Aquatic Chemistry

Meromixis in mining lake Waldsee, Germany: hydrological and geochemical aspects of stratification

by Klaus Joehnk

Dietz, S., Seebach, A., Jöhnk, K.D., Boehrer, B, Knöller, K, Lessmann, D., 2008. Meromixis in mining lake Waldsee, Germany: hydrological and geochemical aspects of stratification. Verh. Intern. Verein. Limnol. 30 (3): 485-488

Observation of a monimolimnetic overturn and its potential role in triggering limnic eruptions from meromictic lakes

by Klaus Joehnk

Boehrer, B., Dietz, S., von Rohden, C., Kiwel, U., Jöhnk, K.D., Naujoks, S., Ilmberger, J., Lessmann, D., 2009. Observation of a monimolimnetic overturn and its potential role in triggering limnic eruptions from meromictic lakes. Geochemistry, Geophysics, Geosystems 10, doi:10.1029/2009GC002389.

Biozönotische und stoffliche Gewässerklassifikation – Untersuchungen zur Morphometrie und Hydrologie sowie limnophysikalische Modellierung von Tagebauseen

by Klaus Joehnk

Jöhnk, K.D., Hofmann, H., Hemm, M., Lessmann, D., Nixdorf, B., 2004. Biozönotische und stoffliche Gewässerklassifikation – Untersuchungen zur Morphometrie und Hydrologie sowie limnophysikalische Modellierung von Tagebauseen. In: Hüttl, R.F. & Gerwin, W. (Hrsg.), Entwicklung und Bewertung gestörter Kulturlandschaften, Forschungszentrum Bergbaulandschaften, ISBN 3-937728-01-5, 331-357.

Evidence for bias in C/N, δ13C and δ15N values of bulk organic matter, and on environmental interpretation, from a lake sedimentary sequence by pre-analysis acid treatment methods.

by Chris Brodie

In Press - Quaternary Science Reviews

Lead Author: Dr. Chris Brodie (Hong Kong University, Hong Kong)

Co-Authors: Dr. James Casford (Durham University); Dr. Jeremy Lloyd (Durham University, UK); Prof. Melanie Leng (NIGL, UK); Dr. Timothy Heaton (NIGL, UK); Christopher Kendrick (NIGL, UK); Dr. Zong Yongqiang (Hong Kong University, Hong Kong).

ABSTRACT:
There is a known bias in C/N, d13C and d15N values of organic matter (OM) due to pre-analysis acid... more ABSTRACT:
There is a known bias in C/N, d13C and d15N values of organic matter (OM) due to pre-analysis acid treatment methods. We report here, for the first time, the results of a pre-analysis acid treatment method comparison of measured C/N, d13C and d15N values in bulk OM from a sedimentary sequence of samples to illustrate this bias. Here we show that acid treatment significantly reduces the accuracy (between method biases) and precision (within method bias) of C/N, d13C and d15N values of OM, suggesting a differential response of sample OM between methods and sample horizons, and in some cases inefficient removal of inorganic carbon. We show that different methods can significantly influence environmental interpretation in some of our sample horizons (i.e. interpretation of aquatic vs. terrestrial OM source; C3 vs. C4 vegetation). Specifically, there are unpredictable and non-linear differences between methods for C/N values in the range of ~ 1 – 100; d13C values in the range of 0.2 – 6.8 ‰ and; d15Nvalues in the range of 0.3 – 0.7 ‰. Importantly, these ranges are mostly much greater than the instrument precision (defined as the standard deviation of replicate analysis of standard reference materials; for this study, ± 0.5 for C/N values, ± 0.1 ‰ for d13C values and; ± 0.1 ‰ for d15N). The accuracy and precision of measured C/N, d13C and d15N values of bulk OM is not just dependent upon environmental variability, but on acid pre-treatment, residual inorganic carbon and organic matter state and composition. Collectively, this makes the correlation between samples prepared in different ways, including those from down core reconstructions, highly questionable.

