Molecular Understanding of Humic Acid-Limited Phosphate Precipitation and Transformation

Phosphorus recovery via struvite crystallization in batch and fluidized-bed reactors: Roles of microplastics and dissolved organic matter

Struvite crystallization, a promising technology for nutrient recovery from wastewater, is facing considerable challenges due to the presence of emerging contaminants such as microplastics (MPs) ubiquitously found in wastewater. Here, we investigate the roles of MPs and humic acid (HA) in struvite crystallization in batch and fluidized-bed reactors (FBRs) using synthetic and real wastewater with a Mg:N:P molar ratio of 1:3:(1-1.3) at an initial pH of 11. Batch reactor (BR) experiment results show that MPs expedited the nucleation and growth rates of struvite (e.g., the rate of crystal growth in the presence of 30 mg L-1 of polyethylene terephthalate (PET) was 1.43 times higher than that in the blank system), while HA hindered the formation of struvite. X-ray diffraction and the Rietveld refinement analysis revealed that the presence of MPs and HA can result in significant changes in phase compositions of the reclaimed precipitates, with over 80 % purity of struvite found in the precipitates from suspensions in the presence of 30 mg L-1 of MPs. Further characterizations demonstrated that MPs act as seeds of struvite nucleation, spurring the formation of well-defined struvite, while HA favors the formation of newberyite rather than struvite in both reactors. These findings highlight the need for a more comprehensive understanding of the interactions between emerging contaminants and struvite crystallization processes to optimize nutrient recovery strategies for mitigating their adverse impact on the quality and yield of struvite-based fertilizers. ENVIRONMENTAL IMPLICATION: The presence of microplastics in wastewater poses a significant challenge to struvite crystallization for nutrient recovery, as it accelerates nucleation and growth rates of struvite crystals. This can lead to changes in the phase compositions of the reclaimed precipitates, with implications for the quality and yield of struvite-based fertilizers. Additionally, the presence of humic acid hinders the formation of struvite, favoring the formation of other minerals like newberyite. Understanding the interactions between emerging contaminants and struvite crystallization processes is crucial for optimizing nutrient recovery strategies and mitigating the environmental impact of these contaminants on water quality and struvite-based fertilizers.

Molecular interaction mechanism for humic acids fouling resistance on charged, zwitterion-like and zwitterionic surfaces

Humic acids (HA) are ubiquitous in surface waters, leading to significant fouling challenges. While zwitterion-like and zwitterionic surfaces have emerged as promising candidates for antifouling, a quantitative understanding of molecular interaction mechanism, particularly at the nanoscale, still remains elusive. In this work, the intermolecular forces between HA and charged, zwitterion-like or zwitterionic monolayers in aqueous environments were quantified using atomic force microscope. Compared to cationic MTAC ([2-(methacryloyloxy)ethyl]trimethylammonium chloride), which exhibited an adhesion energy of ∼1.342 mJ/m2 with HA due to the synergistic effect of electrostatic attraction and possible cation-π interaction, anionic SPMA (3-sulfopropyl methacrylate) showed a weaker adhesion energy (∼0.258 mJ/m2) attributed to the electrostatic repulsion. Zwitterion-like MTAC/SPMA mixture, driven by electrostatic attraction between opposite charges, formed a hydration layer that prevented the interaction with HA, thereby considerably reducing adhesion energy to ∼0.123 mJ/m2. In contrast, zwitterionic MPC (2-methacryloyloxyethyl phosphorylcholine) and DMAPS ([2-(methacryloyloxy)ethyl]dimethyl-(3-sulfopropyl) ammonium hydroxide) displayed ultralow adhesion energy (0.06-0.07 mJ/m2) with HA, arising from their strong dipole moments which could induce a tight hydration layer that effectively inhibited HA fouling. The pH-mediated electrostatic interaction resulted in the increased adhesion energy for MTAC but decreased adhesion energy for SPMA with elevated pH, while the adhesion energy for zwitterion-like and zwitterionic surfaces was independent of environmental pH. Density functional theory (DFT) simulation confirmed the strong binding capability of MPC and DMAPS with water molecules (∼-12 kcal mol-1). This work provides valuable insights into the molecular interaction mechanisms underlying humic-substance-fouling resistance of charged, zwitterion-like and zwitterionic materials at the nanoscale, shedding light on developing more effective strategy for HA antifouling in water treatment.

Iron-containing nanominerals for sustainable phosphate management: A comprehensive review and future perspectives

Adsorption, which is a quick and effective method for phosphate management, can effectively address the crisis of phosphorus mineral resources and control eutrophication. Phosphate management systems typically use iron-containing nanominerals (ICNs) with large surface areas and high activity, as well as modified ICNs (mICNs). This paper comprehensively reviews phosphate management by ICNs and mICNs in different water environments. mICNs have a higher affinity for phosphates than ICNs. Phosphate adsorption on ICNs and mICNs occurs through mechanisms such as surface complexation, surface precipitation, electrostatic ligand exchange, and electrostatic attraction. Ionic strength influences phosphate adsorption by changing the surface potential and isoelectric point of ICNs and mICNs. Anions exhibit inhibitory effects on ICNs and mICNs in phosphate adsorption, while cations display a promoting effect. More importantly, high concentrations and molecular weights of natural organic matter can inhibit phosphate adsorption by ICNs and mICNs. Sodium hydroxide has high regeneration capability for ICNs and mICNs. Compared to ICNs with high crystallinity, those with low crystallinity are less likely to desorb. ICNs and mICNs can effectively manage municipal wastewater, eutrophic seawater, and eutrophic lakes. Adsorption of ICNs and mICNs saturated with phosphate can be used as fertilizers in agricultural production. Notably, mICNs and ICNs have positive and negative effects on microorganisms and aquatic organisms in soil. Finally, this study introduces the following: trends and prospects of machine learning-guided mICN design, novel methods for modified ICNs, mICN regeneration, development of mICNs with high adsorption capacity and selectivity for phosphate, investigation of competing ions in different water environments by mICNs, and trends and prospects of in-depth research on the adsorption mechanism of phosphate by weakly crystalline ferrihydrite. This comprehensive review can provide novel insights into the research on high-performance mICNs for phosphate management in the future.

