Residual Reactants Research Articles

Methyl ester production process from palm fatty acid distillate using hydrodynamic cavitation reactors in series with solid acid catalyst

This study aims to optimize the reduction of free fatty acids (FFAs) in palm fatty acid distillate (PFAD) using hydrodynamic cavitation reactors (HCRs) in series and a solid acid catalyst for biodiesel production. Hydrodynamic cavitation is used to accelerate the esterification of FFAs using a heterogeneous acid catalyst. There are three HCRs units, and each HCR composed of a 3D-printed rotor and stator, is separated by flanges and equipped with a basket for holding Amberlyst-15 catalyst. Through response surface methodology (RSM), the esterification process is optimized by adjusting its optimal parameters, namely, methanol (2–12 wt%), circulation time (30–170 min), and rotor speed (1000–3000 rpm). The optimal conditions for achieving a maximum methyl ester purity of 89.76 wt% in converting FFA in first-step esterified oil are 9 wt% methanol (molar ratio of methanol to oil of 4:1), 133 min of circulation time, and 2000 rpm of rotor speed. An 82.48 wt% biodiesel yield is achieved from the HCRs in series under the optimal conditions. Scanning electron microscope images reveal that after the esterification process, there are minor cracks and defects on the catalyst’s resin surface, indicating the presence of residual reactants. Further examination of the catalyst after the esterification process, reveals an average absorption pore diameter of 341.41 Å and BET surface area of approximately 41.68 m2/g. Although there were slight physical changes in the catalyst, HCRs technology offers a viable FFA reduction process that could enhance biodiesel production efficiency. Moreover, the optimized conditions achieved in this study contribute to the advancement of biodiesel production processes and provide insights into the performance of the catalyst used.

Carbothermal-sulfurization of TiO2 into Ti2SC MAX phase in TiO2/C/FeS2 ternary system

The conventional method for synthesizing Ti2SC depends on the reaction between TiS and TiC, which requires the utilization of costly Ti, TiC, or TiS2 powders. Utilizing TiO2 as the raw material for the synthesis of Ti2SC enables the cost-effective preparation of this compound. However, the optimization of the TiO2/C/XnSm (sulfides) system is of utmost importance when aiming to synthesize Ti2SC through carbothermal-sulfurization of TiO2, as the formation temperature of TiS and TiC greatly depends on the XnSm. In this study, the identification of the thermodynamically dominant region for the formation of TiS and TiC in the TiO2/C/XnSm systems was conducted by utilizing various sulfides (63 types). Considering factors such as the challenges associated with removing by-products, accessibility, and toxicity, FeS2 was ultimately chosen as a highly promising candidate. Thanks to the low melting point of FeS2, Ti3O5, the preliminary carbothermal reduction product of TiO2, has achieved rapid-deep deoxidation and phase reconstruction processes in the molten phase. Meanwhile, unlike the conventional solid-solid sintering reaction, which often results in residual reactants that are challenging to eliminate. This novel one-step phase transformation process that occurs in the molten phase exhibits minimal impurity phases, apart from the by-product Fe. Ti2SC with a purity greater than 99 wt% was obtained when the TiO2/C/FeS2 was pickled to remove Fe after sintering at 1600 °C for 5 min. This work will provide the theoretical basis and a new path for the low-cost preparation of sulfur-containing MAX phases.

Greatly Intensified Guest Exchange Strategy for Highly-Efficient Activation of Metal-Organic Frameworks.

The preservation and accessibility of pores are prerequisites to the application of metal-organic frameworks (MOFs). Activation is a key step to eliciting rich features of pores, but it needs a repeated solvent-exchange process which is tedious and time/cost-consuming. Herein, a facile strategy for highly-efficient activation of MOFs utilizing rotating packed bedis proposed. With the tremendous enhancement of molecular mixing and mass transfer in high-gravity and strong-shearing surrounding, nine representative MOFs are completely activated within 2h without structural change. Compared with conventional process, this activation displays surprising efficiency by accelerating the diffusion of solvents and redissolution of residual reactants in the pores. The complete activation time can be significantly shortened by over 90%. As a proof-of-concept, the methane storage of as-activated UiO-66 is five times that of as-synthesized UiO-66. This strategy provides a potential platform with industrial worth for the activation of MOF materials with ultra-high efficiency and versatility.

