeled by exponential functions of wavelength, either separately or as a sum. We present a new representation of NAP-plus-CDOM absorption adg based on the stretched exponential function adg(λ) = A exp{-[s(λ - λo)]β}, whose parameter β can be considered a measure of optical heterogeneity. A double exponential representation of adg can be fit extremely well by a stretched exponential for all plausible parameter combinations, despite having one fewer free parameter than a double exponential. Fitting two published compilations of in situ adg data - one at low spectral resolution (n = 5, λ = 412-555 nm) and one at high spectral resolution (n = 201, λ = 300-700 nm) - the stretched exponential outperforms the single exponential, double exponential, and a power law. We thereby conclude that the stretched exponential is the preferred model for adg absorption in circumstances when NAP and CDOM cannot be separated, such as in remote sensing inversions.

Microcystis aeruginosa flour as carbon and nitrogen source for aerobic denitrification and algicidal effect of Raoultella sp. R11

Harmful algal blooms (HABs) could be deemed hazardous materials in aquatic environments. In this study, we investigated aerobic denitrification and algicidal effects of the Raoultella sp. R11. Based on PCR amplification, the denitrification genes napA was detected. A response surface methodology (RSM) analysis demonstrated that the maximum nitrate removal ratio was 96.06%, and optimal conditions occurred for inoculum of 13.02% (v/v), initial pH of 6.65, C/N ratio of 7.59 and temperature of 27.08°C based on a ridge analysis. When the Microcystis aeruginosa flour as carbon and nitrogen source for denitrification in aerobic conditions, the NO3−-N and TOC removal ratios were 72.36% and 85.56%, respectively. Additionly, the denitrification rate was 0.0123mgL−1h−1. Furthermore, the infrared spectrogram and SEM images showed the M. aeruginosa cell structure was destroyed, indicating that R11 could use algal death for denitrification. Therefore, Raoultella sp. R11 played a crucial role for simultaneous denitrification and algicidal effects in wastewater treatment.

Algicidal effects and denitrification activities of Acinetobacter sp. J25 against Microcystis aeruginosa

Harmful algal blooms (HABs) could be deemed hazardous materials in aquatic environment. Microcystis aeruginosa is a toxic HAB causing alga, which causes serious economic losses and health problems. In this study, a strain J25 with the high algicidal activity isolated from an eutrophic Qu Jiang Lake, was identified as Acinetobacter sp. based on its morphological features and phylogenetic analysis by sequencing of 16S rDNA. Its algae-lysing characteristics on M. aeruginosa, was tested under the different conditions. The results showed that the strain J25 had good tolerance of acid and alkali, and full illumination played a positive role in the algae removal. Furthermore, this was the first report about the relationship between algicidal and denitrification activities. The maximum denitrification rate (1.62mg NO3−–NL−1h−1) was observed within 6 days at 15% of inoculum.

The first evidence of deinoxanthin from Deinococcus sp. Y35 with strong algicidal effect on the toxic dinoflagellate Alexandrium tamarense

Harmful algal blooms (HABs) could be deemed hazardous materials in aquatic environment. Alexandrium tamarense is a toxic HAB causing alga, which causes serious economic losses and health problems. In this study, the bacterium Deinococcus xianganensis Y35 produced a new algicide, showing a high algicidal effect on A. tamarense. The algicidal compound was identified as deinoxanthin, a red pigment, based on high resolution mass spectrometry and NMR after the active compound was isolated and purified. Deinoxanthin exhibited an obvious inhibitory effect on algal growth, and showed algicidal activity against A. tamarense with an EC50 of 5.636μg/mL with 12h treatment time. Based on the unique structure and characteristics of deinoxanthin, the content of reactive oxygen species (ROS) increased after 0.5h exposure, the structure of organelles including chloroplasts and mitochondria were seriously damaged. All these results firstly confirmed that deinoxanthin as the efficient and eco-environmental algicidal compound has potential to be used for controlling harmful algal blooms through overproduction of ROS.