Direct Nanoscale Imaging Reveals the Mechanism by Which Organic Acids Dissolve Vivianite through Proton and Ligand Effects

The coprecipitation of iron (Fe) and phosphorus (P) in natural environments limits their bioavailability. Plant root-secreted organic acids can dissolve Fe-P precipitates, but the molecular mechanism underlying mobilizing biogenic elements from highly insoluble inorganic minerals remains poorly understood. Here, we investigated vivianite (Fe3(PO4)2·8H2O) dissolution by organic acids (oxalic acid (OA), citric acid (CA), and 2'-dehydroxymugineic acid (DMA)) at three different pH values (4.0, 6.0, and 8.0). With increasing pH, the vivianite dissolution efficiency by OA and CA was decreased while that by DMA was increased, indicating various dissolution mechanisms of different organic acids. Under acidic conditions, weak ligand OA (HC2O4- > C2O42- at pH 4.0 and C2O42- at pH 6.0) dissolved vivianite through the H+ effect to form irregular pits, but under alkaline condition (pH 8.0), the completely deprotonated OA was insufficient to dissolve vivianite. At pH 4.0, CA (H2Cit- > HCit2- > H3Cit) dissolved vivianite to form irregular pits through a proton-promoted mechanism, while at pH 6.0 (HCit2- > Cit3-) and pH 8.0 (Cit3-), CA dissolved vivianite to form near-rhombohedral pits through a ligand-promoted mechanism. At three pH values ((H0)DMA3- > (H1)DMA2- at pH 4.0, (H0)DMA3- at pH 6.0, and (H0)DMA3- and one deprotonated imino at pH 8.0), strong ligand DMA dissolved vivianite to form near-rhombohedral pits via ligand-promoted mechanisms. Raman spectroscopy showed that the deprotonated carboxyl groups (COO-) and imino groups were bound to Fe on the vivianite (010) face. The surface free energy of vivianite coated with OA decreased from 29.32 mJ m-2 to 24.23 mJ m-2 and then to 13.47 mJ m-2 with increasing pH, and that coated with CA resulted in a similar pH-dependent vivianite surface free-energy decrease while that coated with DMA increased the vivianite surface free energy from 31.92 mJ m-2 to 39.26 mJ m-2 and then to 49.93 mJ m-2. Density functional theory (DFT)-based calculations confirmed these findings. Our findings provide insight into the mechanism by which organic acids dissolved vivianite through proton and ligand effects.

Contrasting fertilization response of soil phosphorus forms and functional bacteria in two newly reclaimed vegetable soils

Fertilization is a pervasive approach to agricultural production enhancing vegetable nutrients such as phosphorus (P) absorption. However, unreasonable fertilization strategies result in high levels of residual P in vegetable planting systems. To better understand the mechanisms of soil phosphorus dynamics responding to inorganic/organic fertilization, we conducted a 3-year field experiment in two newly reclaimed vegetable fields in southern China. The results revealed that soil Olsen-P in CF (mineral fertilization) and OF (Combined application of organic and inorganic fertilizers) increased by approximately 210.6 % and 183.6 %, respectively, while stable P proportion decreased by approximately 9.2 % and 18.1 %, respectively, compared with CK. Combined application of organic and inorganic fertilizer increased the proportion of moderately labile P (NaOH-P) by 1-6 % in comparison with chemical fertilizer and facilitated the conversion from diester-P to monoester-P, indicating that applying pig manure enhanced the potential soil P bioavailability. Besides, organic-inorganic fertilization shaped a bacterial community with more connectivity and stability and changed keystone taxa related to the P transformation of the network. Phenylobacterium, Solirubrobacter, and Modestobacter were regarded as core genera for mobilizing soil phosphorus. However, residual P content in newly reclaimed soils under fertilization, especially for chemical fertilizer, remained non-negligible and may cause potential environmental risks. The partial least squares path modeling results demonstrated that fertilization management had both direct and indirect positive effects on P fraction through the improvement of soil nutrients e.g. total N and soil organic carbon, and bacterial community, while soil properties mainly determined the variation of soil P species. Our results provide comprehensive insights into the current status of legacy P forms and the vital role of fertilizer, key soil properties and bacteria in P dynamics in newly reclaimed vegetable field.

Field application of hydroxyapatite and humic acid for remediation of metal-contaminated alkaline soil

The quality of soil is essential for ensuring the safety and quality of agricultural products. However, soils contaminated with toxic metals pose a significant threat to agricultural production and human health. Therefore, remediation of contaminated soils is an urgent task, and humic acid (HA) with hydroxyapatite (HAP) materials was applied for this study in contaminated alkaline soils to remediate Cd, Pb, Cu, and Zn. Physiochemical properties, improved BCR sequential extraction, microbial community composition in soils with superoxide dismutase (SOD), peroxidase (POD), and chlorophyll content in plants were determined. Among the studied treatments, application of HAP-HA (2:1) (T7) had the most significant impact on reducing the active forms of toxic metals from soil such as Cd, Pb, Cu, and Zn decreased by 18.59%, 9.12%, 11.83%, and 3.33%, respectively, but HAP and HA had a minor impact on metal accumulation in Juncao. HAP (T2) had a beneficial impact on reducing the TCleaf/root of Cd, Cu, and Zn, whereas HAP-HA (T5) showed the best performance for reducing Cd and Cu in EFleaf/soil. HAP-HA (T5 and T7) showed higher biomass (57.3%) and chlorophyll (17.9%), whereas HAP (T4) showed better performance in POD (25.8%) than T0 in Juncao. The bacterial diversity in soil was increased after applying amendments of various treatments and enhancing metal remediation. The combined application of HAP and HA effectively reduced active toxic metals in alkaline soil. HAP-HA mixtures notably improved soil health, plant growth, and microbial diversity, advocating for their use in remediating contaminated soils.

Differential Adsorption of Dissolved Organic Matter and Phosphorus on Clay Mineral in Water-Sediment System

Lake sediments connection to the biogeochemical cycling of phosphorus (P) and carbon (C) influences streamwater quality. However, it is unclear whether and how the type of sediment controls P and C cycling in water. Here, the adsorption behavior of montmorillonite (Mt) with different interlayer cations (Na+, Ca2+, or Fe3+) on dissolved organic matter (DOM) and P was investigated to understand the role of Mt in regulating the organic carbon-to-phosphate (OC/P) ratio within freshwater systems. The adsorption capacity of Fe-Mt for P was 3.2-fold higher than that of Ca-Mt, while it was 1/3 lower for DOM. This dissimilarity in adsorption led to an increased OC/P in Fe-Mt-dominated water and a decreased OC/P in Ca-Mt-dominated water. Moreover, an in situ atomic force microscope and high-resolution mass spectrometry revealed molecular fractionation mechanisms and adsorptive processes. It was observed that DOM inhibited the nucleation and crystallization processes of P on the Mt surface, and P affected the binding energy of DOM on Mt through competitive adsorption, thereby governing the interfacial P/DOM dynamics on Mt substrates at a molecular level. These findings have important implications for water quality management, by highlighting the role of clay minerals as nutrient sinks and providing new strategies for controlling P and C dynamics in freshwater systems.