Modified Sulfanilamide Release from Intelligent Poly(N-isopropylacrylamide) Hydrogels.

The aim of this study was to examine homopolymeric poly(N-isopropylacrylamide), p(NIPAM), hydrogels cross-linked with ethylene glycol dimethacrylate as carriers for sulfanilamide. Using FTIR, XRD and SEM methods, structural characterization of synthesized hydrogels before and after sulfanilamide incorporation was performed. The residual reactants content was analyzed using the HPLC method. The swelling behavior of p(NIPAM) hydrogels of different crosslinking degrees was monitored in relation to the temperature and pH values of the surrounding medium. The effect of temperature, pH, and crosslinker content on the sulfanilamide release from hydrogels was also examined. The results of the FTIR, XRD, and SEM analysis showed that sulfanilamide is incorporated into the p(NIPAM) hydrogels. The swelling of p(NIPAM) hydrogels depended on the temperature and crosslinker content while pH had no significant effect. The sulfanilamide loading efficiency increased with increasing hydrogel crosslinking degree, ranging from 87.36% to 95.29%. The sulfanilamide release from hydrogels was consistent with the swelling results-the increase of crosslinker content reduced the amount of released sulfanilamide. After 24 h, 73.3-93.5% of incorporated sulfanilamide was released from the hydrogels. Considering the thermosensitivity of hydrogels, volume phase transition temperature close to the physiological temperature, and the satisfactory results achieved for sulfanilamide incorporation and release, it can be concluded that p(NIPAM) based hydrogels are promising carriers for sulfanilamide.

Nanosphere size control by varying the ratio of poly(ester amide) block copolymer blends

HypothesisBlending amphiphilic triblock (A-B-A) and diblock (A-B) copolymers comprised of the same hydrophobic tyrosine-derived oligomeric B-block and hydrophilic poly(ethylene glycol) methyl ether (mPEG) A-block can provide highly tunable self-assembled nanosphere particle sizes suitable for biomedical applications. ExperimentTriblock and diblock copolymers were synthesized via carbodiimide chemistry and were characterized by nuclear magnetic resonance (NMR) and gel permeation chromatography (GPC). The amount of free PEG present in the purified copolymers was determined using a standard addition calibration curve and GPC peak deconvolution methods. Nanospheres were prepared by co-precipitation of each copolymer and of copolymer blends over a range of mole ratios. Nanospheres were characterized by dynamic light scattering (DLS), transmission electron microscopy (TEM) and % polymer recovery post-preparation. FindingPrecise synthesis control produced triblock and diblock copolymers with narrow molecular weight distributions and minimal residual reactants. Self-assembled nanosphere particle sizes were 33 nm for the triblock and 129 nm for the diblock, and the size of their blends increased continuously as a function of mole ratio within that biomedically relevant range. Addition of unreacted PEG had minimal impact on either triblock or diblock nanosphere particle sizes whereas addition of unreacted oligomeric B-block increased nanosphere sizes.

Green Stereoregular Polymerization of Poly(methyl methacrylate)s Through Vesicular Catalysis

Green Stereoregular Polymerization of Poly(methyl methacrylate)s Through Vesicular Catalysis

Recent Progress on Perovskite Surfaces and Interfaces in Optoelectronic Devices.

Surfaces and heterojunction interfaces, where defects and energy levels dictate charge-carrier dynamics in optoelectronic devices, are critical for unlocking the full potential of perovskite semiconductors. In this progress report, chemical structures of perovskite surfaces are discussed and basic physical rules for the band alignment are summarized at various perovskite interfaces. Common perovskite surfaces are typically decorated by various compositional and structural defects such as residual surface reactants, discrete nanoclusters, reactions by products, vacancies, interstitials, antisites, etc. Some of these surface species induce deep-level defect states in the forbidden band forming very harmful charge-carrier traps and affect negatively the interface band alignments for achieving optimal device performance. Herein, an overview of research progresses on surface and interface engineering is provided to minimize deep-level defect states. The reviewed subjects include selection of interface and substrate buffer layers for growing better crystals, materials and processing methods for surface passivation, the surface catalyst for microstructure transformations, organic semiconductors for charge extraction or injection, heterojunctions with wide bandgap perovskites or nanocrystals for mitigating defects, and electrode interlayer for preventing interdiffusion and reactions. These surface and interface engineering strategies are shown to be critical in boosting device performance for both solar cells and light-emitting diodes.