Keywords: C/N ratios, δ13C, δ15N, organic matter; pre-analysis acid treatment methods, environmental interpretation, palaeoclimate.

Evidence for bias in C and N concentrations and δ13C composition of terrestrial and aquatic organic materials due to pre-analysis acid preparation techniques

by Chris Brodie

Chemical Geology. 282, 67 - 83.

Lead Author: Dr. Chris Brodie (Durham University, UK; Hong Kong University, Hong Kong)

Co-Authors: Prof. Melanie Leng (NIGL, UK), Dr. James Casford (Durham University, UK), Christopher Kendrick (NIGL, UK), Dr. Jeremy Lloyd (Durham University, UK), Dr. Zong Yongqiang (Hong Kong University, Hong Kong), Prof. Michael Bird (James Cook University, Australia).

ABSTRACT:
This study is the first systematic comparison of the effect of acid treatment methods on the... more ABSTRACT:
This study is the first systematic comparison of the effect of acid treatment methods on the reliability of organic carbon [C] and nitrogen [N], and carbon isotope (δ13C) values on a range of terrestrial and aquatic, modern and geological environmental materials. We investigated the 3 most common methods; (i) acidification followed by sequential deionised water rinses (“rinse method”); (ii) acidification in silver capsules (“capsule method”); and (iii) acidification by exposure to an acid vapour (“fumigation method”). We also investigated the effect of sample size and capsule type (silver and tin) on C/N ratio and δ13C values. We find (i) %C, %N, C/N and δ13C showed significant within and between method variability; (ii) disproportionate and non-linear offsets of %C, %N and C/N values after acidification within and between methods and within and between sample materials; (iii) that alterations in %C did not necessarily manifest themselves in shifts in δ13C, and vice-versa; (iv) small (~ 90 μg C) sample sizes showed consistent overestimations and inaccuracies after acidification; (v) The effect of capsule type was not significant on most samples, but did show a notable effect on our aquatic materials, generally increasing %C and %N, and producing depleted δ13C values. These findings raise cause for concern on the interpretative nature of C/N ratios and their support for carbon isotope values. The comparability between laboratories (different preparation methods) and environmental settings (amount, type and nature of OM) are also likely to be problematic. We conclude that the response of C and N concentrations in organic matter to acid treatment in environmental materials is neither negligible nor systematic.

Keywords: δ13C; C/N ratio; Method comparison; Rinse method; Capsule method; Fumigation method; environmental reconstruction

Coupled feldspar dissolution-clay precipitation kinetics and lead sorption onto ferrihydrite nano-particles

by Peng Lu

Ph.D. dissertation, Indiana University, August 2010.

One of the fundamental problems in modern geochemistry is the significant discrepancy between laboratory-measured and... more One of the fundamental problems in modern geochemistry is the significant discrepancy between laboratory-measured and field derived feldspar dissolution rates. Zhu et al. (2004) proposed a new hypothesis for explaining the laboratory-field discrepancy wherein the slow kinetics of secondary clay precipitation is the rate limiting step and thus controls the overall feldspar dissolution rate. We conducted new feldspar dissolution batch experiments and performed geochemical modeling to test this hypothesis. The experimental results show that partial equilibrium was not attained between secondary minerals and aqueous solutions for the feldspar hydrolysis batch systems. Modeling results show that a quasi-steady state was reached. At the quasi-steady state, dissolution reactions proceeded at rates that are orders of magnitude slower than the rates measured at far-from-equilibrium. Results reported in this dissertation lend support to Zhu et al. (2004) hypothesis and showed how the slow secondary mineral precipitation provides a regulator to explain why the systems are held close to equilibrium and show how the most often-quoted "near equilibrium" explanation for an apparent field-lab discrepancy can work quantitatively.