Cooperative roles of phosphate and dissolved organic matter in inhibiting ferrihydrite transformation and their distinct fates

Phosphate and dissolved organic matter (DOM) mediate the crystalline transformation of ferrihydrite catalyzed by Fe(II) in subsurface environments such as soils and groundwater. However, the cooperative mechanisms underlying the mediation of phosphate and DOM in crystalline transformation of ferrihydrite and the feedback effects on their own distribution and speciation remain unresolved. In this study, solid characterization indicates that phosphate and DOM can collectively inhibit the crystalline transformation of ferrihydrite to lepidocrocite and thus goethite, via synergetic effects of inhibiting recrystallization and electron transfer. Phosphate can be retained on the surface or transformed to a nonextractable form within Fe oxyhydroxides; DOM is either released into the solution or preserved in an extractable form, while it is not incorporated or retained in the interior. Element distribution and DOM composition analysis on Fe oxyhydroxides reveals even distribution of phosphate on the newly formed Fe oxyhydroxides, while the distribution of DOM depends on its specific species. Electrochemical and dynamic force spectroscopic results provide molecular-scale thermodynamic evidence explaining the inhibition of electron transfer between Fe(II) and ferrihydrite by phosphate and DOM, thus influecing the crystalline transformation of ferrihydrite and the distribution of phosphate and DOM. This study provides new insights into the coupled biogeological cycle of Fe with phosphate and DOM in aquatic and terrestrial environments.

Direct observations of nanoscale brushite dissolution by the concentration-dependent adsorption of phosphate or phytate

With the development of agricultural intensification, phosphorus (P) accumulation in croplands and sediments has resulted in the increasingly widespread interaction between inorganic and organic P species, which has been, previously, underestimated or even ignored. We quantified the nanoscale dissolution kinetics of sparingly soluble brushite (CaHPO4·2H2O, DCPD) over a broad range of phosphate and/or phytate concentrations by using in situ atomic force microscopy (AFM). Compared to water, we found that low concentrations of phosphate (1-1000 µM) or phytate (1-100 µM) inhibited brushite dissolution by slowing single step retraction. However, with increasing phosphate or phytate concentrations to 10 mM, there was a reverse effect of dissolution promotion at brushite-water interfaces. In situ observations of the coupled dissolution-reprecipitation showed that phosphate precipitated more readily than phytate on brushite surfaces, with the formation of amorphous calcium phosphate (ACP). For a fundamental understanding, zeta potential and in situ Raman spectroscopy (RS) revealed that the concentration-dependent dissolution is attributed to the reverse of outer-sphere to inner-sphere adsorption with increasing phosphate or phytate concentrations. In addition, the mineralization of phytate with outer-sphere adsorption by phytase was higher than that with inner-spere adsorption, and the presence of phytate delayed ACP phase transformation to hydroxylapatite (HAP). These in situ observations and analyses may fill the knowledge gaps of interaction between inorganic and organic P species in P-rich terrestrial and aquatic environments, thereby implicating their biogeochemical cycling and the associated availability.

Applied phosphorus is maintained in labile and moderately occluded fractions in a typical meadow steppe with the addition of multiple nutrients

Phosphorus (P) is a limiting nutrient second only to nitrogen (N) in the drylands of the world. Most previous studies have focused on N transformation processes in grassland ecosystems, particularly under artificial fertilization with N and atmospheric N deposition. However, P cycling processes under natural conditions and when P is applied as an inorganic P fertilizer have been understudied. Therefore, it is essential to examine the fate of applied P in grassland ecosystems that have experienced long-term grazing and, under certain circumstances, continuous hay harvest. We conducted a 3-year field experiment with the addition of multiple nutrient elements in a typical meadow steppe to investigate the fate of the applied P in various fractions of P pools in the top soil. We found that the addition of multiple nutrients significantly increased P concentrations in the labile inorganic P (Lab-Pi) and moderately occluded inorganic P (Mod-Pi) fractions but not in the recalcitrant inorganic P (Rec-Pi) fraction. An increase in the concentration of total inorganic P was found only when P and N were applied together. However, the addition of other nutrients did not change P concentrations in any fraction of the mineral soil. The addition of P and N significantly increased the total amount of P taken up by the aboveground plants but had no effect on the levels of organic and microbial P in the soil. Together, our results indicate that the P applied in this grassland ecosystem is taken up by plants, leaving most of the unutilized P as Lab-Pi and Mod-Pi rather than being immobilized in Rec-Pi or by microbial biomass. This implies that the grassland ecosystem that we studied has a relatively low P adsorption capacity, and the application of inorganic P to replenish soil P deficiency in degraded grasslands due to long-term grazing of livestock or continuous harvest of forage in the region could be a practical management strategy to maintain soil P fertility.

Understanding the Nanoscale Affinity between Dissolved Organic Matter and Noncrystalline Mineral with the Implication for Water Treatment

In recent decades, the concentration of dissolved organic matter (DOM) in aquatic ecosystems has gradually increased, leading to water pollution problems. Understanding the interfacial chemical processes of DOM on natural minerals is important to the exploration of high-efficiency absorbents. However, studying DOM chemical processes and adsorption mechanisms are still challenging due to the complex DOM structure and environmental system. Hence, we characterized the microstructure changes after the formation of amorphous calcium phosphate (ACP) at the interface of montmorillonite (Mt) minerals in a simulated environment system. Combined with atomic force microscopy and density functional theory (DFT) simulation, the mechanism of interfacial interaction between Mt-ACP and DOM was characterized at the molecular level. Moreover, we further evaluated the adsorption behavior of Mt-ACP as a potential adsorbent for organic matter. The comprehensive investigation of humic acid adsorption, intermolecular force, and DFT simulation is conducive to our understanding of the interfacial interaction mechanism between organic matter and noncrystalline minerals in aquatic environments and provides new perspectives on the application of clay-based mineral materials in pollutant removal under exposure from DOM.

Calcium phosphate hybrid micelles inhibit orthotopic bone metastasis from triple negative breast cancer by simultaneously killing cancer cells and reprogramming the microenvironment of bone resorption and immunosuppression

