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•The effects of citrate, oxalate and phosphate on Cr sorption by EPS was studied.•Oxalate and citrate enhanced the sorption of Cr by EPS compared to phosphate.•The zeta potential of ...EPS became less negative after interacting with Cr.•The adsorption of Cr by EPS was both spontaneous and exothermic.•Oxalate and citrate enhanced the reduction of Cr(VI) compared to phosphate.
Environmental pollution caused by heavy metals such as Cr(VI) is posing serious threats to humans and other living organisms. Bioreduction of toxic Cr(VI) to low toxic Cr(III) using extracellular polymeric substances (EPS) has gained attention in recent decades due to its cost-effectiveness and environmentally friendly nature. However, the effect of naturally occurring anions such as oxalate, citrate, and phosphate which have the potential to alter the sorption process needs to be investigated. Thus, the adsorption of Cr(VI) by EPS of Pseudomonas fluorescens was investigated in the presence of citrate, oxalate, and phosphate. Zeta potential measurements, spectroscopic and thermodynamic analyses were performed to study the biosorption process. After interacting with Cr(VI), the zeta potential of EPS became less negative suggesting the presence of cationic Cr species. The adsorption capacity of EPS for Cr(VI) was significantly (p < 0.05) enhanced in the presence of oxalate and citrate relative to free EPS and EPS-phosphate. This observation was attributed to the ability of oxalate and citrate to enhance Cr(VI) bioreduction to Cr(III). The maximum adsorption capacities were respectively 91.1, 48.4, 38, 30.2 mg g−1 for EPS-oxalate, EPS-citrate, free EPS, and EPS-phosphate. Cr adsorption isotherms fitted the Langmuir, Dubinin-Radushkevich (D-R), and Freundlich equations well. The adsorption process was both spontaneous and exothermic with ΔGads values of -19.0, -21.5, -20.8, and -26.7 kJ mol−1 and ΔHads of -165.8, -100.9, -274.6, and -104.4 kJ mol−1 for EPS-Cr(VI), EPS-Cr(VI)-phosphate, EPS-Cr(VI)-oxalate, and EPS-Cr(VI)-citrate, respectively. This study serves as a reference for optimizing the adsorption and bioreduction of Cr(VI).
More recent works implied that biochar enhanced P availability in variable charge soils. However, the involved mechanisms have not been clearly defined. In the present study, we evaluated the effects ...of peanut and rice straw biochars on improving P availability (Olsen-P) in an Oxisol and an Ultisol. Incubation experiments, with different amendments including peanut straw biochar (PC), rice straw biochar (RC), acid-washed peanut straw biochar (APC), acid-washed rice straw biochar (ARC), and Ca(OH)
2
, were performed. After 56-day incubation without and with 50 mg P kg
–1
loading, Olsen-P, pH, electrical conductivity (EC), and exchangeable acidity in the amended soils were analyzed. Results showed that PC greatly increased soil pH, EC, cation exchange capacity (CEC), and Olsen-P and decreased exchangeable acidity in biochar-amended soils followed by RC when compared with the control and other treatments. However, APC, ARC, and Ca(OH)
2
application did not increase P availability greatly. The results manifested that an adequate amount of functional groups together with high alkalinity in PC and RC was responsible for the increase in Olsen-P in both soils. Thus, application of biochar with higher functional groups and alkali mutually inhibited P adsorption and increased its availability in variable charge soils. The increase in soil pH enhanced the dissociation of acidic functional groups on the biochars and thus increased the ability of these anionic functional groups to compete for adsorption sites on the soils with phosphate and decreased phosphate adsorption by the soils, which was the main mechanism for the increase in Olsen-P content in the soils induced by the biochars.