Chlortetracycline degradation by photocatalytic ozonation in the aqueous phase: mineralization and the effects on biodegradability

Chlortetracycline (CTC) is a hazardous material in aquatic environments. This study was focused on optimization of photocatalytic ozonation processes for removal of CTC from wastewater at pH 2.2 and 7.0. In this study, the tested processes for CTC removal were arranged from the least efficient to the most efficient as: UV, UV/TiO2, O3, O3/UV and O3/UV/TiO2. Ozonation efficiency was due to ozone affinity for electron-rich sites on the CTC molecule. In the O3/UV and O3/UV/TiO2 processes, efficiency was increased by the photolysis of CTC and generation of •OH. At pH 7.0, all the processes were more efficient for CTC degradation than at pH 2.2 due to CTC speciation, ozone decay to •OH and the attractions between ionized CTC and TiO2 particles. UV/O3 at pH 7.0 showed an additive effect while other combination processes showed a synergistic effect that resulted in higher rates of reactions than the sums of individual reaction rates. The TOC removal ranged from 8% to 41% after one hour of reaction, with the above-mentioned order of efficiency. The biodegradability increased rapidly during the early minutes of the reaction. A reaction time of 10–15 min was sufficient for near maximum biodegradability, making these processes good pretreatments for the biological processes.

Cellulose degradation by one mesophilic strain Caulobacter sp. FMC1 under both aerobic and anaerobic conditions

Caulobacteria are presumed to be responsible for considerable mineralization of organic material in aquatic environments. In this study, a facultative, mesophilic and cellulolytic bacterium Caulobacter sp. FMC1 was isolated from sediments which were taken from a shallow freshwater lake and then enriched with amendment of submerged macrophyte for three months. This strain seemed to evolve a capacity to adapt redox-fluctuating environments, and could degrade cellulose both aerobically and anaerobically. Cellulose degradation percentages under aerobic and anaerobic conditions were approximately 27% and 10% after a 240-h incubation in liquid mediums containing 0.5% cellulose, respectively. Either cellulose or cellobiose alone was able to induce activities of endoglucanase, exoglucanase, and β-1,4-glucosidase. Interestingly, ethanol was produced as the main fermentative product under anaerobic incubation on cellulose. These results could improve our understanding about cellulose-degrading process in aquatic environments, and were also useful in optimizing cellulose bioconversion process for bioethanol production.

Toxicity of raw and neutralized bauxite refinery residue liquors to the freshwater cladoceran Ceriodaphnia dubia and the marine amphipod Paracalliope australis

The extraction of alumina from bauxite produces a highly toxic residue, termed bauxite refinery residue (BRR) or red mud. The toxicity of this material is due to chemical and biological effects of high pH, alkalinity, electrical conductivity (EC), and Na(+) and Al(3+) concentrations. Several neutralization techniques may allow BRR to be used for environmental remediation. The present study investigated standardized 48-h acute toxicity tests with a freshwater cladoceran, Ceriodaphnia dubia, and a marine amphipod, Paracalliope australis, against raw supernatant BRR liquor (RL) versus liquors neutralized with acid (ANL), CO(2) (CNL), seawater (SNL), and a hybrid method (HNL). Based on 48-h LC50 values, the toxicity of the liquors to C. dubia increased in the following order; HNL ≤ SNL< ANL ≤ CNL < RL, with comparable responses from P. australis. The high toxicity of RL likely is due to high pH (≈ 12), alkalinity, and Al concentration. Toxicity of CNL likely is due to high EC and alkalinity. Sulfate and Na(+) concentrations are considered sources of toxicity in ANL. Seawater-neutralized liquor and HNL were considerably less toxic to both test species. These data provide evidence of the acute lethal toxicity of raw supernatant liquor from BRR and four neutralized supernatant liquors to the freshwater cladoceran C. dubia and the marine amphipod P. australis, providing valuable baselines for further ecotoxicological investigations of BRR materials in aquatic environments.

Photo-dissolution of flocculent, detrital material in aquatic environments: Contributions to the dissolved organic matter pool

This study shows that light exposure of flocculent material (floc) from the Florida Coastal Everglades (FCE) results in significant dissolved organic matter (DOM) generation through photo-dissolution processes. Floc was collected at two sites along the Shark River Slough (SRS) and irradiated with artificial sunlight. The DOM generated was characterized using elemental analysis and excitation emission matrix fluorescence coupled with parallel factor analysis. To investigate the seasonal variations of DOM photo-generation from floc, this experiment was performed in typical dry (April) and wet (October) seasons for the FCE. Our results show that the dissolved organic carbon (DOC) for samples incubated under dark conditions displayed a relatively small increase, suggesting that microbial processes and/or leaching might be minor processes in comparison to photo-dissolution for the generation of DOM from floc. On the other hand, DOC increased substantially (as much as 259 mgC gC −1) for samples exposed to artificial sunlight, indicating the release of DOM through photo-induced alterations of floc. The fluorescence intensity of both humic-like and protein-like components also increased with light exposure. Terrestrial humic-like components were found to be the main contributors (up to 70%) to the chromophoric DOM (CDOM) pool, while protein-like components comprised a relatively small percentage (up to 16%) of the total CDOM. Simultaneously to the generation of DOC, both total dissolved nitrogen and soluble reactive phosphorus also increased substantially during the photo-incubation period. Thus, the photo-dissolution of floc can be an important source of DOM to the FCE environment, with the potential to influence nutrient dynamics in this system.