Synthesis and characterization of poly(N-isopropylmethacrylamide-co-N-isopropylacrylamide) copolymers

Copolymeric hydrogels of poly(N-isopropylmethacrylamide-co-N-isopropylacrylamide), p(NIPMAM/NIPAM), are synthesized by radical polymerization of N-isopropylmethacryl-amide (NIPMAM) and N-isopropylacrylamide (NIPAM) monomers by using the cross-linker ethylen glycol dimethacrylate (EGDM). The synthesized copolymeric p(NIPMAM/NIPAM) hydrogels, starting monomers and the cross-linker were structurally characterized by using Fourier transform infrared spectroscopy (FTIR). The amounts of residual reactants in the synthesized hydrogels were determined by high-pressure liquid chromatography (HPLC). Swelling of p(NIPMAM/NIPAM) hydrogels was investigated in relation to the temperature and pH value of the solution. The obtained values of residual monomer quantities are within acceptable limits and in the range from 2.69 to 5.25 mg g-1 for NIPMAM and 14.55 to 30.80 mg g-1 for NIPAM. The synthesized p(NIPMAM/NIPAM) hydrogels are negatively thermosensitive. The most common mechanisms of transport of a swelling solution in p(NIPMAM/NIPAM) hydrogels are polymer chain relaxation, (Case III), and the anomalous type of diffusion (non-Fickian diffusion). The maximal equilibrium swelling degree of 51.19 was reached by the p(NIPMAM/NIPAM) hydrogel with 1.5 mol% of EGDM at the temperature of 25?C and pH 4, whereas the lowest one of 0.98 was exhibited by the hydrogel with 3 mol% of EGDM at the temperature of 80?C and pH 7. Due to their low content of residual reactants and a satisfactory degree of swelling at various pH values, synthesized p(NIPMAM/NIPAM) hydrogels can be applied as carriers for the controlled release of pharmaceutically active substances.

Determination of residual monomer content in dental acrylic polymers and effect after tissues implantation

The aim of the present study was to determine the content of residual monomer methyl methacrylate (MMA) in cold and hot polymerized poly(methyl methacrylate) (PMMA), both widely applicable in dentistry, by using MHE-GC-MS (multiple headspace extraction analysis by gas chromatography-mass spectrometry) in combination with GC-FID (gas chromatography with flame-ionization detection). The samples of PMMA were obtained by free-radical polymerization of MMA at room temperature (cold polymerized PMMA) and at 100 °C (hot polymerized PMMA), according to the manufacturer’s instructions. The MHE-GC-MS method consisted of ten successive extractions of MMA from the polymerized samples. According to the constructed calibration curve, the content of residual MMA in PMMA samples (mgMMA/gPMMA) was determined. Intramuscular implantation of sample materials was performed in experimental animals (rats) in order to demonstrate potential adverse effects of the unpolymerized residual reactants to the tissue in direct contact with polymeric implants. The content of residual monomer in cold polymerized PMMA was higher (15.75 mgMMA/gPMMA) in comparison to hot polymerized PMMA (10.96 mgMMA/gPMMA). The implanted samples of cold-polymerized acrylic resin showed intense inflammatory response of the surrounding tissue. According to the obtained results, hot polymerized PMMA is safer for use in dentistry.

Study of factors influencing the fabrication of Co‐porphyrin porous coordination polymer via metal–organic gel intermediate

In this work, 5, 10, 15, 20‐Tetra‐(4‐aminophenyl) porphyrin (TAPP) was used as gelator to prepare metal‐porphyrin porous coordination polymer (PCP) via solvothermal process, Soxhlet extraction and supercritical CO2 extraction. Firstly, the metal‐porphyrin organic gel (MOG) was prepared as intermediate with solvothermal method. The generation of gels is associated with many factors. When four acetates [Co(Ac)2•4H2O, Zn(Ac)2•2H2O, Mn(Ac)2•4H2O and Ni(Ac)2•7H2O] reacted with TAPP, only the reaction between Co(Ac)2•4H2O and TAPP could form desired metal‐porphyrin organic gel. The influences of solvent, concentration and anions were investigated in the gelation process. Secondly, the residual reactants and solvent molecules in MOG were removed through Soxhlet extraction and supercritical CO2 extraction. The Co‐PCP is an amorphous material with a hierarchical porous structure can effectively catalyze the oxidation of ethylbenzene and also exhibits a strong adsorptive capacity for the strong‐polar solvent molecules.