Triple negative breast cancer (TNBC) is prone to develop drug resistance and metastasis. Bone is the most common distant metastasis site of breast cancer cell. Patients with bone metastasis from TNBC suffer from unbearable pain due to the growth of bone metastasis and bone destruction. Simultaneously blocking the growth of bone metastasis and reprogramming the microenvironment of bone resorption and immunosuppression is a promising strategy to treat bone metastasis from TNBC. Herein, we prepared a pH and redox responsive drug delivery system, named DZ@CPH, by encapsulating docetaxel (DTX) with hyaluronic acid-polylactic acid micelle then reinforcing with calcium phosphate and zoledronate for targeting to bone metastasis from TNBC. DZ@CPH reduced the activation of osteoclast and inhibited bone resorption by decreasing the expression of nuclear factor κB receptor ligand and increasing the expression of osteoprotegerin in drug-resistant bone metastasis tissue. At the same time, DZ@CPH inhibited the invasion of bone metastatic TNBC cells by regulating the apoptosis-related and invasion-related protein expression. It also increased the sensitivity of orthotopic drug-resistant bone metastasis to DTX by inhibiting the expression of P-glycoprotein, Bcl-2 and transforming growth factor-β in tissue of drug-resistant bone metastasis. Moreover, the ratio between M1 type macrophage to M2 type macrophage in bone metastasis tissue was increased by DZ@CPH. In a word, DZ@CPH blocked the growth of bone metastasis from drug-resistant TNBC through inducing the apoptosis of drug-resistant TNBC cells and reprogramming the microenvironment of bone resorption and immunosuppression. DZ@CPH has a great potential in clinical application for the treatment of bone metastasis from drug-resistant TNBC. STATEMENT OF SIGNIFICANCE: Triple negative breast cancer (TNBC) is prone to develop bone metastasis. Now bone metastasis is still an intractable disease. In this study, docetaxel and zoledronate co-loaded calcium phosphate hybrid micelles (DZ@CPH) were prepared. DZ@CPH reduced the activation of osteoclasts and inhibited bone resorption. At the same time, DZ@CPH inhibited the invasion of bone metastatic TNBC cells by regulating the expression of apoptosis and invasion related protein in bone metastasis tissue. Moreover, the ratio between M1 type macrophages to M2 type macrophages in bone metastases tissue was increased by DZ@CPH. In a word, DZ@CPH blocked vicious cycle between the growth of bone metastasis and bone resorption, which greatly improved the therapeutic effect on bone metastasis from drug-resistant TNBC.

Phosphorus recovery from wastewater via calcium phosphate precipitation: A critical review of methods, progress, and insights

Phosphorus (P) is one of the important elements for human, animal, and plant life. Due to the development of the circular economy in recent years, the recovery of P from wastewater has received more attention. Recovery of P from domestic, industrial, and agricultural wastewater in the form of calcium phosphate (CaP) by precipitation/crystallization process presents a low-cost and effective method. Recovered CaP could be used as P fertilizer and for other industrial applications. This review summarizes the effects of supersaturation, pH, seed materials, calcium (Ca) source, and wastewater composition, on the precipitation/crystallization process. The recovery efficiency and value proposition of recovered CaP were assessed. This in-depth analysis of the literature reports identified the process parameters that are worth further optimization. The review also provides perspectives on future research needs on expanding the application field of recovered CaP and finding other more economical and environmentally friendly Ca sources.

A novel heterogeneous catalytic system (AC/ZVI/CaO2) promotes simultaneous removal of phosphate and sulfamethazine: Performance, mechanism and application feasibility verification

The eutrophication and pharmaceutical residue are the key issues to the treatment of rural non-point source pollution, concerning risks to aquatic ecosystems and human health. In this study, a novel activated carbon/zero-valent iron/calcium peroxide (AC/ZVI/CaO₂) catalytic system was constructed to remove simultaneously typical rural non-point source pollutants: phosphate and sulfamethazine (SMZ). The optimal mass ratio of the system was determined as 20% AC, 48% ZVI and 32% CaO₂. It was shown that the removal efficiency of phosphorus (P) and SMZ exceeded 65% and 40% in pH 2-11, respectively. It worked well in the presence of typical anions and humic acid. The mechanistic analyses for P removal indicated that AC/ZVI/CaO₂ system can effectively load P by the formation of crystalline state Ca-P species and Fe-P/Ca-P amorphous state coprecipitates under neutral and acidic conditions, respectively. The presence of AC in AC/ZVI/CaO₂ system could form iron-carbon micro-electrolysis process for accelerating Fenton reaction in acidic environment. And AC also can produce reactive oxygen species for the SMZ degradation by relying on persistent free radicals/graphitic carbon catalysis under environmental condition. In addition, we developed a low-impact development stormwater filter for application feasibility verification of the system. Feasibility analysis showed that the system could save up to ∼50% cost in contrast with the price of Phoslock (a commercial P load product) and presented advantages of non-toxicity, long-acting, stability and the potential to promote biodegradation by provision of aerobic environment.

Biotic and abiotic effects of soil organic matter on the phytoavailable phosphorus in soils: a review

Soil organic matter (SOM) has a critical role in regulating soil phosphorus (P) dynamics and producing phytoavailable P. However, soil P dynamics are often explained mainly by the effects of soil pH, clay contents, and elemental compositions, such as calcium, iron, and aluminum. Therefore, a better understanding of the mechanisms of how SOM influences phytoavailable P in soils is required for establishing effective agricultural management for soil health and enhancement of soil fertility, especially P-use efficiency. In this review, the following abiotic and biotic mechanisms are discussed; (1) competitive sorption between SOM with P for positively charged adsorption sites of clays and metal oxides (abiotic reaction), (2) competitive complexations between SOM with P for cations (abiotic reaction), (3) competitive complexations between incorporation of P by binary complexations of SOM and bridging cations with the formation of stable P minerals (abiotic reaction), (4) enhanced activities of enzymes, which affects soil P dynamics (biotic reaction), (5) mineralization/immobilization of P during the decay of SOM (biotic reaction), and (6) solubilization of inorganic P mediated by organic acids released by microbes (biotic reaction).

Nanoscale Interactions of Humic Acid and Minerals Reveal Mechanisms of Carbon Protection in Soil

The concentrations of terrestrially sourced dissolved organic matter (DOM) have expanded throughout aquatic ecosystems in recent decades. Although sorption to minerals in soils is one major pathway to sequestrate soil organic matter, the mechanisms of organic matter-mineral interactions are not thoroughly understood. Here, we investigated the effect of calcium phosphate mineralization on humic acid (HA) fixation in simulated soil solutions, either with or without clay mineral montmorillonite (Mt). We found that Mt in solution promoted nucleation and crystallization of calcium phosphate (CaP) due to amorphous calcium phosphate clustering and coalescence on Mt surface, which contributed to the long-term persistence and accumulation of HA. Organic ligands with specific chemical groups on HA have higher binding energies to CaP-Mt than to CaP/Mt, according to dynamic force spectroscopy observations. Moreover, CaP-Mt formed in solution showed a great capacity for HA adsorption with a maximum adsorption quantity of 156.89 mg/g. Our findings directly support that Mt is crucial for DOM sequestration by facilitating CaP precipitation/transformation. This has an impact on how effectively we understand the long-term turnover of DOM and highlights knowledge gaps that might assist in resolving essential soil C sequestration issues.