The second topic of this dissertation is Pb sorption onto ferrihydrite nano-particles. The differences of adsorption and coprecipitation of Pb with iron oxyhydroxide are studied with sorption edge measurements, High Resolution Transmission and Analytical Electron Microscopy (HR TEM-AEM), and geochemical modeling. Coprecipitation of Pb2+ with ferric oxyhydroxides occurred at ~ pH 4, about 0.5-1.0 pH unit higher than Fe3+ precipitation. Coprecipitation is more efficient than adsorption in removing Pb2+ from aqueous solutions at similar sorbate/sorbent ratios. X-ray Diffraction and HRTEM shows Pb-Fe coprecipitates are 2-line ferrihydrite (2LFh) and lepidocrocite. Geochemical modeling shows that a surface complexation model can explain adsorption experimental data well. In contrast, a solid solution model or a model of combined solid solution formation and surface complexation can fit coprecipitation experimental data sets well. Hence, coprecipitation and adsorption experiments resulted in different Pb2+ incorporation mechanisms, which could result in different mobility, bioavailability, and long-term stability of Pb2+ in the environment.

Alkali feldspar dissolution and secondary mineral precipitation in batch systems: 3. Saturation states of product minerals and reaction paths

by Peng Lu

Zhu, C., and Lu, P. (2009) Geochimica et Cosmochimica Acta 73 (11): 3171-3200.

In order to evaluate the complex interplay between dissolution and precipitation reaction kinetics, we examined the... more In order to evaluate the complex interplay between dissolution and precipitation reaction kinetics, we examined the hypothesis of partial equilibria between secondary mineral products and aqueous solutions in feldspar–water systems. Speciation and solubility geochemical modeling was used to compute the saturation indices (SI) for product minerals in batch feldspar dissolution experiments at elevated temperatures and pressures and to trace the reaction paths on activity–activity diagrams. The modeling results demonstrated: (1) the experimental aqueous solutions were supersaturated with respect to product minerals for almost the entire duration of the experiments; (2) the aqueous solution chemistry did not evolve along the phase boundaries but crossed the phase boundaries at oblique angles; and (3) the earlier precipitated product minerals did not dissolve but continued to precipitate even after the solution chemistry had evolved into the stability fields of minerals lower in the paragenesis sequence. These three lines of evidence signify that product mineral precipitation is a slow kinetic process and partial equilibria between aqueous solution and product minerals were not held. In contrast, the experimental evidences are consistent with the hypothesis of strong coupling of mineral dissolution/precipitation kinetics [e.g., Zhu C., Blum A. E. and Veblen D. R. (2004a) Feldspar dissolution rates and clay precipitation in the Navajo aquifer at Black Mesa, Arizona, USA. In Water–Rock Interaction (eds. R. B. Wanty and R. R. I. Seal). A.A. Balkema, Saratoga Springs, New York. pp. 895–899]. In all batch experiments examined, the time of congruent feldspar dissolution was short and supersaturation with respect to the product minerals was reached within a short period of time. The experimental system progressed from a dissolution driven regime to a precipitation limited regime in a short order. The results of this study suggest a complex feedback between dissolution and precipitation reaction kinetics, which needs to be considered in the interpretation of field based dissolution rates.

Coupled alkali-feldspar dissolution and secondary mineral precipitation in batch systems: 1. New experiments at 200 °C and 300 bars

by Peng Lu

Fu, Q., Lu, P., Konishi, H., Dilmore, R., Xu, H., Seyfried, W. E. and Zhu, C. (2009) Chemical Geology 258 (3-4): 125-135.

Batch reactor experiments were conducted to assess perthitic alkali-feldspar dissolution and secondary mineral... more Batch reactor experiments were conducted to assess perthitic alkali-feldspar dissolution and secondary mineral formation in an initially acidic fluid (pH = 3.1) at 200 °C and 300 bars. Temporal evolution of fluid chemistry was monitored by major element analysis of in situ fluid samples. Solid reaction products were retrieved from two identical experiments terminated after 5 and 78 days. Scanning electron microscopy revealed dissolution features and significant secondary mineral coverage on feldspar surfaces. Boehmite and kaolinite were identified as secondary minerals by X-ray diffraction and transmission electron microscopy. X-ray photoelectron spectroscopy analysis of alkali-feldspar surfaces before and after reaction showed a trend of increasing Al/Si ratios and decreasing K/Al ratios with reaction progress, consistent with the formation of boehmite and kaolinite.