Metalloporphyrins‐Al3+ porous coordination polymers: Preparations, Characterizations and Catalytic Properties

In this work, metalloporphyrins‐based MOGs were synthesized as precursor to prepare porous coordination polymers (PCPs). Firstly, the carboxyl metalloporphyrins MCPp (M = Zn (II), Co (II), Cu (II), Mn (III)OAc; CPp = 5,10,15,20‐tetra(4‐(3‐carboxy)phenoxy)phenylporphyrin) were synthesized as gelator. Then, the metalloporphyrin‐based MOGs were prepared with MCPp and Al(NO3)3·9H2O by sol–gel method. Moreover, the residual reactants and solvent molecules in MOGs were removed by Soxhlet extraction and supercritical CO2 extraction to get the finial porous coordination polymers. This methodology effectually avoids the collapse of frame construction. Compared with the reference material prepared by the oven‐dry method, these PCPs exhibit much higher BET surface ranging from 398 to 439 m2 g−1. The PCPs were carefully characterized by FT‐IR, UV–vis, SEM, TEM and PXRD, and exhibit excellent catalytic activities and stabilities for the oxidation of ethylbenzene.

Research on the preparation and properties of a novel grouting material based on magnesium phosphate cement

Abstract A novel grouting material based on magnesium phosphate cement (MPC) was prepared by two-component grouting method with additions of fly ash (FA) and ground granulated blast furnace slag (GGBS). The main concern of this research is to study the influences of the addition of GGBS on the properties of MPC-grouting material, which were determined in terms of the properties of the fresh slurry, mechanical properties, water resistance, hydration products and microstructures. The results showed that GGBS reduce the setting time obviously and increase the fluidity in a certain amount, reaching to a maximum of 302 mm at a level of 20% content of GGBS. The addition of GGBS can improve the mechanical properties and water resistance of the samples. Moreover, the hydration products analyzed by X-ray diffraction (XRD) and the microstructures observed by Scanning Electronic Microscopic (SEM) illustrated that GGBS promote the hydration to produce more struvite during the late stage, and the new hydration products were formed surrounded by the residual reactants, making the structure denser. The pH value of the resulting grouting slurry was around 7.0 and the stone ratio was high, which indicated that the novel MPC-grouting material blended with GGBS and FA could meet the requirements well and was friendly to the environment.

Desalination for enhancing the recovery of creatine from its wastewater by foam fractionation

Creatine has been regarded as an essential dietary supplement due to its function of auxiliary energy supplement for muscles and neurons. Therefore, it is economical to effectively recover creatine from its wastewater by a separation technology with the characters of low energy consumption and environmental compatibility. In this work, a novel two-stage foam fractionation technology was developed for the desalination of the residual reactants and the effective recovery of creatine from its wastewater, successively. In the first stage, S-methylisothiourea sulfate was effectively removed into the foamate. Then, creatine was diluted and discharged into the storage tank with the creatine concentration of 1.5g/L. In the second stage, a foam fractionation column with a vertical ellipsoid-shaped foam channel was used to concentrate creatine from the residual solution of the first stage. Under the optimal operating conditions, the enrichment ratio and the recovery percentage of creatine could reach as high as 25.3 and 83.3%, respectively. Through the two-stage foam fractionation, the total recovery percentage of creatine could achieve above 77.9%.

29Si{27Al}, 27Al{29Si} and 27Al{1H} double-resonance NMR spectroscopy study of cementitious sodium aluminosilicate gels (geopolymers) and gel-zeolite composites.

