Chemical Coagulation (CC) and Electrocoagulation (EC) processes using iron as coagulating agent were compared while treating aquifer water contaminated by a relatively high concentration of total chromium (CrT =19.0 mg L-1). A continuous semipilot comprising of EC (or CC), followed by flocculation, sedimentation and sandfiltration steps was used to remove CrT from water in order to get drinking water. A ratio of Fe/CrT ≥ 3 was required to reach the best efficiency of treatment using EC and CC. More than 99% of CrT was removed and residual CrT concentration below 0.05 mg L-1 (the limiting value recommended by WHO) could be obtained using the both configurations. However, CC increased the concentration of dissolved solids above the guideline recommended for drinking water.

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While arsenous acid, As(OH)3, has been the subject of a plethora of studies due to its worldwide ubiquity and its toxicity, pentavalent As in the form of arsenic acid, AsO(OH)3, has recently been found in rivers in central Mexico as the most abundant naturally occurring arsenic species. To better understand the solvation patterns of both toxic acids at the molecular level, we report the results of Born-Oppenheimer molecular dynamics simulations on the aqueous solvation of the AsO(OH)3 and As(OH)3 molecules at room temperature using the cluster microsolvation approach including 30 water molecules at the B3LYP/6-31G** level of theory. We found that the average per-molecule water binding energy is ca. 1 kcal mol-1 larger for the As(v) species as compared to the As(iii) one. To account for the asymmetry of both molecules, the hydration patterns were studied separately for a "lower" hemisphere, defined by the initially protonated oxygens, and for the opposite "upper" hemisphere. Similar lower hydration patterns were found for both As(iii) and As(v), with the same coordination number CN = 7. The upper pattern for As(iii) was found to be of a hydrophobic type, whereas that for As(v) showed the fourth oxygen to be hydrogen-bonded to the water network, yielding CN = 3.7; moreover, a proton "hopped" from the lower to the upper side, through the Grotthuss mechanism. Theoretical EXAFS spectra were obtained that showed good agreement with experimental data for As(iii) and As(v) in liquid water, albeit with somewhat longer As-O distances due to the level of theory employed. Proton transfer processes were also addressed; we found that the singly deprotonated H2AsO3- species largely dominated (99% of the simulation) for the As(iii) case, and that the deprotonated H2AsO4- and HAsO42- species were almost equally present (45% and 55%, respectively) for the As(v) case, which is in line with the experimental data pKa1 = 2.24 and pKa2 = 6.96. Through vibrational analysis the features of the Eigen and Zundel ions were found in the spectra of the microsolvated As(iii) and As(v) species, in good agreement with experimental data in aqueous solutions.

Arsenic has been classified as a toxic and carcinogenic chemical element. It therefore presents a serious environmental problem in different regions of the country and the world. In the present work, two adsorbent media were developed and evaluated to remove arsenic from water in the Pájaro Verde mine shaft, Huautla, Tlaquiltenango, Morelos. The media were synthesized and characterized, obtaining a surface area of 43.04 m2·g−1 for the goethite and 2.44 m2·g−1 for silica sand coated with Fe(III). To conduct the sorption kinetics and isotherms, a 23 factorial design was performed for each medium in order to obtain the optimal conditions for the factors of arsenic concentration, pH and mass of the adsorbent. The best results were obtained for goethite, with a removal efficiency of 98.61% (C0 of As(V) 0.360 mg·L−1), and an effluent concentration of 0.005 mg·L−1, a value that complies with the modified Official Mexican Standard NOM-127-SSA1-1994 [1] and WHO guidelines (2004) [2]. The kinetic equation that best fit the experimental data was the pseudo-second-order, resulting in the highest values for the constants for synthetic goethite, with a rate constant sorption of 4.019·g·mg−1·min−1. With respect to the sorption isotherms, both media were fitted to the Langmuir-II linear model with a sorption capacity (qm) of 0.4822 mg·g−1 for goethite and 0.2494 mg·g−1 for silica sand coated with Fe(III).

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Arsenic contamination of groundwater is still a global problem due to the toxicity at low dose on human health confirmed by epidemiological studies. Magnetic xerogel monoliths (MXs) were synthesized by the sol-gel polymerization using resorcinol, formaldehyde, alkaline catalyst and magnetite. The varying molar ratios of magnetite and resorcinol (M/R) in the gel were evaluated for As(V) removal from groundwater. The surface chemistry, structure and morphology of MXs related to arsenic adsorption were characterized by X-ray diffraction, scanning electron microscopy, energy-dispersive X-ray spectroscopy, Fourier transform infrared spectroscopy and point of zero charge. Batch adsorption experiments were carried out to investigate the effects of Fe contents, initial pH and adsorbent dose on As(V) removal performance. The MXs with molar ratio of M/R at 0.15 gave the maximum As(V) adsorption capacity and removal with values of 62.8 µg/g and 86.7%, respectively. The adsorption data were well described by the Elovich equation of the kinetic model and the Freundlich isotherm. The thermodynamic studies demonstrated that the adsorption process was endothermic and spontaneous in nature. MXs showed to be a good alternative for As(V) removal from groundwater and achieving the efficient desorption, and thus fulfilled the Mexican standard for drinking water.

Magnetic xerogels Monoliths (MCs) were simultaneously prepared and formed by the cross-linking polymerization of resorcinol and formaldehyde using the alkaline catalyst and magnetite. The varying of molar ratio of resorcinol and catalyst (R/C) was studied and characterized by IEP, pHpzc, SEM-EDX, XRD, N2 adsorption and FTIR. The result of XRD and EDX confirmed the presence of magnetite into the gel at 1.19% with low molar ratio of magnetite and resorcinol ratio at 0.01. The surface morphology and textural properties of MCs affect directly with SBET, total pore volume and volume of mesopore increase when molar of R/C increases. The behavior of arsenic (As(V)) adsorption by using MCs, was studied in groundwater into the ranges of pH from 2.0 to 7.0. MC50 shows the maximum As(V) uptake and removal were 72 μg/g and 73.5% at pH 5, respectively, while MC100 gave the best performance within the application range of pH both of acidic and neutral region. Furthermore, the prediction technique based on an adaptive fuzzy rules emulated network was utilized for evaluation the arsenic removal performance.