Saturation indices of feldspars and secondary minerals suggest that albite dissolution occurred throughout the experiments, while K-feldspar exceeded saturation after 216 h of reaction. Reactions proceeded slowly and full equilibrium was not achieved, the relatively high temperature of the experiments notwithstanding. Thus, time series observations indicate continuous supersaturation with respect to boehmite and kaolinite, although the extent of this decreased with reaction progress as the driving force for albite dissolution decreased. The first experimental evidence of metastable co-existence of boehmite, kaolinite and alkali feldspar in the feldspar hydrolysis system is consistent with theoretical models of mineral dissolution/precipitation kinetics where the ratio of the secondary mineral precipitation rate constant to the rate constant of feldspar dissolution is well below unity. This has important implications for modeling the time-dependent evolution of feldspar dissolution and secondary mineral formation in natural systems.

Arsenic Eh-pH Diagrams at 25 oC and 1 bar

by Peng Lu

Lu, P., and Zhu, C., Environmental Earth Sciences, 2011, 62(8): 1673-1683.

A thermodynamic dataset for arsenic species in As–O–H–S–Fe–Ba system was compiled from the literature. Using this... more A thermodynamic dataset for arsenic species in As–O–H–S–Fe–Ba system was compiled from the literature. Using this dataset, Eh–pH diagrams for the systems As–O–H, As–O–H–S, As–O–H–S–Fe, As–O–H–Ba, and As–O–H–S–Fe–Ba were constructed at 25°C and 1 bar. The inclusion of thioarsenite species in the systems As–O–H–S and As–O–H–S–Fe results in substantial differences from previously published Eh–pH diagrams. There are considerable differences in the thermodynamic properties for orpiment, realgar, scorodite, arsenopyrite, barium arsenate, and barium hydrogen arsenate, which result in vastly different stability fields when different values are adopted.

Bridging the gap between laboratory measurements and field estimations of silicate weathering using simple calculations

by Peng Lu

Jiwchar Ganor, Peng Lu, Zuoping Zheng and Chen Zhu (2007) Environmental Geology Volume 53, Number 3, 599-610, DOI: 10.1007/s00254-007-0675-0

Weathering rates of silicate minerals observed in the laboratory are in general up to five orders of magnitude higher... more Weathering rates of silicate minerals observed in the laboratory are in general up to five orders of magnitude higher than those inferred from field studies. Simple calculations show that even if the field conditions were fully simulated in standard laboratory experiments, it would be impossible to measure the slow rates of mineral dissolution that are observed in the field. As it is not possible to measure the dissolution rates under typical field conditions, one should extrapolate the available data to the field conditions. To do this, a rate law for the dissolution of plagioclase in the field was formulated by combining the far from equilibrium dissolution rate of weathered natural oligoclase at 25°C with the effect of deviation from equilibrium on dissolution rate of fresh albite at 80°C. In contrast to the common view that laboratory experiments predict dissolution rates that are faster than those in the field, the simulation based on this rate law indicates that laboratory dissolution experiments actually predict slower rates than those observed in the field. This discrepancy is explained by the effect of precipitation of secondary minerals on the degree of saturation of the primary minerals and therefore on their dissolution rate. Indeed, adding kaolinite precipitation to the simulation significantly enhances the dissolution rate of the plagioclase. Moreover, a strong coupling between oligoclase dissolution and kaolinite precipitation was observed in the simulation. We suggest that such a coupling must exist in the field as well. Therefore, any attempt to predict the dissolution rate in the field requires knowledge of the rate of the secondary mineral precipitation.