Extracellular polymeric substances (EPS) found in soils can reduce the mobility of heavy metals through the use of both electrostatic and non-electrostatic mechanisms. Their effects vary from one ...soil type to another. The influence of EPS from Escherichia coli on the adsorption behaviors of Cu(II) and Cd(II) by two bulk variable charge soils, Oxisol and Ultisol, was studied at constant and varied pH, and the results were compared to a constant charge Alfisol. The maximum adsorption capacities of the soils were significantly (P < 0.05) enhanced in the presence of EPS, with Cu(II) adsorption being greater. Interaction of EPS with soils made the soil surface charge more negative by neutralizing positive charges and shifting the zeta potentials in a negative direction: from −18.6 to −26.4 mV for Alfisol, +5.1 to −22.2 mV for Oxisol, and +0.3 to −28.0 mV for Ultisol at pH 5.0. The adsorption data fitted both the Freundlich and Langmuir isotherms well. Preadsorbed Cd(II) was more easily desorbed by KNO3 than preadsorbed Cu(II) from both the control and EPS treated soils. The adsorption of both metals was governed by electrostatic and non-electrostatic mechanisms, although more Cu(II) was adsorbed through the non-electrostatic mechanism. The information obtained in this study will improve our understanding of the mechanisms involved in reducing heavy metals mobility in variable charge soils and hence, their bioavailability.
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•EPS enhanced adsorption of Cu(II) and Cd(II) by Oxisol, Ultisol and Alfisol.•EPS showed greater effect on metal adsorption by Oxisol and Ultisol than Alfisol.•Adsorption of EPS led to shift of zeta potentials of soil colloids to negative side.•Electrostatic and non-electrostatic reactions involved in enhanced metal adsorption.•Cd(II) was more mobile than Cu(II) in both soils and soil-EPS complexes.
The presence of EPS increased adsorption and immobilization of Cu(II) and Cd(II) in variable charge soils. In addition to electrostatic and non-electrostatic mechanisms, hydrophobic and van der Waals interactions also enhanced metal adsorption.
Soil arsenic (As) contamination is a serious concern because of its mark negative impacts on plant growth and physiological processes. In plant-soil system, As competes against phosphorus (P) which ...depends on charge component of different soil types. The main objective of this study was to investigate the influence of ((NH4)3PO4 (PO43-) and Ca5(PO4)3(OH) (phosphorite)) in ameliorating As stress on plant physiological process against As toxicity and their role in As accumulation. We performed eighteen treatments with different levels of As (0, 35, and 70 mg/kg) and P (0, 100, and 200 mg/kg) against two P sources of PO43- and phosphorite. Overall, more improvement in plant growth was observed by addition of PO43- than phosphorite. Significant increases in plant height (51%), dry biomass (root (49%) and shoot (40%)), chlorophyll contents (88%), total soluble sugars (58%) and plant functional leaves (51%) were observed by PO43- application as compared to their corresponding un-fertilized treatment under As stress conditions. However, proline and MDA contents were decreased by 49% and 71% with PO43- applied, respectively, under As stress. The As and P uptake by soybean were remarkably enhanced by the application of PO43− than phosphorite. Therefore, highly soluble P supplementation has great potential to minimize As-induced damage to plant growth in acidic soils and improve As uptake by plants. The findings obtained in present study will be used as an important tool for amelioration of As polluted acidic soils.
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•As contamination inhibited soybean growth in acidic Ultisol.•Addition of phosphate improved soybean growth in As contaminated treatments.•PO43− improved soybean growth more than phosphorite under As stress conditions.•Highly soluble phosphate decreased oxidative damage in soybean by As contamination.
To evaluate the effects of biochar on pH buffering capacity (pHBC) of paddy soils, rice straw biochar (RC) was modified by hydrogen peroxide (H
2
O
2
) and nitric acid/sulphuric acid (HNO
3
/H
2
SO
4
...), respectively, and then added to the paddy soils developed from sandy Ultisol, clay Ultisol and granite Ultisol at the same proportion. After submerging and draining incubation, soil pHBC, soil pH and the available cadmium (Cd) extracted with calcium chloride were determined. The results indicated that the two modified biochars effectively promoted pHBC of three paddy soils and HNO
3
/H
2
SO
4
modified biochar (HNO
3
/H
2
SO
4
-RC) led to greater increase in soil pHBC. Compared with control, HNO
3
/H
2
SO
4
-RC apparently increased pHBC of the paddy soils from sandy Ultisol, clay Ultisol and granite Ultisol by 32.7%, 37.4% and 25.3%, respectively. Correspondingly, soil pH increased by 1.59, 1.15 and 0.88 units, and the available Cd decreased by 74.1%, 67.5% and 75.0% in the paddy soils due to the incorporation of HNO
3
/H
2
SO
4
-RC. HNO
3
/H
2
SO
4
modification significantly increased the carboxyl groups on the surface of RC. The dissociation of these functional groups produced a large number of organic anions, which could consume exogenous protons, and thus retarding the decline of soil pH and decreased soil available Cd.