Root response and phosphorus uptake with enhancement in available phosphorus level in soil in the presence of water-soluble organic matter deriving from organic material

Understanding the available phosphorus (P) levels in the presence of water-soluble organic matter (WSOM) deriving from organic materials can be important for the improvement of the P use efficiency. This study aimed to: (i) determine which types of WSOM (deriving from the organic material) can suppress P immobilization, and (ii) understand whether plants can uptake P that the immobilization is suppressed by the presence of WSOM, as well as how the plant roots response depending on the available P levels. The P sorption test revealed that the presence of WSOM deriving from cattle manure compost (CM), sewage sludge compost (SSC), and hydrothermal decomposed liquid fertilizer (HDLF) can suppress the P sorption by 44, 44, and 24%, respectively, as compared to single P. In the incubation test, the percentage of the available P to that added as P fertilizer was found to be >21% higher in the presence of a CM- or a SSC-derived WSOM than those of single P, but the effect of the HDLF-derived WSOM was not. In the cultivation test, P uptake was found to be improved in the CM-, the SSC-, and the HDLF-deriving WSOM by 17, 13, and 11%, respectively, as compared to single P. Moreover, the root weight was found to decrease along with an increase in the amount of P uptaken by the plant. These findings provide the first experimental evidence that the presence of the WSOM deriving from CM, SSC, and HDLF simultaneously enhance the available P level in the soil and P uptake by the plant at the lab-scale test. In addition, the higher the available P levels in the presence of WSOM, the lower the root developments. The presence of WSOM, particularly of one of high maturity, can suppress the P sorption by 24-44%; as a result, >20% of the P added remains as the available P depending on the type of organic material used.

Effect of nanohydroxyapatite/biochar/sodium humate composite on phosphorus availability and microbial community in sandy soils

Phosphorus (P) is essential for crop growth as an indispensable nutrient; however, there has been growing concern over the low use efficiency of P used in current fertilizers. We synthesized and characterized a potential P fertilizer nanohydroxyapatite/biochar/sodium humate (nHAP/BC/HANa) composite. To study the impact of the composite on soil chemical properties and microbial community in sandy soils, we set up four treatments as follows: (1) biochar (BC), (2) nanohydroxyapatite (nHAP), (3) nHAP/BC/HANa composite, and (4) sodium humate (HANa) was added separately into soils amended with nHAP/BC (nHAP/BC + HANa) to compare its performance with that of the nHAP/BC/HANa composite. A key finding was that the nHAP/BC/HANa composite not only significantly increased the soil available P content and alkaline phosphatase activity but also the increased organic matter content compared to the control. Additionally, leaching losses of P in soils amended with the nHAP/BC/HANa composite were lower than those in soils amended with the nHAP/BC + HANa, which suggested that the nHAP/BC/HANa composite had great potential to decrease P loss in sandy soils. Moreover, bacterial communities were more sensitive than fungal communities to all treatments. The bacterial communities showed the most significant changes in the nHAP/BC/HANa treatments. Results from Mantel tests further indicated that the strongest correlation between bacterial communities and soil properties occurring in the nHAP/BC/HANa treatments. Random forest analysis was conducted to identify the dominant microbial taxa, such as Proteobacteria, Acidobacteria, and Gemmatimonadetes, for predicting changes in soil properties. There was an asymptotical transition in bacterial community assembly processes from stochastic to deterministic in the nHAP/BC/HANa treatments. In conclusion, we demonstrated that nHAP/BC/HANa composite had the remarkable contribution to soil P availability in sandy soils, and simultaneously promoted the bacterial functions potential for P cycling, which present valuable insights to the development of potential P fertilizer.

The spatio-temporal change in soil P and P-solubilizing bacteria under clover mulching in apple orchards of Loess Plateau

Cover crop is an effective practice for improving soil quality and increase soil nutrients. However, the spatio-temporal change of soil phosphorus (P) components and P-solubilizing microorganisms in the process of grass succession is not evident. Here, we studied the variation of soil P components and P-solubilizing bacteria at 0-60 cm soil layer under clean tillage (CT) and white clover (WC, Trifolium repens L.) grown for 5, 9, and 14 years in an apple test station on the Loess Plateau, China. This study suggested that clover cover could effectively increase the total P, available P (AP), microbial P, organic P (Po), and inorganic P (Al-P, Ca₂-P, Ca₈-P and Fe-P) in topsoil (0-20 cm) and AP, Po and inorganic P at 20-40 cm soil layer to improve the soil P bioavailability. The effects of WC living mulch on the soil P forms were more significant with the increase in grass growing years, but this effect was difficult to extend to deep soil. In addition, the WC treatments were beneficial to the growth of P-solubilizing microorganisms in surface soil and improved the alkaline phosphatase activity at 0-40 cm soil layer, mainly including Bacillus, Bradyrhizobium, Nocardioides, Sphingomonas and Streptomyces. This study provided a perspective on the dynamic changes of soil P forms and P-solubilizing microorganisms and under long-term cover crop.

Direct observation of humic acid-promoted hydrolysis of phytate through stabilizing a conserved catalytic domain in phytase

As a potential phosphorus (P) pool, the enzymatic hydrolysis of organic phosphorus (P_o) is of fundamental importance due to the release of bioavailable inorganic phosphate (P_i) for agronomic P sustainability. However, little is known about the role of soil organic matter (SOM) in the hydrolysis process of phytate by phytase and the subsequent chemical behaviors involving the hydrolysis product (P_i) at different soil interfaces. Here, by using liquid-cell atomic force microscopy (AFM), we present a model system to in situ quantify the nucleation kinetics of phytase-released P_i when precipitating with representative soil multivalent cations (Ca²⁺/Fe³⁺) on typical soil mineral/organic interfaces in the presence/absence of humic acid (HA), which involves complex phytase-interface-HA interactions. We observed that a higher HA concentration resulted in a faster nucleation rate of amorphous calcium/iron phosphate (ACP/AIP) on bare and organically-coated (-OH/-COOH) mica surfaces compared with the HA-free control. Besides, the nucleation rate of ACP/AIP induced by organic interfaces was much more significant than that induced by clay mineral interfaces. By combining enzyme activity/stability experiments and AFM-based PeakForce quantitative nanomechanical mapping (PF-QNM) measurements, we directly quantified the contribution of noncovalent phytase-HA interaction to the increase in enzymatic activity from complex phytase-interface-HA interactions. Furthermore, the direct complexation of phytase-HA resulted in the stabilization of a conserved active catalytic domain (ACD) in phytase through the enhanced formation of both an ordered, stereochemically-favored catalytic domain and an unordered non-catalytic domain, which was revealed by Raman secondary structure determination. The results provide direct insights into how HA regulates the catalytic activity of phytase controlling P_o fates and how soil interfaces determine the behaviors of released P_i to affect its availability, and thereby contribute to P sustainability in soils.