Influence of extreme storm events on West Florida Shelf CDOM distributions

Colored Dissolved Organic Matter (CDOM) distribution and signatures provide vital information about the amount and composition of organic material in aquatic environments. This information is critical for deciphering the sources and biogeochemical pathways of organic carbon, and thus vital to the understanding of carbon cycling and budgets. Waters of the West Florida Shelf are heavily influenced by many river systems on Florida's Gulf Coast that, to the first order, control CDOM distributions on the shelf. Three storm events during 2004 and 2005 (Hurricane Charley, Hurricane Wilma, and a Winter Storm) profoundly altered the typical distribution of CDOM fluorescence and absorption properties on the Southern West Florida Shelf. Seasonal surveys revealed that changes in the underwater light field as a result of major hurricanes and resuspension events are linked closely with a number of factors prior to a storm's passing such as the presence of persistent blooms, rainfall and discharge. Additionally, storm track and wind direction were found to play a significant role in CDOM signatures.

Adsorption of Natural Organic Matter onto Carbonaceous Surfaces: Atomic Force Microscopy Study

The microscopic structure of carbonaceous surfaces exposed to natural organic matter (NOM) under aqueous conditions has been explored using atomic force microscopy (AFM). Dismal Swamp Water was used as the NOM source, while highly ordered pyrolytic graphite (HOPG) served as a surrogate for the graphene sheets that characterize the surface of many carbonaceous materials in aquatic environments. Under acidic conditions, the HOPG surface was covered with a densely packed monolayer of NOM molecules. In some cases, aggregates of well-defined, individual NOM molecules were observed that exhibited a degree of registry with respect to the HOPG substrate. This suggests that adsorbate-substrate interactions play a role in moderating the structure of the adsorbate layer. As the pH increased, the concentration of adsorbed NOM decreased systematically because of increasingly repulsive interactions between adsorbates. Increasing the ionic strength produced a modest increase in the concentration of adsorbed NOM. Ca2+ ions exerted a more pronounced influence on both the surface coverage of adsorbed NOM molecules and the size of individual adsorbates because of the effects of intermolecular complexation. In contrast to the spherical structures observed by AFM under aqueous conditions, adsorbed NOM formed a mixture of "ringlike" assemblies and larger aggregates upon drying.

Influence of pH and ionic strength on removal processes of a sedimentary humic acid in a suspension of vermiculite

Understanding the interactions of humic acids and clay minerals is important for modeling the geochemical fate and transport of nutrients and pollutants in soil and natural waters upon changes of environmental parameters such as ionic strength and pH. This paper describes the adsorption/precipitation properties of a sedimentary humic acid under different conditions of pH and ionic strength in presence of the clay mineral vermiculite. The clay suspension was prepared to have a distribution of grain size composed mainly of particles <63 μm, which is believed to play a major role on transport and fate of adsorbed materials in aquatic environments. The pHs studied were 5.0, 6.0 and 7.0 (±0.1 pH units), in ionic medium of 0.002 and 0.020 mol l−1 KNO3, using initial HA concentrations from 0 to 100 mg l−1. The maximum removal of HA from solution occurred at pH 5.0 in medium of 0.020 mol l−1 KNO3. This removal is mostly due to precipitation, which occurred at similar rates in both ionic strength, so that the additional HA removal at higher ionic strength (0.020 mol l−1 KNO3) was attributed to adsorption. Adsorption processes intensified at higher ionic strength suggest the occurrence of chemical interactions between the interfaces since under these conditions the electrostatic interactions are minimized. A similar effect was observed at pH 6.0, but in this case adsorption prevailed as the removal mechanism in medium of 0.020 mol l−1 KNO3. At pH 7, adsorption is the predominant process removing HA from solution, with increase of adsorption upon decreasing the ionic strength, suggesting that electrostatic attractions play the major role in the adsorption mechanism under these conditions. Comparison of the results obtained using a re-precipitated commercial HA with low N content suggested that the presence of positively charged amine groups increase the adsorption and decrease desorption.