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In this paper, low temperature, economical sol–gel combustion method was adopted to synthesize wollastonite ceramic. Calcium nitrate tetrahydrate and tetraethyl orthosilicate were taken as source for Ca and Si, while citric acid and nitric acid were used as chelating/combustion agents. The yielded powder calcined at 600${}^{\circ }$C for 4 h was characterized by FTIR, XRD and SEM techniques. Results showed that the citrate combustion method was the most efficient method to prepare wollastonite at low temperature. Moreover, in vitro bioactivity test performed in simulated body fluid (SBF) showed good bioactivity of synthesized wollastonite ceramics.

Surface modification by using citric acid (CA) in the graphene is a process to modify the physicochemical properties of graphene oxide. The strategy that has been proposed depends upon the electrochemical exfoliation of reduced graphene oxide (rGO), and simultaneously, the surface modification of rGO with CA carried out in accordance with the green technique. The synthesis of graphene oxide that has been doped with CA was accomplished via an electrochemical process in an aqueous medium containing fresh lime juice and sulphuric acid (electrolyte heating aided method at $60^{\circ }$C) as an electrolyte. The electrolyte has been prepared using CA & H₂SO₄ (sulphuric acid), and both were mixed in a proportion of 1:2. In order to dilute the H₂SO₄ and perform the sonication, the water that has been pasteurized (according to the USP standards for irrigation) was used. The crystallite size, structural disorder, structure and surface morphology of the CA-doped graphene oxide were identified through X-ray diffraction (XRD) analysis, Raman spectroscopy, Field emission scanning electron microscope (FE-SEM). The presence of oxygen-containing functional group and adsorption has been analyzed using Fourier transform infrared (FTIR), and UV–Vis spectroscopy. The thermal stability of the CA-doped, and without CA-doped thermally reduced graphene oxide (TRGO) has been analyzed via thermogravimetric analysis (TGA). A green, simple, and environmentally friendly method has been demonstrated for the synthesis of CA-doped TRGO by electrochemical synthesis method by using natural dopant.

Effect of pH on the one-step electrodeposition of Cu(In, Al)Se₂ chalcopyrite layer in the presence of citric acid has been investigated by applying different electrochemical and characterization techniques. It has been observed that at pH of 1.5, nanocrystalline phase of chalcopyrite and small amount of binary phase of Cu₂Se with overall composition of Cu_0.91In_0.32Al_0.39Se₂ have been deposited. On the other hand, at pH of 4, the film composition changed to Cu_1.9In_0.05Al_0.21Se₂ and an additional binary phase of copper selenide (CuSe) has also been formed. Morphological investigation illustrated that smooth and compact layer with fine spherical particles having the size of ~20 nm has been obtained at pH of 1.5 whereas mixture of planar and spherical particles with size of 450–550 nm have been formed at pH of 4. In alkaline environment (pH~9), the deposition current has been noticeably decreased and no deposition occurred due to the formation of a stable complex of citric acid with metal ions. The mechanism of citric acid interaction with metal ions at different pH has also been studied by cyclic voltammetry measurement.

A simple method has been developed for decorating multi wall carbon nanotubes (MWCNTs) with palladium and silver nanoparticles. In this method, MWCNT was opened and functionalized using nitric and sulfuric acid mixture. Then citric acid was polymerized in the presence of functionalized MWCNT and MWCNT-graft-poly (citric acid) (MWCNT-g-PCA) was obtained. The mixing of MWCNT-g-PCA with metal salts, such as palladium chloride and silver nitrate, leads to encapsulation of metal nanoparticles in the polymeric shell (MWCNT/Pd, AgNPs). The structure of MWCNT/Pd and AgNPs were characterized by usual spectroscopy and microscopy methods. The influence of nanoparticles on the electrical conductivity of MWCNT was also investigated.

In the atmosphere carbon dioxide (also called carbonic anhydride: CO₂) is the most abundant form of carbon (Section 4.1). In the lithosphere and hydrosphere carbon is found in the form of carbonates ${(\text{CO}}_3^{2-})$ and bicarbonates (hydrogen carbonates: ${\text{HCO}}_3^{-})$, bound to mono-cations such as sodium Na⁺, potassium, K⁺, ammonium cation, ${\text{NH}}_4^{+}$, or dications such as calcium Ca²⁺ and magnesium Mg²⁺. Carbon dioxide, carbonates and bicarbonates are mineral carbon compounds (Sections 4.3, 4.5 and 4.6). These compounds are described, their uses and their most important properties are presented. This exercise will allow us to introduce fundamental notions about chemical equilibria (Section 4.2).