Control the greenhouse gas emission via mediating the dissimilatory iron reduction: Fulvic acid inhibit secondary mineralization of ferrihydrite

Reducing methane emission is of great importance to control the global greenhouse effect. Dissimilatory iron reduction (DIR) coupling of organic matter decomposition may suppress methane production via reducing primary electron donors available for methanogenesis. However, during DIR, the amorphous iron oxides (e.g., ferrihydrite) are easy to transform into more stable crystalline iron minerals, which slowdowns the rate of DIR. Humic substance (HS) with redox activity has been extensively reported to facilitate DIR via "electron shuttles" mechanism, yet little known about the effect of HS on mediating the mineralization of iron oxides and the subsequent influences on DIR and methanogenesis. To clarify this, ferrihydrite and fulvic acid (FA) (as the model substance of HS) were supplied to anaerobic methanogenesis systems. Results showed that FA could significantly decrease the formation of crystalline iron oxides, enhance DIR rate by 13.72% and suppress methanogenesis by 25.13% compared to ferrihydrite supplemented only. By X-ray absorption spectra analysis, it was found that FA could complex with ferrihydrite via forming a Fe-C/O structure on the second shell of Fe atom. Quantum chemical calculation further confirmed that FA reduced the adsorption energy between Fe(II) and ferrihydrite. Our study suggested that rational use of HS to mediate mineralization pathway of iron oxides could efficiently improve the availability of iron oxides to drive DIR and control the conversion of organics into CH₄ in natural or engineered systems.

Molecular mechanisms for the humic acid-enhanced formation of the ordered secondary structure of a conserved catalytic domain in phytase

Changes in the secondary structure of phytase, particularly the conserved active catalytic domain (ACD, SRHGVRAPHD) are extremely important for the varied catalytic activity during hydrolyzing phytate in the presence of humic acid (HA). However, little is known about the molecular-scale mechanisms of how HA influences the secondary structure of ACD found in phytase. First, in situ surface-enhanced Raman spectroscopy (SERS) results show the secondary structure transformation of ACD from the unordered random coil to the ordered β-sheet structure after treatment with HA. Then, we use an atomic force microscopy (AFM)-based single-molecule force spectroscopy (SMFS) technique that can in situ directly probe the single-molecule interaction of ACD with HA and underlying changes in ACD secondary structure in the approach-retraction cycles in real time. Based on the SMFS results, we further detect the HA-enhanced formation of H-bonding between amide groups in the ACD backbone after noncovalently interacting with HA in the absence of phytate. Following the addition of phytate, the calculated contour length (L_c) and the free energies (ΔG_b) of functional groups within ACD(-1/2) binding to mica/HA collectively demonstrate the formation of the organized intermediate structural state of ACD following its covalent binding to phytate. These spectroscopic and single-molecule determinations provide the molecular-scale understanding regarding the detailed mechanisms of HA-enhancement of the ordered β-sheet secondary structure of ACD through chemical functionalities in ACD noncovalently interacting with HA. Therefore, we suggest that similar studies of the interactions of other soil enzymes and plant nutrients may reveal predominant roles of dissolved organic matter (DOM) in controlling elemental cycling and fate for sustainable agriculture development.

Humus Acids in the Digested Sludge and Their Properties

Fulvic acids, alpha (α) humic acids and hymatomelanic acids were extracted digested sludge in two Cracow sewage treatment plants: Kujawy and Płaszów. Their elemental composition was examined and micropollution and ash content were determined. Based on the IR and UV-VIS spectrum, their similarities were determined with the occurring interactions with micropollution. Strong correlations between the acids coming from different sources depend on acid type and micropollution accompanying them, depending on concentration, influences to a specific extent their IR and UV-VIS spectra. Absorption analysis in infrared constitutes a simple method for characterizing fulvic and humic acids from wastewater treatment plants. The extracted fulvic acids were characterized by moderate maturity, while humus acids were well developed. In the fermentation process, the N bond increases together with the level of humification of the humus acid. The characteristics of the extracted humus acids comply with other humic substances presented in the literature. Quantitative analysis showed that digested sludge contains, on average: FA from 5.07 to 5.30 g/kg dry matter, αHA from 59.22 to 74.72 g/kg dry matter, HMA from 20.31 to 43.66 g/kg dry matter. It was thus demonstrated that wastewater treatment, in particular digested sludge, constitutes an attractive source of humus acids with a wide range of applications in numerous areas, such as agriculture, ecological rehabilitation, environmental protection, animal breeding, aquaculture, veterinary as well as medicine and is a precious source of soil fertilizers.

Biochemical Fulvic Acid Modification for Phosphate Crystal Inhibition in Water and Fertilizer Integration

Biochemical fulvic acid (BFA), produced by organic wastes composting, is the complex organic matter with various functional groups. A novel modified biochemical fulvic acid (MBFA) which possessed stronger chelating ability had been synthesized by the grafting copolymerization of BFA and acrylic acid (AA). Results showed that MBFA effectively inhibited the crystallization of calcium phosphate and increased the concentration of phosphate in water solution. The optimum reaction conditions optimized by Box–Behnken design and response surface methodology were reaction temperature 69.24 °C, the mass of monomer to fulvic acid ratio 0.713, the initiator dosage 19.78%, and phosphate crystal-inhibition extent was 96.89%. IR spectra demonstrated AA was grafted onto BFA. XRD data and SEM images appeared the formation and growth of calcium phosphate crystals was effectively inhibited by MBFA.

Effect of dissolved organic matter on the phytoremediation of Cd-contaminated soil by cotton

Dissolved organic matter (DOM) assists in the phytoremediation of heavy-metal-contaminated soils, but the effect of synergistic remediation of DOM on plants is unclear. This study investigated the effect of two DOM sources (cotton straw (CM) DOM and farmyard manure (FM) DOM) on cadmium (Cd) accumulation in Cd-contaminated soil by cotton and evaluated the phytoremediation effect of DOM. The results showed that adding DOM reduced the available nitrogen and increased organic matter, available phosphorus and available potassium. Applying DOM increased the proportions of Cd acid soluble fractions and reduced the proportions of Cd residual fractions by 1-7%. DOM application increased root length, root surface area and root volume compared to the control and had a promoting or inhibiting effect on cotton biomass, depending on the soil Cd concentration. Furthermore, applying DOM improved the Cd content and bioconcentration factor of cotton. The lower the molecular weight, hydrophilic components and aromaticity of DOM, the more conducive to Cd accumulation is in cotton. The correlation and random forest analyses also showed that CM showed high remediation potential. According to our study, DOM can improve the phytoremediation efficiency of cotton, especially in low-concentration contaminated soils. This study provides a basis for applying DOM in the phytoremediation of Cd-contaminated soils.