The influence of starting materials and synthesis route on the properties and the structure of cementitious sodium aluminosilicate gels is not fully understood, partly due their amorphous nature and the fact that they often contain residual reactants, which can make the results of single-pulse NMR spectroscopy applied to these materials difficult to interpret or ambiguous. To overcome some of these limitations, 29Si{27Al} TRAPDOR NMR as well as 27Al{29Si} and 27Al{1H} REDOR NMR spectroscopy were applied to materials synthesized by the one-part alkali-activation route from three different amorphous silica starting materials, including rice husk ash. The latter led to formation of a fully amorphous sodium aluminosilicate gel (geopolymer), while the materials produced from the other silicas contained amorphous phase and crystalline zeolites. Application of the double-resonance NMR methods allowed to identify hydrous alumina gel domains in the rice husk ash-based material as well as significantly differing amounts of residual silica in the three cured materials. Four-coordinated Al existed not only in the aluminosilicate gel framework but also in a water-rich chemical environment with only a small amount of Si in proximity, likely in the alumina gel or possibly present as extra-framework Al in the aluminosilicate gel. The results demonstrate how the employment of different silica starting materials determines the phase assemblage of one-part alkali-activated materials, which in turn influences their engineering properties such as the resistance against chemically/biologically aggressive media.

Study on the synergistic effect of UV/Fenton oxidation and mass transfer enhancement with addition of activated carbon in the bubble column reactor

For the UV/Fenton treatment of waste gas, low solubility of volatile organic compounds (VOCs) was a significant factor of limiting gas–liquid mass transfer and thus restricted the removal efficiency. In this study, synergistic effect of UV/Fenton oxidation and mass transfer enhancement with addition of activated carbon (AC) was observed through a drastic improvement of n-octane removal in a bubble column reactor. Firstly, the effect of AC on mass transfer was investigated. Result revealed that the addition of AC could change bubble behavior and largely enhance the mass transfer efficiency (volumetric mass transfer coefficient (kLa) increased from 0.0057 to 0.14 s−1 as 0–0.1% AC was added) due to the particle shuttle effect. Then AC was introduced into the UV/Fenton reaction system, and the removal efficiency of n-octane had been remarkably improved (maintained above 50% in 60 min with addition of 0.04% AC, compared with below 10% in 20 min for the UV/Fenton alone). Moreover, we comprehensively detected the concentration of residual reactants, intermediates, final products and hydroxyl radicals in the gas and liquid phase. Results indicated that addition of AC could mildly and stably generate hydroxyl radicals (10−8 M), and the mass balance showed that the mineralization efficiency had increased from 42.4% to 57.9%. For theoretical calculations, the value of the cooperation factor raising from 0.5 to 11.7 in 40 min could offer direct evidence for confirming the synergistic effect. Finally, the reaction pathways of n-octane in UV/Fenton or AC-UV/Fenton were proposed. It can be induced that the latter ones were more likely to first break the CC bond and then oxidize into small molecules acids or CO2.

In Situ Two-Step Photoreduced SERS Materials for On-Chip Single-Molecule Spectroscopy with High Reproducibility.

A method is developed to synthesize surface-enhanced Raman scattering (SERS) materials capable of single-molecule detection, integrated with a microfluidic system. Using a focused laser, silver nanoparticle aggregates as SERS monitors are fabricated in a microfluidic channel through photochemical reduction. After washing out the monitor, the aggregates are irradiated again by the same laser. This key step leads to full reduction of the residual reactants, which generates numerous small silver nanoparticles on the former nanoaggregates. Consequently, the enhancement ability of the SERS monitor is greatly boosted due to the emergence of new "hot spots." At the same time, the influence of the notorious "memory effect" in microfluidics is substantially suppressed due to the depletion of surface residues. Taking these advantages, two-step photoreduced SERS materials are able to detect different types of molecules with the concentration down to 10-13 m. Based on a well-accepted bianalyte approach, it is proved that the detection limit reaches the single-molecule level. From a practical point of view, the detection reproducibility at different probing concentrations is also investigated. It is found that the effective single-molecule SERS measurements can be raised up to ≈50%. This microfluidic SERS with high reproducibility and ultrasensitivity will find promising applications in on-chip single-molecule spectroscopy.

Nanoporous Atomically Thin Graphene Membranes for Desalting and Dialysis Applications.

