Ni sorption

Almost 100 % of the Ni was sorbed on the fresh waste rock after 400 minutes, while Ni retention reaches only 50 % after 1500 minutes on the weathered waste rock. The resulting Ni sorption capacities from these batch tests are 0.34 mg/g for the fresh and 0.19 mg/g for the weathered waste rock. However, the Ni sorption capacity of the fresh waste rock is probably greater, since all the Ni was retained in the batch test and the sorption sites were not fully saturated. 

Perrone et al. (2001) modelled Ni(II) adsorp-tion to synthetic carbonate fluoroapatite (CaI0 ((P04)5(C03))(0H,F). The solid phase had a pHIEP of 6.3 and a ZPC of 6.4 with an SSA of 8.8m2/g, an estimated sorption site density of 3.1 sites/nm2. They conducted 8-day isotherms in closed vessels at Ni concentrations of 5 x 10-10 to 1 x 10 8 M, constant I (0.05, 0.1 or 0.5 M), constant solid phase concentrations of 10 g/dm3 at pH values of 4 to 12. As Ni sorption occurred, no significant release of Ca was seen. Sorption was reversible. Rather than precisely characterize surface functional groups, they elected to describe their sorbent surfaces using acid-base reactions for the average behaviour of all sites involved in protonation and deprotonation. Potentiametric titration data were used to estimate the constants with the FTTEQL computer code ... 

An example of actual experimental data (Ni sorption on alumina, low initial Ni concentration, different solid to liquid ratios) plotted as percentage of uptake on the one hand and as A o on the other is presented in Fig. 4.8. The results plotted as... 

Figure 3.1. Kinetics of Ni sorption (%) on pyrophyllite, kaolinite, gibbsite, and montmorillonite from a 3 mM Ni solution, an ionic strength I = 0.1 NaNOs, and a pH of 7.5. (From Scheidegger et al., 1997a.)...

XAFS data, showing the formation of Ni-Al LDH phases on soil components, are shown in Figure 3.5 and Table 3.1. Radial structure functions (RSFs), collected from XAFS analyses, for Ni sorption on pyrophyllite, kaolinite, gibbsite, and montmorillonite were compared to the spectra of crystalline Ni(OH)2 and takovite. All spectra showed a peak at R 0.18 nm, which represents the first... 

TABLE 3.1. Structural Information Derived from XAFS Analysis for Ni Sorption on Various Sorbonents and for Known Ni Hydoxides ... 

Figure 3.7. Radial structure functions (achieved from XAFS analyses) for Ni sorption on pyrophyllite for reaction times up to 24 hours, demonstrating the appearance and growth of the second shell (peak at 2.8 A) contributions due to surface precipitation and growth of a mixed Ni-Al phase. (From Scheidegger et ah, 1998.)...

The formation and subsequent aging of the metal hydroxide surface precipitate can have a significant effect on metal release. In Figure 3.8 one sees that as residence time (aging) increased from 1 hour to 2 years, Ni release from pyrophyllite, as a percentage of total Ni sorption, decreased from 23 to 0% when HNO3 (at pH 6.0) was employed as a dissolution agent for 14 days. This... 

Competitive sorption of trace elements to organic soil components has also been studied. Kinniburgh et al. (1996) demonstrated that Cd sorption on a humic acid was reduced by Ca, but in contrast, Cu sorption was poorly reduced. Mandel et al. (2000) showed clear competitive effects of Ca and Mg on Ni sorption to a soil fulvic acid. Many studies have showed evidence that there may be differences in competition between selected trace elements depending on the functional group composition of the humic substances (Kretzschmar and Voegelin, 2001). 

Having successfully modeled the data, there is a temptation to consider the point to be proven and the work to be done. Unfortunately, individual models are not always unique to a data set, so a mechanism consistent with the model must be postulated and confirmed by independent means. (While agreement of a model with a mechanism does not provide a priori knowledge of the kinetic system, inability to match the two does argue for rejection of the theory developed.) For the data illustrated (Fig. 6-2), cation exchange fits the known system characteristics. To confirm a cation exchange hypothesis, Ni sorption by an exchange reaction should be accompanied... 

Fig. 6-7. Kinetics of Ni + sorption and associated cation release by Paxton A horizon (0.040 mol L Ni " " added, soil/solution ratio = 1 100). (a) Ratio of Ni " remaining to added as a function of time. The model represents a first-order reversible reaction with kJ = 0.65 and kj 0.95 K = 0.68). (b) Difference between Ni remaining in solution and the first-order kinetic model (Fig. 7a), as a function of The line indicates the area of direct relationship, (c) Cation release during the reaction. The solid line represents (1 — C/Q) or the inverse of the first-order model presented as a solid line in Fig. 6-7a. (d) Difference between cation gain to solution and the first-order kinetic model (Fig. 6-7c), as a function of The line indicates the area of direct relationship. 

