Modeling pedogenization of zeolitized tuffs. II: medium-term weathering of phlegraean yellow tuff and red tuff with black scoriae by water and humic acids

2092

From Zeolites to Porous MOF Materials – the 40th Anniversary of International Zeolite Conference R. Xu, Z. Gao, J. Chen and W. Yan (Editors) © 2007 Elsevier B.V. All rights reserved.

Modeling pedogenization of zeolitized tuffs. II: medium-term weathering of phlegraean yellow tuff and red tuff with black scoriae by water and humic acids A. Buondonnoa, A. Colellab, C. Colellac, E. Coppolad, B. de’ Gennaroc, M. de’ Gennarob, N. Gargiuloc, E. Grillid, A. Langellae and M. Rubinod a

CRA-Istituto Sperimentale Agronomico, Via Celso Ulpiani 5, 70125 Bari, Italy

b

Dipartimento di Scienze della Terra, Università Federico II, Via Mezzocannone 8, 80134 Napoli, Italy c

Dipartimento di Ingegneria dei Materiali e della Produzione, Università Federico II, P.le V. Tecchio 80, 80125 Napoli, Italy d

Dipartimento di Scienze Ambientali, polo Scientifico di Caserta, Seconda Università di Napoli, Via Vivaldi 43, 81100 Caserta, Italy e

Dipartimento di Studi Geologici e Ambientali, Università del Sannio, Via Port’Arsa 11, 82100 Benevento, Italy ABSTRACT An experimental pedology research program was started aiming at modeling the potential pedogenization of zeolitized tuffs. The present study deals with a medium-term weathering of Phlegraean Yellow Tuff (PYT, with phillipsite>chabazite) and Red Tuff with Black Scoriae (RTBS, with chabazite>phillipsite) under discontinuous treatment by water (W) and by humic acids (HA). Significant amounts of cations, increasing in the sequence Al
2093 matter is also formed and stabilized. The humic component of the organic soil matter then plays a crucial role in complexing of polyvalent cations and their mobilization. This enhances a distinctive form of mineral weathering, in which weathering products [2] are removed by chelation from their equilibrium state at the mineral/solution interface. The pedomineralogical evolution of the principal earth-crust rocks has already been studied [3], whereas very limited information on the alteration and transformation of zeolitized tuffs is available, notwithstanding the worldwide outcrop occurring of such rocks and their relevant presence as soil substrates in volcanic districts. On this basis, an experimental pedology program was started aiming at modeling the effect of water and dissolved humic acids on the alteration of zeolitized tuffs and on the subsequent cations release. In a previous work, the short-term (1704 h) simulated weathering of Neapolitan Yellow Tuff (NYT), a material similar to PYT, and Red Tuff with Black Scoriae (RTBS) by water (W) and humic acids (HA) was studied [4], showing that cations release was different and depending on the nature of the investigated tuffs as well as on the applied extracting media. The present work provides additional information on Ca, Mg, Al, and Fe removal, and on the susceptibility to alteration of PYT and RTBS after a medium-term (2904 h) simulated weathering by W and HA. 2. MATERIALS AND METHODS 2.1. Starting materials Two different zeolitized tuffs were used: (i) A Phlegraean yellow tuff, PYT, (surroundings of Naples, Italy), marketed by I.Z.Italiana Zeoliti (Pigneto, Modena, Italy) with prevailing phillipsite over chabazite content. The PYT sample used for the present investigation had the following chemical composition (wt%): SiO2=53.75, TiO2=0.40, Al2O3=16.45, Fe2O3=3.91, MnO=0.14, MgO=1.11, CaO=5.98, K2O=6.98, Na2O=1.35, P2O5=0.06, H2O=10.10. The chemical analysis was performed through X-ray Fluorescence Spectrometry (Philips TW1400 apparatus, equipped with a tungsten tube). Zeolite content was 51% (38% phillipsite, 13% chabazite); other tuff components were smectite 4%, biotite 1%, feldspar 33% and calcite 1%, plus amorphous phases. The mineral composition of the material was estimated using the Reference Intensity Ratio (RIR) procedure [5]. (ii) Red Tuff with Black Scoriae, RTBS, from Vulsini formation of Orvieto-Bagnoregio (Lazio-Umbria-Toscana regions), with prevailing chabazite over phillipsite content. This formation is much older than PYT and represents the pyroclastic flow of the OrvietoBagnoregio Ignimbrite, (333,000±4,000yb.p.) [6], covering an area of about 200 km2 [7]. The RTBS sample used for the present investigation had the following chemical composition (wt%): SiO2=51.98, TiO2=0.50, Al2O3=16.79, Fe2O3=4.53, MnO=0.13, MgO=1.33, CaO=4.92, K2O=6.22, Na2O=1.43, P2O5=0.06, H2O=12.35. This sample contained 41% chabazite, 17% phillipsite, 4% analcime, 4% smectite, 1% biotite, 29% feldspar, plus amorphous components. 2.2. Experimental weathering Representative samples of PYT and RTBS were crushed and sieved at 0.3”‡”2.0 mm. 5 g of each material were shaken in polypropylene stoppered tubes with 150 mL deionized water (W) or with a solution containing 2 g L-1 of commercial humic acids (HA, Fluka; oxidizable organic C content=507.11 g kg-1) at 25 °C. The HA concentration, 2 g L-1, has previously been used in cases of clay minerals investigations [8]. The pH of HA solution was