Characterization of humic acids from original coal and its oxidization production

Five coal samples obtained from Chinese coal-producing areas were oxidized by hydrogen peroxide (H₂O₂), and humic acids (HAs) were derived from original coal and its oxidizition samples. HAs were characterized by physical and chemical methods, between which was also comparison. Yield, ash, aromaticity, molecular weight and functional group of HAs showed variance between original coals. While, yield, molecular weight, and the quantity of oxygen-containing groups of HAs increased more from coals oxidized with H₂O₂. However, the increase of oxygen-containing functional groups depended on original coals. For Yimin lignite, the oxidation of H₂O₂ could obviously improve the carboxyl group content of HAs, thus promoting the adsorption of nitrogen. This study demonstrated that oxidation of coal by using H₂O₂ was one pretreatment way to obtain and modify HAs which could be used as prerequisite and functional material in agricultural field.

Short-chain soluble polyphosphate fertilizers increased soil P availability and mobility by reducing P fixation in two contrasting calcareous soils

Short-chain polyphosphate fertilizers have been increasingly applied in agriculture, but little is known about the chemical behaviors of polyphosphate in soils. Herein, a cylinder experiment was carried out to investigate the influences of different P types (i.e., mono-ammonium phosphate (MAP), phosphoric acid (PA) and ammonium polyphosphate (poly-P)) and their application methods (single vs split) on the mobility and availability of P in soil through a column millimeter-scale slice cutting method; meanwhile a soil microcosm experiment (560-day) was conducted to investigate the effects of different P types on phosphorus dynamic transformation. Polyphosphate addition significantly increased P mobility. The average distance of P downward movement (81.5 mm) in soil profile in the poly-P application treatment increased by 33.6% and 81.1%, respectively, compared to the MAP and PA treatments. Different P application methods also markedly influenced phosphorus mobility. For instance, the average distance of P vertical movement in the split P application treatment was 21.2% higher than in the single application treatment, indicating that split P addition significantly increased P downward movement. Moreover, polyphosphate application decreased soil P fixation by blocking the transformation of the applied-P from labile to recalcitrant forms (HCl-P and residual-P). Overall, our findings provide meaningful information to current phosphorus fertilization practice in increasing soil P mobility and bioavailability. We suggest that polyphosphate could be regarded as an alternative P source used in agriculture, and split polyphosphate application is recommended as an effective P fertilization strategy.

Long-term cover crops improved soil phosphorus availability in a rain-fed apple orchard

The objective of this present study was to understand the distribution patterns of various forms of soil phosphorus (P) and the biotic and abiotic factors affecting the soil P fractions under long-term cover crops. Here, we investigated the characteristics of soil P forms, community structure of P-solubilizing bacteria (using 16S rRNA) and the related enzyme activity under clean tillage (CT), 14 years of white clover (WC, Trifolium repens L.) and orchard grass (OG, Dactylis glomerata L.) cover crops in a rain-fed apple orchard on the Weibei Loess Plateau, China. Relative to CT treatment, long-term cover crops enhanced the bioavailability of soil P by increasing the contents of total phosphorus (TP), microbial phosphorus (MBP), organic phosphorus (Po) and certain forms of inorganic phosphorus (e.g. Al-P, Ca₂-P, Ca₈-P and Fe-P) in the surface soil, in addition, WC treatment also increase the available P (AP) contents in the topsoil. A redundant analysis (RDA) showed that soil organic matter (SOM), NH₄⁺-N and pH were the key environmental factors affecting the morphological changes of soil P. In addition, the effects of long-term cover crops on soil P forms were mainly concentrated in the topsoil, and the WC treatment had a greater impact on soil P composition than the OG treatment. Interestingly, long-term cover crops effectively increased the abundances of P-solubilizing bacteria, such as Streptomyces, Sphingomonas, Nocardioides and Haliangium, and enhanced the alkaline phosphatase (ALP) activity. Overall, long-term cover crops were an effective strategy to activate soil P as they improve the soil environment.

Comparative effectiveness of activated dolomite phosphate rock and biochar for immobilizing cadmium and lead in soils

Sandy soils in Florida are vulnerable to toxic metal pollution, and it is necessary to identify desirable amendments for the remediation of metal contaminated soils. Sorption and incubation experiments were conducted to compare the effectiveness of dolomite phosphate rock (DPR), humic acid activated dolomite phosphate rock (ADPR) and biochar (BC) in immobilizing Cd²⁺ and Pb²⁺ in two representative agricultural soils in south Florida (Alfisol-Riviera and Spodosol -Ankona series). The results showed that the soils had a low sorption capacity for metals with maximum sorption of 0.767-3.30 mg/g. Application of amendments increased the maximum sorption by 4.2-4.8 times for Pb²⁺ and 1.5-2.2 times for Cd²⁺ in Alfisol soil, and 7.1-7.9 times for Pb²⁺ and 1.7-3.1 times for Cd²⁺ in Spodosol soil. ADPR was the most effective amendment for increasing the soil's sorption capacity for Cd²⁺ and Pb²⁺. 0.01 M CaCl₂ extractable metals in the contaminated soils were significantly decreased by all the amendments, especially ADPR, which reduced extractable Cd²⁺ and Pb²⁺by 87.2 and 76.0% in Alfisol and 91.3 and 76.3% in Spodosol soil as compared to control. The amounts of extractable Cd²⁺ and Pb²⁺ were negatively correlated with soil pH and available P, indicating that the change of soil characteristics by amendments was the dominant mechanism for enhanced immobilization of metals in the contaminated soils. These results indicate that ADPR has great potential for remediating toxic levels of Cd²⁺ and Pb²⁺ in contaminated soils.

Structural Evolution of Lanthanum Hydroxides during Long-Term Phosphate Mitigation: Effect of Nanoconfinement

Lanthanum (La)-based materials are effective in removing phosphate (P) from water to prevent eutrophication. Compared to their bulky analogues, La(OH)₃ nanoparticles exhibit a higher P removal efficiency and a more stable P removal ability when spatially confined inside the host. Consequently, the understanding of the nanoconfinement effects on the long-term evolution of La-P structures is crucial for their practical use in P sequestration and recycle, which, however, is still missing. Here, we describe an attempt to explore the evolution of La-P structures, the P environment, and the status of La(OH)₃ nanoparticles confined in the nanopores of the D201 resin, compared to a nonconfined analogue, over a P adsorption period of 25 days in both simulated wastewater and the real bioeffluent. A combinative use of X-ray diffraction (XRD), cross-polarization nuclear magnetic resonance (CP-NMR), and X-ray photoelectron spectroscopy (XPS) techniques confirms the transition from La-P inner-sphere complexation to the formation of LaPO₄·xH₂O and finally to LaPO₄ in both samples. Interestingly, the rate of structural transformation in the real bioeffluent is substantially reduced. Nevertheless, in both conditions, nanoconfinement results in a much faster rate and larger extent of the structural transition. Moreover, nanoconfinement also facilitates the reverse transformation of stable LaPO₄ back to La(OH)₃. Our work provides the scientific basis of nanoconfinement for the preferable use of La-based nanocomposites in P mitigation, immobilization, and recycle application.