Ni Sorption to iron hydroxides, carbonate minerals Iron hydroxide availability pH, alkalinity, and Ca levels to answer if calcium carbonate is stable. Eh, and, if E is low, sulfide levels. 

The potential for AFM to significantly contribute further to sorptive geochemistry research is very high. There are many interesting and important systems to explore. Just as one example, Scheidegger and others (23-25) have performed some fascinating studies on Ni sorption on clays and aluminum oxides... 

Ni Sorption on Clay Minerals A Case Study. Initial research with Co/clay mineral systems demonstrated the formation of nucleation products using XAFS spectroscopy, but the stmcture was not strictly identified and was referred to as a Co hydroxide-like stmcture (11,12). Thus, the exact mechanism for surface precipitate formation remained unknown. Recent research in our laboratory and elsewhere suggests that during sorption of Ni and Co metal ions, dissolution of the clay mineral or aluminum oxide surface can lead to precipitation of mixed Ni/Al and Co/Al hydroxide phases at the mineral/water interface (14,16,17,67,71). This process could act as a significant sink for metals in soils. The following discussion focuses on some of the recent research of our group on the formation kinetics of mixed cation hydroxide phases, using a combination of macroscopic and molecular approaches (14-17). 

Figure 3 shows the kinetics of Ni sorption on pyrophyllite, kaolinite, gibbsite, and montmorillonite from a 3 mMNi solution at pH = 7.5 (16). For kaolinite and pyrophyllite relative Ni removal from solution follows a similar sorption trend with -90% Ni sorbed within the first 24 hours. At the end of the experiments, relative Ni removal from solution was almost complete (Ni/kaolinite system, 97% sorbed after 70 hours Ni/pyrophyllite system, 98% sorbed after 200 hours). Nickel sorption on gibbsite and montmorillonite exhibited a fast initial step. Thereafter, relative Ni removal from solution distinctively slowed down. Relative Ni sorption increased from 42-58% for the Ni/montmorillonite system (time range 0.5-7Q hours) and from 15-41% for the Ni/gibbsite system (time range... 

Figure 7 illustrates the kinetics of Ni sorption on pyrophyllite, along with dissolved Si data from the Ni-treated pyrophyllite and from an untreated pyrophyllite (16). The release of Si into solution shows a similar kinetic behavior as Ni sorption on pyrophyllite. When one compares the Si release rate with the dissolution rate of the clay alone, the Si release rate in the Ni-treated system is strongly enhanced as long as Ni removal from solution is pronounced (Figure 7). Although not shown, a similar correlation between Ni sorption and Si release was observed for the Ni/kaolinite system but not for the Ni/montmorillonite system. The dissolution rate of the Ni/gibbsite system was not determined since the [Al] in solution was too low (<50 ppb) to produce reliable ICP measurements.

Figure 9 illustrates radial structure fimetions (RSFs) produced by forward Fourier transforms of the XAFS spectra represented in Figure 8 (76). The spectra were uncorrected for phase shift. All spectra showed a peak of R 1.8A, which represents the first coordination shell of Ni. A second peak representing the second Ni shell can be observed at R 2.8A in the spectra of the Ni sorption samples and takovite (Figure 9). These spectra also showed peaks beyond the second shell at R == 5-6A (Figure 9) these peaks resulted from multiple scattering among Ni atoms (72).

The structural parameters derived from XAFS analysis are summarized in Table II (76). Least-square fits of filtered XAFS for the first RSF peak reveal that in the first coordination shell Ni is surrounded by six O atoms. This behavior indicates that Ni(II) is in an octahedral environment. The Ni-0 distances for the Ni sorption samples are 2.02-2.03A and are similar to those in takovite (2.03A). The Ni-0 distances in crystalline Ni(OH)2(s) are distinctly longer (2.06A). 

Figure 7. The kinetics of Ni sorption on pyrophyllite from a 3mMNi solution at pH 7.5. ( ) denotes the amount of sorbed Ni (pmol m ) and (a) the amount of simultaneous dissolved Si (pmol m ). The dissolution of untreated pyrophyllite at pH 7.5 is shown for comparison ( ). From Scheidegger et 2il. (16), with permission.

Nickel sorption on pyrophyllite, kaolinite, gibbsite, and montmorillonite at pH 7.5 results in formation of Ni-nucleation products from solutions which are undersaturated with respect to the thermodynamic solubility product of Ni(OH)2(s). An important finding of the study of Scheidegger et al. (16) is that the structural environment of Ni in all Ni sorption samples is similar. There is also an obvious similarity among the spectra of the Ni sorption samples and the spectrum of takovite, suggesting the presence of Ni phases of similar structure (Table II). 

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