2094 7.0r0.1. After fixed periods, the suspensions were centrifuged, the supernatant solution carefully removed, and quantitatively replaced by fresh W or HA. Measurements of pH, electrical conductivity (EC) and concentration of polyvalent cations as Ca, Mg, Al, Fe, measured by Atomic Absorption Spectrophotometry (AAS, Perkin-Elmer AAnalist 100), were then performed. Five replicates of each material were prepared. The reported results refer to a total weathering of 2904 h, subdivided into 23 steps. 3. RESULTS AND DISCUSSION 3.1. pH and electrical conductivity (EC) During leaching experiments by water (W), pH was always higher for the PYT (range 6.9-8.6, mean value 7.8±0.1) than for the RTBS (range 5.8-8.0, mean value 7.0±0.1) solutions. In the presence of humic acid (HA), the pH was usually significantly lower than the corresponding of W, but still higher for the PYT (range 6.8-7.5, mean value 7.1±0.1) than for the RTBS (range 6.4-7.2, mean value 6.7±0.1). Generally, the average pH of leachates significantly decreased in sequence PYT-W>PYT-HA>RTBS-W>RTBS-HA. Taking into account that the HA solutions were neutral, such a trend reveals a sub-alkaline hydrolysis for PYT, and a sub-acidic hydrolysis for RTBS. A wider variation was observed in the EC values ranging in the cases of W experiments between 0.033 and 0.092 dS m-1 for the PYT , with a mean value of 0.042±0.002, and 0.020 and 0.041 dS m-1 for the RTBS, with a mean value of 0.026±0.001, respectively. In the case of the HA experiments the corresponding values were much higher and ranged between 0.256 and 0.366 dS m-1 (mean value 0.320±0.005), and 0.261 and 0.350 dS m-1 (mean value 0.309±0.003), respectively for the PYT and RTBS series. In any case, the PYT leachates showed EC values always higher than the RTBS ones. 3.2. Removal of Ca, Mg, Al and Fe Table 1 shows the amounts (in mmol kg-1±standard error) of Ca, Mg, Al, and Fe cumulatively removed from PYT and RTBS. In any case, HA was a more efficient extractant than the W. On the other hand, whatever the extractant, PYT released larger amounts of cations (especially cations of alkaline earths). This is in agreement with the results of the EC measurements. Generally, the amount of cations extracted increased in the sequence Al
2095 The HA could preferentially remove Ca and Mg from PYT and Al and Fe from RTBS. Considering that the Al and Fe hydrolysis products produce a much more acidic environment than that created by Ca and Mg, this consequently accounts for the sub-acidity of RTBS leachates compared with the sub-alkalinity of PYT ones, even when Ca is always the ion preferentially removed from both materials by W and HA. Table 2 shows the amounts of Ca, Mg, Al, and Fe cumulatively removed from PYT and RTBS recalculated as relative percentage on the basis of their initial content. Table 2 Amounts of Ca, Mg, Al, and Fe cumulatively removed from PYT and RTBS, expressed as percentage of the respective total initial content. Cation PYT-W RTBS-W PYT-HA RTBS-HA Ca 3.16 1.46 18.62 13.94 Mg 4.17 2.36 9.11 7.03 Al 0.18 0.17 0.26 0.26 Fe 1.55 1.14 3.90 4.68 Tables 1 and 2 additionally provide some interesting information. As expected, the HA extracting efficiency was always higher than of W. On the other hand, a higher percentage of cations were extracted from PYT in comparison with RTBS. The only exception observed was that of Fe-HA. In particular, huge amounts of Ca and Mg could be removed from PYT, substantially larger than the respective ones removed from RTBS by HA or W. Furthermore, there is no relation either between the relative efficiency of HA with respect to W in extracting a given cation from the same tuff, or between the relative amounts of the cations extracted, by W or HA, from PYT with respect to RTBS. To make the comparison quicker and easier, aiming at understanding and assessing the susceptibility of the investigated tuffs to weathering, two experimental indexes were devised: (i) a Relative Extracting Efficiency Index of HA on W, REEIHA/W, and, (ii) a Relative Cation Removal Index from PYT on RTBS, RCRIPYT/RTBS. The former was calculated as REEIHA/W=[M]-HA/[M]-W

(1)

where [M]-HA and [M]-W respectively represent the cumulative percentage of a given cation extracted by HA or W. Such index then identifies, for each tuff and each cation, the relative extracting power of HA with respect to W. The latter index was calculated as RCRIPYT/RTBS=[M]-RTBS/ [M]-PYT

(2)

where [M]-RTBS and [M]-PYT respectively represent the cumulative percentage of a given cation removed from RTBS on PYT. Such index then identifies, for each extractant and each cation, the relative susceptibility of a given cation to be removed from PYT with respect to RTBS. Table 3 shows the REEIHA/W, calculated for PYT and RTBS, and the RCRIPYT/RTBS, calculated for HA and W.