The relationship between the chemical forms of Cu2+ and Cd2+ adsorbed on the roots of different wheat cultivars and their phytotoxic effects on the plants were investigated. The wheat varieties ...Dunmaiwang (DMW), Tekang 6 (TK6), Zhongmai895 (ZM895), and Chaojixiaomai (AK68) were used. The zeta potentials of wheat roots, measured by the streaming potential method, were used to characterize root charge properties. Results indicated that the changes in zeta potential at pH 4.01–6.61 were 14.7, 15.53, 13.01, and 12.06 mV for ZM895, AK68, DMW, and TK6, respectively. The negative charge and functional groups on ZM895 and AK68 roots were greater than on DMW and TK6 roots, which led to more exchangeable and complexed Cu2+ and Cd2+ on ZM895 and AK68 roots and increased Cu2+ and Cd2+ toxicity compared to DMW and TK6. Coexisting cations, such as Ca2+, Mg2+, K+, and NH4+, alleviated Cu2+ and Cd2+ toxicity to wheat roots through competition for adsorption sites on the roots, which decreased exchangeable and complexed Cu2+ and Cd2+ on wheat roots. The Ca2+ and Mg2+ were most effective in alleviating heavy metal toxicity and they decreased exchangeable Cu2+ on AK68 roots by 39.14% and 47.82%, and exchangeable Cd2+ by 8.51% and 28.23%, respectively.
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•The roots of heavy metal-sensitive wheat varieties carry more negative charge.•More negative charge led to greater exchangeable Cu(II) and Cd(II) on wheat roots.•More functional groups led to greater complexed Cu(II) and Cd(II) on wheat roots.•More exchangeable and complexed Cu(II)/Cd(II) led to greater toxicity to wheat.•Coexisting cations alleviated Cu(II) and Cd(II) toxicity to wheat roots.
Phosphorus (P) availability in highly weathered soils is significantly influenced by the contents of iron (Fe)/aluminum (Al) oxides, clay minerals, and organic matter. With the increasing interest in ...biofertilizers (e.g. chitosan), it is important to understand how they affect P adsorption profiles on colloids of weathered soils rich in Fe/Al oxides. Thus, the effect of chitosan on the adsorption of P to colloids of hematite, gibbsite, Oxisol, and Ultisol was studied through electrokinetic measurements, spectroscopic analysis, and adsorption edge/isotherm profiles. The presence of chitosan significantly improved the surface positive charge and the decreasing trend of surface positive charge was slower for chitosan-treated colloids compared to the control with increasing pH. At pH 5.0, all the colloids were positively charged, with the oxides containing more positve charges than the soil colloids. At this pH value, the surface coverage capacity of P was 99.1, 61.6, 50.5, and 37.5 mmol kg⁻1 for Oxisol, Ultisol, hematite, and gibbsite, respectively. This suggests that clay minerals in soil colloids were vital in enhancing P adsorption. In the presence of chitosan, the surface coverage capacity of P was increased by 111%, 173%, 647%, and 488% for Oxisol, Ultisol, gibbsite, and hematite, respectively. Drawing inferences from spectroscopic analysis, citric acid desorption profile, and zeta potential analysis, we suggest that chitosan (CH) enhanced P adsorption by promoting the formation of (i) citric acid “undisplaceable” inner-sphere P complexes such as Colloid-OP-O-CH and Colloid-OP-N-CH, (ii) citric acid “displaceable” outer-sphere P complexes such as {Colloid-O-CH-OP} and {Colloid-N-CH-OP}, and (iii) water “leachable or soluble” P complexes such as {Colloid-CH+PO4³⁻} and {Colloid-OP⁻CH+}. Thus, applying chitosan as a biofertilizer (source of N) along with P in highly weathered soils could improve P availability while reducing P leaching.