Phosphorus binding to soil organic matter via ternary complexes with calcium

Soil organic matter (SOM) is known to exert a major control on the mobility and bioavailability of cationic nutrients. However, the role of SOM in the fate of anionic nutrients, especially phosphorus (P), is less well characterized. The objectives of this study were to (1) compare the formation of binary complexes of calcium (Ca) with humic acids (HA) extracted from two contrasting soils, and (2) determine if binary HA-Ca complexes could incorporate P by forming ternary HA-Ca-P complexes. The Ca binding capacities of the HA extracted from an agricultural organic soil (AOS) and a pristine riparian soil (RS) were measured via potentiometric titrations; the formation of ternary complexes was analyzed by size fractionation using MWCO tubes. Proton and Ca binding capacities of RS-HA were higher than AOS-HA, and pH had a weaker effect on Ca binding to RS-HA. These differences are consistent with lower proportions of aromatic groups, and a higher proportion of alkyl groups derived from ¹³C NMR spectroscopy. Together, the NMR, titration and MWCO data indicate that Ca binds to RS-HA through monodentate complexes and electrostatic attraction that are capable of binding P producing ternary complexes. In contrast, at pH 8.5 Ca forms bidentate complexes with AOS-HA, which do not provide bridging positions to incorporate P. Overall, our results imply that the formation of HA-Ca and HA-Ca-P complexes depend on the structure of the HA, and that complexation to HA may play an important role in the fate of P in terrestrial and aquatic environments.

Molecular insight into the interfacial chemical functionalities regulating heterogeneous calcium-arsenate nucleation

Heterogeneous nucleation induced by natural organic matter (NOM) can lower the energy barrier for calcium arsenate (Ca-As) precipitation, which aids in immobilizing arsenate (As^Ⅴ). However, it remains unclear how certain chemical functionalities of NOM affect Ca-As nucleation at the molecular scale. By analyzing changes in the local supersaturation and/or interfacial energy, the present work investigates the Ca-As heterogeneous nucleation kinetics and mechanisms on functional-group-modified model surfaces. Mica surfaces modified by functional groups of amine (NH₂), hydroxyl (OH), or carboxyl (COOH) through self-assembled monolayers were used to investigate how chemical functionalities affect the Ca-As heterogeneous nucleation, in which the distributions of formation kinetics and size (as measured by the change in particle height) of nucleated Ca-As particles were measured by using in situ atomic force microscopy. In a parallel analysis, a quartz-crystal microbalance with dissipation was used to detect the buildup of Ca²⁺ and/or HAsO₄^2- ions at the solid-fluid interface. PeakForce quantitative nanomechanical mapping and dynamic force spectroscopy using functional-group-modified tips made it possible to calculate the binding energies holding functional groups to Ca-As particles. Nucleated Ca-As particles were characterized by using Raman spectroscopy and high-resolution transmission electron microscopy. The results indicate that the height of amorphous Ca-As particles formed on a modified mica surface may be ranked in descending order as NH₂ > OH > bare mica > COOH, as determined by the buildup of Ca²⁺ and HAsO₄^2- ions at the solid-fluid interface and the decrease of interfacial energy due to the functional groups. These nanoscale observations and molecular-scale determinations improve our understanding of the roles played by chemical functionalities on NOM in immobilizing dissolved As through heterogeneous nucleation in soil and water.

Dissolution and Precipitation Dynamics at Environmental Mineral Interfaces Imaged by In Situ Atomic Force Microscopy

Chemical reactions at the mineral-solution interface control important interfacial processes, such as geochemical element cycling, nutrient recovery from eutrophicated waters, sequestration of toxic contaminants, and geological carbon storage by mineral carbonation. By time-resolved in situ imaging of nanoscale mineral interfacial reactions, it is possible to clarify the mechanisms governing mineral-fluid reactions.In this Account, we present a concise summary of this topic that addresses a current challenge at the frontier of understanding mineral interfaces and their importance to a wide range of mineral re-equilibration processes in the presence of a fluid aqueous phase. We have used real-time nanoscale imaging of liquid-cell atomic force microscopy (AFM) to observe the in situ coupling of the dissolution-precipitation process, whereby the dissolution of a parent mineral phase is coupled at mineral interfaces with the precipitation of another product phase, chemically different from the parent. These nanoscale observations allow for the identification of dissolution and growth rates through systematically investigating various minerals, including calcite (CaCO₃), siderite (FeCO₃), cerussite (PbCO₃), magnesite (MgCO₃), dolomite (CaMg(CO₃)₂), brushite (CaHPO₄·2H₂O), brucite (Mg(OH)₂), portlandite (Ca(OH)₂), and goethite (α-FeOOH), in various reacting aqueous fluids containing solution species, such as arsenic, phosphate, organo- or pyrophosphate, CO₂, selenium, lead, cadmium, iron, chromium, and antimony. We detected the in situ replacement of these parent mineral phases by product phases, identified through a variety of analytical methods such as Raman spectroscopy, high-resolution transmission electron microscopy, and various X-ray techniques, as well as modeling by geochemical simulation using PHREEQC. As a consequence of the coupled processes, sequestration of toxic elements and hazardous species and inorganic and organic carbon, and limiting or promoting recovery of nutrients can be achieved at nano- and macroscopic scales.We also used in situ AFM to quantitatively measure the retreat rates of molecular steps and directly observe the morphology changes of dissolution etch pits on calcium phosphates in organic acid solutions present in most rhizosphere environments. By molecular modeling using density functional theory (DFT), we explain the origin of dissolution etch pit evolution through specific stereochemistry and molecular recognition and provide an energetic basis by calculating the binding energies of chemical functionalities on organic acids to direction-specific steps on mineral surfaces. In addition, we further quantified precipitation kinetics of calcium phosphates (Ca-P's) on typical mineral surfaces at the nanoscale in environmentally relevant solutions with various organic molecules, by measurements obtained from sequential images obtained by liquid-cell AFM. In situ dynamic force spectroscopy (DFS) was used to determine binding energies of single-molecules with different chemical functionalities found in natural organic matter at mineral-fluid interfaces. Quantifying molecular organo-mineral bonding or binding energies mechanistically explains phosphate precipitation and transformation. From DFS measurements, molecular-scale interactions of mineral-natural organic matter (DNA, proteins, and polysaccharides) associations were determined. With this powerful tool, single-molecule determinations of polysaccharide-amorphous iron oxide or hematite interactions provided the mechanistic origin of the phase- or facet-dependent adsorption. These systematic investigations and findings significantly contribute to a more quantitative prediction of the fate of nutrients and contaminants, chemical element cycling, and potential geological carbon capture and nuclear waste storage in aqueous environments.