Molecular Structure of Lead(II) Coprecipitated with Iron(III) Oxyhydroxide

by Peng Lu

Kelly, S., Lu, P., Bolin, T., Chattopadhyay, S., Newville, M. G., Shibata, T., and Zhu, C., 2008, In Mark Barnett and Douglas Kent (Ed.) Adsorption of Metals by Geomedia II, 67-94. Developments in Earth & Environmental Sciences 7, Elsevier.

Detailed knowledge of the uptake mechanisms of trace metals in geomedia is fundamental to environmental geochemical... more Detailed knowledge of the uptake mechanisms of trace metals in geomedia is fundamental to environmental geochemical processes, particularly to the prediction of contaminant transport and mobility in geological media. We investigated the molecule structure by using macroscopic and microscopic structural probes including X-ray diffraction (XRD), high-resolution transmission and analytical electron microscopy (HRTEM-AEM), and extended X-ray absorption fine structure (EXAFS) spectroscopy. The long-range crystalline properties of the PbFe coprecipitate, as measured by XRD, were consistent with poorly crystalline lepidocrocite and two-line ferrihydrite (2LFh). The particle size and shape of the PbFe coprecipitate, as measured by HRTEM, showed a mixture of spheres (2–6 nm in diameter) and needles (20–80 nm×200–300 nm) composed of aggregated crystallites 2–3 nm in diameter. The local atomic structure about Pb and Fe in the PbFe coprecipitate was further elucidated through a series of EXAFS modeling efforts, including molecular moiety modeling, linear combination fitting, and co-refinement based on a FeO6 sheet structure for lepidocrocite. The larger atomic size of Pb as compared to Fe was accounted for in the model of the FeO6 sheet structure by displacing the PbO6 unit perpendicular to the sheet by 0.30±0.02 Å from the FeO6 unit position. The co-refinement of the Pb LIII-edge and the Fe K-edge EXAFS spectra with the same local atomic environment and the additional PbO6 displacement suggested that Pb formed a solid solution in the PbFe coprecipitate. The coprecipitation of Pb by Fe oxyhydroxides may decrease the mobility and bioavailability of Pb2+ by incorporating Pb into the Fe oxyhydroxide structure.

On the potential of CO2–water–rock interactions for CO2 storage using a modified kinetic model

by Peng Lu

V.T.H. Phama, P. Lu, P. Aagaarda, C. Zhu and H. Hellevang. International Journal of Greenhouse Gas Control. 5(4): 1002-1015.

During CO2 storage, mineral trapping is the safest long-term storage mechanism, and it is therefore important to... more During CO2 storage, mineral trapping is the safest long-term storage mechanism, and it is therefore important to estimate the correct CO2 portion trapped in secondary mineral phases. The storage potential for cold, quartz-rich reservoirs, hereafter termed Utsira-type reservoirs, were solved using the numerical code PHREEQC, using a rate model that took into account both nucleation and growth of secondary mineral phases. This represented a modification of earlier simulations where growth rates were calculated from dissolution rate data. Because growth rate and nucleation rate parameters were largely unknown for the secondary carbonates, we did a sensitivity study on the potential for carbonate growth on rate parameters.

The simulations suggest that the total amount of CO2 trapped as mineral carbonates is given by the amount of glauconite, chlorite, and smectite present in the reservoir prior to injection, as they were nearly completely dissolved. The fast dissolution of the silicates provided divalent cations for the growth of ankerite and siderite. The timing of precipitation and the secondary mineral assemblage were seen to be highly sensitive to the nucleation and growth rates. Moreover, at high nucleation rates, the secondary carbonates started to precipitate at fairly low supersaturations and formed rapidly after the dissolution of the primary minerals.

Finally, a comparison of earlier simulations on the Utsira-type system with the present model and natural analogues, suggests that the earlier models have largely overestimated the growth potential of carbonates such as dolomite, magnesite and dawsonite.