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•Soil colloids adsorbed more phosphate than gibbsite/hematite due to clay minerals.•Chitosan enhanced positive charge of soil colloids more than for gibbsite/hematite.•Chitosan enhanced P adsorption through electrostatic and chemical complexation.•Effect of chitosan on P adsorption was largely dependent on chemical complexation.
Antibiotic pollution is an ever-growing concern that affects the growth of plants and the well-being of animals and humans. Research on antibiotics remediation from aqueous media has grown over the ...years and previous reviews have highlighted recent advances in antibiotics remediation technologies, perspectives on antibiotics ecotoxicity, and the development of antibiotic-resistant genes. Nevertheless, the relationship between antibiotics solution chemistry, remediation technology, and the interactions between antibiotics and adsorbents at the molecular level is still elusive. Thus, this review summarizes recent literature on antibiotics remediation from aqueous media and the adsorption perspective. The review discusses the principles, mechanisms, and solution chemistry of antibiotics and how they affect remediation and the type of adsorbents used for antibiotic adsorption processes. The literature analysis revealed that: (i) Although antibiotics extraction and detection techniques have evolved from single-substrate-oriented to multi-substrates-oriented detection technologies, antibiotics pollution remains a great danger to the environment due to its trace level; (ii) Some of the most effective antibiotic remediation technologies are still at the laboratory scale. Thus, upscaling these technologies to field level will require funding, which brings in more constraints and doubts patterning to whether the technology will achieve the same performance as in the laboratory; and (iii) Adsorption technologies remain the most affordable for antibiotic remediation. However, the recent trends show more focus on developing high-end adsorbents which are expensive and sometimes less efficient compared to existing adsorbents. Thus, more research needs to focus on developing cheaper and less complex adsorbents from readily available raw materials. This review will be beneficial to stakeholders, researchers, and public health professionals for the efficient management of antibiotics for a refined decision.
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•Antibiotics pollution has raised global environmental concerns.•Advanced technologies have been developed to detect, analyze and remediate antibiotics.•Advanced technologies combine two or more simpler processes to achieve better results.•The adsorption behavior of antibiotics differs on different surfaces.
The rapid increase in soil acidification rate has led to a decrease in global agricultural productivity owing to the debilitating effects of Al and Mn toxicities. In this study, we investigated the ...adaptation of plants to acidic conditions by examining the behavior of plant roots grown in hydroponic solution and pot experiments at different pHs. The Mn(II) sorption by the roots was investigated and the mechanisms involved were deduced by analyzing the changes in the zeta potential and functional groups on the root surface. The exchangeable, complexed, and precipitated Mn(II) on plant roots were extracted sequentially with 1 M KNO3, 0.05 M EDTA-2Na, and 0.01 M HCl. The results of hydroponic experiment indicated that plant roots subjected to NH4+ treatment carried lower negative charge and fewer functional groups owing to acidic pH condition induced by NH4+ uptake of roots, when compared with plant roots treated with NO3−. Similarly, in pot experiments, the surface negative charge and functional groups of plant roots cultured in soils with lower pH were fewer than those on plant roots cultured in soils with higher pH, with the former presenting less exchangeable and complexed Mn(II) sorption than the latter. Thus, alterations in the charge properties and number of functional groups on the surface of plant roots are some of the mechanisms used by plants to adapt to acidic soil condition.
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•Plants grown in acid solutions and soils carry less negative charge on their roots.•Plants grown in acid solutions and soils have few functional groups on their roots.•The roots of plants grown in acid conditions adsorbed less exchangeable Mn(II).•Plant roots reduce negative charges and functional groups to adapt acidic soils.
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