2096 Table 3 Relative Extracting Efficiency Index of HA on W, REEIHA/W, and a Relative Cation Removal Index from PYT on RTBS, RCRIPYT/RTBS Cation REEIHA/W REEIHA/W RCRIPYT/RTBS RCRIPYT/RTBS for PYT for RTBS for W for HA Ca 5.89 9.55 2.16 1.34 Mg 2.18 2.98 1.77 1.30 Al 1.42 1.50 1.06 1.00 Fe 2.52 4.11 1.36 0.83 With reference to REEIHA/W values, it is quite evident that RTBS shows the largest figures for all cations. It is also clear that, for both tuffs, humic acids are selective extractants of Ca, and that the HA/W efficiency decreases as Ca>Fe>Mg>Al. When the RCRIPYT/RTBS values are considered, all cations usually appear to be more easily removed from PYT than RTBS, especially by W. In particular, the RCRIPYT/RTBS of Ca has a value of 1.34 for HA and 2.16 for W. Besides this, the above data show that Fe is the only cation that can be much more easily removed by HA from RTBS compared to PYT. Concerning the mineralogical features of the tuffs, the only change observed was disappearance of smectite from the PYT samples during the HA and W treatment. The smectite could possibly be removed as suspension in the eluates. 4. CONCLUSIONS The present results, achieved after a medium-term experimental weathering of tuffs, clarify and confirm the findings from a previous short-term study [4], thus providing interesting information on the weatherability and on the pedogenic potential of the investigated zeolitized materials. From this point of view, both Red Tuff with Black Scoriae and Phlegraean Yellow Tuff appear susceptible to weathering. In fact, significant amounts of cations are extracted by both water (mild hydrolysis) and humic acids (powerful chelation). As expected, the humic acids showed the highest extracting efficiency, especially for calcium and iron. However, as a general trend, all cations, with special reference to calcium, were more easily removed from Phlegraean Yellow Tuff, despite its native smaller amount in the bulk rock, than from Red Tuff with Black Scoriae, with the exception of Fe, preferentially removed from RTBS by HA. Consistently with these observations, during the whole experiment the electrical conductivity of HA extracts was considerably higher than the respective W, especially for PYT series. On the other hand, the pH of RTBS extracts was always lower than the respective of PYT series, regardless of the extractant. On the whole, larger amounts of all cations were proportionally removed from PYT by W. In other words, a mild hydrolysis by water appears to be more effective on PYT than RTBS. Furthermore, the disappearance of analcime and smectite from PYT residues after treatment is a clear clue for weathering. All these remarks suggest that PYT is more prone than RTBS to be weathered by humic substances as well as by water, and, in the long run, to undergo pedogenisation. Such behavior could be likely explained taking into account the different mineralogy of the investigated tuffs. The research advances aiming at studying the evolution of parent tuff materials under the dynamics aspects of cations removal, also considering some possible chemical and morpho-mineralogical transformations, including the formation of new mineralogical phases.

2097 REFERENCES [1] H. Jenny, Soil Sci. Soc. Am. Proc., 25 (1961) 385. [2] O. A. Chadwick and R. C. Graham, Pedogenic processes. In: M. E. Sumner (ed.) Handbook of Soil Science, CRC Press, Boca Raton, FL, USA, (2000) E/41. [3] J. B. Dixon and S. B. Weed (Eds.). Minerals in Soil Environments, 2nd ed., SSSA Book Series No. 1, SSSA, Madison, WI, USA., (1989) [4] A. Buondonno, S. Capasso, A. Colella, C. Colella, E. Coppola, E. Grilli, A. Langella, and S. Salvestrini, Proc. VII Conv. Naz. di Scienza e Tecnologia delle Zeoliti, G. Giordano Ed., Centro Editoriale e Librario dell'Università della Calabria, Cosenza , Italy, (2005) pp. 151. [5] S. J. Chipera and D. L. Bish, Powder Diffr., 10 (1995) 47. [6] G. Nappi, A. Renzulli, P. Santi and P. Y: Gillot, Boll. Soc. Geol. It., 114 (1995) 599. [7] G. Nappi, B. Capaccioni, E. Mancini, M. Mattioli, and L. Valentini, Bull. Volcanol., 56 (1994) 502. [8] P. M. Huang and M. Schnitzer (eds.) Interactions of Soil Minerals with Natural Organics and Microbes, SSSA sp. publ. n 17, Madison, WI, U.S.A., (1986).