The clean-up of loess soils contaminated with heavy metals is an urgent problem. Column flushing was conducted to investigate the performance of 0.02, 0.04, 0.06 and 0.08 mol/L citric acid (CA) to enhance the removal of Cu(II) from sandy loess soil. The maximum concentration of Cu(II) in effluent solutions and the removal efficiency of Cu(II) were positively related to CA concentration while the cumulative pore volume number (N_CPV) of eluent was negatively related to CA concentration. The similar pattern of elution curves was found, in which the concentrations of Cu(II) in effluent solutions increased sharply, peaked and decreased with N_CPV increasing. Accordingly, the removal efficiency of Cu(II) increased sharply and then leveled off with N_CPV increasing and the overall removal efficiencies were larger than 91.2% in all cases. Given the removal efficiency, the needed N_CPV was decreased and the eluting duration was shortened when the more concentrated CA was used. A logarithm relationship between the maximum concentration of Cu(II) in effluent solution and CA concentration was found. The overall removal efficiencies of Cu(II) from soil column increased with N_CPV in Sigmoidal model.

The main objective of this laboratory project is to investigate the effectiveness of using polyamide grout injection on sand improvement. Soil samples with different dry densities in their loose, medium and dense state (cylindrical shaped) with 5cm diameter and 10cm height are subjected to polyamide injection. In order to investigate the grout injection effect on soil's strength, the specimens are subjected to one dimensional unconfined compression tests. We know that polyamide doesn't mix with water and we should mix it with citric acid. It has been observed that by increasing the polymer percent in polymer-water-citric acid mixture grout, the soil compressive strength and elastic modulus has been improved significantly. In general denser sands presented higher compressive strength and elastic modulus in comparison with looser ones in same polymer percents.

The Ultra-molecular sieving research is an important research topic. Microwave heating method uses a specific response to the microwave generator system to launch UHF microwave, shock rapidly reactant molecular, so that they bump, extrude, rub each other, reorganize and form zeolite crystals. In this study, microwave heating method is used to synthesize zeolite crystal, add citric acid as a promoter in the process of preparation, makes grain size of molecular sieve crystal smaller, crystal is more uniform. The conclusion shows microwave heating method with citric acid can improve reaction condition clearly, crystallization time is 10 minute, crystallization temperature is 100°C, molecular sieve granular size is about 300nm.

Low molecular weight organic acids (LMWOAs) are known to be effective in enhancing the mobilization of inorganic P, thus increasing plant-available P in soils. But little is known about their effect on organic P, especially in agricultural soils. The aim of this study was to investigate the role of three common organic acids (oxalic, citric and malic acids) and their mixture (ratio, 1:1:1) in the mobilization of soluble inorganic (SIP), soluble organic P (SOP) and available P in a calcareous, neutral and acid agricultural soil. All the three organic acids and their mixture at 10 mmol kg⁻¹ soil enhanced the mobilization of SIP, SOP and available P in soils as compared with water, but no evident combined effects among the three organic acids were observed. The ability of LMWOAs to mobilize SIP was ascribed to its ability to complex or precipitate with cations (e.g. Fe, Al and Ca) bound to P, whereas the release of SOP was linked to the shift in pH values. The results of this study suggested that LMWOAs might have potential as amendments to increase plant available P due to their effectiveness to mobilize SIP, SOP and available P in agricultural soils.

Cu²⁺ was selected as simulative pollutant to study the experimental conditions and adsorption properties. The metal ion adsorbents were prepared by the modification of manganese ore using citric acid method. Results showed that the absorption rate-pH curves of modified manganese ore inclined to the lower pH. The pH50 decreased from 5.2 to 2.6 with the increase of sulfuric acid concentration (within the range of 0–3mol/L). The ratio of citric acid to natural manganese ore was 1:20, and the pH50 were 3.8, 3.0, 2.5 and 2.6, respectively. The ratios of citric acid to natural manganese ore were 0.1:100, 1:50 and 1:20, respectively, with the sulfuric acid concentration of 3mol/L. The saturation adsorption increased to 35.97mg/g when pH was 5.5, the sulfuric acid concentration 3mol/L, the mass ratio of citric acid to natural manganese ore 1:50.