Dialysis is a ubiquitous separation process in biochemical processing and biological research. State-of-the-art dialysis membranes comprise a relatively thick polymer layer with tortuous pores, and suffer from low rates of diffusion leading to extremely long process times (often several days) and poor selectivity, especially in the 0-1000 Da molecular weight cut-off range. Here, the fabrication of large-area (cm2 ) nanoporous atomically thin membranes (NATMs) is reported, by transferring graphene synthesized using scalable chemical vapor deposition (CVD) to polycarbonate track-etched supports. After sealing defects introduced during transfer/handling by interfacial polymerization, a facile oxygen-plasma etch is used to create size-selective pores (≤1 nm) in the CVD graphene. Size-selective separation and desalting of small model molecules (≈200-1355 Da) and proteins (≈14 000 Da) are demonstrated, with ≈1-2 orders of magnitude increase in permeance compared to state-of-the-art commercial membranes. Rapid diffusion and size-selectivity in NATMs offers transformative opportunities in purification of drugs, removal of residual reactants, biochemical analytics, medical diagnostics, therapeutics, and nano-bio separations.

Apply modified cotton by m-DMDHEU and choline cloride to separate ion CrO4 2- , H2AsO4 - in water solution

In this article, cotton is modified by m- DMDHEU and choline chloride. The modified cotton can adsorb and ion exchange ion CrO4 2- , H2AsO4 - in water solution as effectively as anionit resin. The material are activated by soaking in 24 h with mixed m-DMDHEU and choline chloride, then dried and activated in 1 h at 140 oC, all residual reactants was removed by water. Adsorption and ion exchange process were studied with a model solution contained ion CrO4 2- , H2AsO4 - in different conditions. Results showed that the modified cotton is able to adsorb ion CrO4 2- , H2AsO4 - of the model, solution is nearly completely, respectively 99,95 % and 99,60 %; the values of anionit resin is respectively 97,99 % and 97,60 % at pH=7,0.

In Situ Investigations of Laser-Generated Ligand-Free Platinum Nanoparticles by X-ray Absorption Spectroscopy: How Does the Immediate Environment Influence the Particle Surface?

Pulsed laser ablation in liquid (PLAL) has proven its usefulness as a nanoparticle (NP) synthesis method alternative to traditional chemical reduction methods, where the absence of any molecular ligands or residual reactants makes laser-generated nanoparticles ideal reference materials for charge-transfer experiments. We synthesized additive-free platinum nanoparticles by PLAL and in-situ characterized their interaction with H2O, sodium phosphate buffer, and sodium citrate as well as a TiO2 support by X-ray absorption fine structure (XAFS), i.e., X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS). Differences in the white-line intensity among the colloidal particles in the three liquids indicate that the respective NP-solvent interaction varies in strength. The ions added ex situ diffuse through the particles' electric double layer and interact electrostatically with the Stern plane. Consequently, these ions weaken the interaction of the functional OH groups that are bound to the partially oxidized platinum surfaces and cause their partial reduction. Comparing XAFS spectra of laser-generated Pt NPs in citrate with wet-chemically synthesized ones (both ligand-covered) indicates different types of Pt-O bonds: a Pt(IV)O2 type in the case of wet-chemical NPs and a Pt(II)O type in the case of laser-generated NPs. A comparison of unsupported laser-generated platinum NPs in H2O with TiO2-supported ones shows no white-line intensity differences and also an identical number of Pt-O bonds in both cases. This suggests that in the deposition process at least part of the double-layer coating stays intact and that the ligand-free Pt particle properties are preserved in the TiO2-supported Pt particles, relevant for heterogeneous catalysis.

Significant indicators for biomineralisation in sand of varying grain sizes

Microbially induced calcium carbonate precipitation (MICP) is emerging as a sustainable technology for improved construction materials. In this technique, calcium carbonate is deposited in the pores of substrates such as sand and concrete. As the technology moves from laboratory to field new metrics and field measurement techniques are required to ensure that the results achieved at the laboratory are replicated. In the scaled up technique it is not possible to collect samples from deep inside the substrate for direct measurement. This paper explores some indirect indicators of MICP that can be easily monitored in large scale applications. Bacterial fluid has been passed through sand columns of varying grain sizes. The rate of flow is monitored for ten days. The effluent has been tested for pH and electrical conductivity to evaluate their potential for ensuring MICP. The residual urea and calcium in the effluent has been monitored to have a quantitative estimate of MICP. At the end of the tests the quantity of carbonate deposited in the sand columns is measured. A correlation between the residual reactants and the carbonate deposition is developed. An approximate expression for different grain sizes has been developed. This study demonstrates the utility of indirect measurement techniques as flow rate, pH, conductivity, urea and calcium consumption as potential indicators of efficient MICP in sand media.