Identifying the source and fate of boron in geothermal water: Evidence from B/Na and B isotopes.

High level dissolved B, which poses risks to human health, has been widely observed in geothermal water. In the Guide Basin, NW China, a series of geothermal water samples along a fault show a wide range of B contents ranging from 3.14 to 8.33 mg/L, which are higher than the WHO Guideline value equaling 2.4 mg/L in drinking water. To identify the sources and fate of B, we conduct a comprehensive analysis of hydrochemistry and stable isotopes (D, 18O and 11B) of three thermal fields representing three stages of hydrogeochemical evolution (stages I, II and III). From stage I to III, there are trends of increasing mineral dissolution, which is supported by increasing mean reservoir temperature and concentrations of conservative elements (Cl, Na, K, Li and Si). Geothermal water in stage I with meteoric origin and the lowest reservoir temperature has the highest B/Na resulting from silicate dissolution and falls on the mixing line between granitoids and cold water on the plot of δ11B versus 1/B, showing the control of silicate dissolution. However, geothermal water in stage III has lower Ca, B Sr and B/Na than that in stage II. Because of the occurrence of other processes, geothermal water in stages II and III deviates from the LMWL. Compared with geothermal water in stage I, the increased Sr/Ca and decreased B/Ca show that B are removed by both coprecipitation and vapor separation. With the aid of B isotopes, we find vapor separation dominates in stage II, whereas carbonate precipitation dominates in stage III. Overall, a combined use of three isotopes (H, O and B) and three element ratios (B/Na, B/Ca and Sr/Ca) leads to a complete understanding of B cycle and hydrogeochemical evolution in hydrothermal systems.

[Arsenic Adsorption and Its Species on Ferrihydrite and Ferrihydrite Colloid].

Batch experiments were conducted to investigate the adsorption kinetics and adsorption isotherms of As(Ⅲ) and As(Ⅴ) on ferrihydrite and its colloid. A sequential extraction technique and As speciation extraction were used to assess the chemically bound fractions of As and As species in solids, respectively. The kinetic and isotherm data showed that the adsorption was multilevel and readily occurred. The adsorption of As(Ⅲ) and As(Ⅴ) on the ferrihydrite colloid was 194.8 g·kg-1 and 107.3 g·kg-1, respectively, which was higher than that on ferrihydrite (155.2 g·kg-1 and 104.4 g·kg-1). The specifically adsorbed As, amorphous oxide bound As, and crystalline oxide bound As were the major fractions of adsorbed As on ferrihydrite and its colloid. The residual As was absorbed on the ferrihydrite surface instead of on the ferrihydrite colloid. Therefore, As adsorption on ferrihydrite was much stronger than that on ferrihydrite colloid. The ferrihydrite and ferrihydrite colloid could not reduce As(Ⅴ) to As(Ⅲ).

[Modification of natural siderite and enhanced adsorption of arsenic].

Groundwater with high arsenic concentration has widely been found in China. More attention has been paid to economic and efficient arsenic removal technology. Natural siderite, which was abundant and relatively cheap, was used as the main raw material for arsenic adsorption by batch methods. Modified conditions of natural siderite, including temperature and time of calcination and adhesive addition, were carried out for arsenic removal. Results showed that the maximum removal efficiency was reached with the calcination temperature of 350 degrees C for 90 minutes and an adhesive dosage of 10 mg x g(-1). With the ratio of solid to liquid of 0.5 g: 50 mL and the initial concentration of 5 mg x L(-1) for either As(III) or As(V) at 25 degrees C, arsenic concentrations at equilibrium time were lower than 10 microg x L(-1). Characteristics of adsorption kinetics and adsorption isotherm on the optimal modified adsorbent were also evaluated. It was found that the arsenic adsorption kinetics fitted pseudo-second order kinetics equation, and the adsorption achieved equilibrium at about 12 h. The adsorption isotherm could be well described by Langmuir and Freundlich models. The maximum adsorption capacity was 1039 microg x g(-1) for As(III) and 1 026 microg x g(-1) for As(V). Furthermore, X-ray diffraction (XRD), scanning electron microscopy (SEM), and BET method were used to investigate main mechanisms of arsenic removal. Results showed that modified adsorbent had higher specific surface area and contained the spherical coating of Fe(II) and Fe(III) on the surface, in comparison with pristine material, which were believed to contribute to the high adsorption capacity of the modified material. The modified natural siderite appears to be a promising adsorbent that is worthy of further studies and practical application for arsenic removal.

[Characteristics of fluoride removal by activated iron-manganese nodules].

Adsorption of fluoride on FeCl3-activated manganese-iron nodules was investigated in this study. Influences of contact time, temperature and coexistence anions on F- adsorption were carried out in terms of batch tests. It was found that the fluoride adsorptions onto activated material achieved equilibrium gradually at about 48 h. The adsorption capacity increased with the increase in reaction temperature. Furthermore, the adsorption isotherm can be described as Langmuir and Freundlich models. Thermodynamical study reveals that fluoride adsorptions on the activated material are spontaneous and endothermic. In addition, the presence of coexistence anions shows negative effect on removal of fluoride. Column studies demonstrate that the activated adsorbent could efficiently adsorb F-. Concentrations of Fe and Mn in effluents meet Chinese sanitary standard for drinking water. Effectiveness of adsorbent for fluoride removal increases after regeneration, and the average capacity is up to 1.340 mg/g with 2 cycles of desorption and regeneration. Results of X-ray diffraction (XRD) and scanning electron microscopy (SEM) manifest that high adsorption capacity arose from conglutination of Fe oxides/ hydroxides to the surface of nodules and subsequently adsorption of F- on the fresh Fe hydroxides. Therefore, the activated absorbent has high adsorption capacity and stable property for F- removal, which shows that it is a potential material for F- removal in practice.