Environ. Sei. Technol. 1982, 16, 441
SIR: We are pleased to have the opportunity of responding to the questions raised by Schecher et al. concerning the article on the "Nature of the Bonding between Metallic Ions and Algal Cell Walls" (1). 1. We welcome their observations on the ZPC of Chlorella. Recent work here with the ApH effect on addition of NaN03 (2) using the alga Tribonema indicates a ZPC around pH 3.5. The data on Vancheria shown in Figure 1 represent an early attempt to show reversibility in the pH titration that would indicate a degree of structural integrity during the process. The figure demonstrates this point. The data were not corrected, however, for a water blank; this would bring Figure 1 more in line with Figure 3. 2. The use of fragmented cells raises the question of applicability to cells that are intact but nonfunctional due to the experimental environment as well as to those that are fully viable. The technique was developed because early work was not sufficiently reproducible to establish physical-chemical relationships. Data on fragmented cells were more reproducible and at the same time were comparable to those obtained with intact cells. This is supported by present results. (Cell Wall Fragments vs. Intact Cells, p 1213 (1)). Recently, Tribonema fragment preparations have been used for pH titration studies. This alga has a considerably smaller cross section than Vaucheria, and though the fibers are broken up by the Thomas tissue grinder the cells appear quite intact. Results here are in line with those reported for the Vaucheria fragments. Also with a water blank correction as noted above, Figure 1would be more like the corresponding curve of Figure 3, suggesting com-
0013-936X/82/0916-0441$01.25/0
parable buffering capacities. 3. The adsorption of metallic ions was relatively fast, being complete in 5-10 min, with a half hour generally allowed for equilibration. Since the pH decreases on adsorption, periodic adjustment to a stated pH gave a good indication of completeness of reaction. 4. The stochiometry of eq 2 represents the apparent first-power dependence that was obtained for both sodium and zinc. As for electroneutrality we have at present no basis for specifying species that would represent this, though obvious suggestions could be Na+
+ (ZnA) = (NaA-) + Zn2+
or Na+ + H+ + ZnA = (NaHA) + Zn2+ where for the latter a pH change is indicated. The data for Tribonema are from work in this laboratory by Joanna Lehman.
Literature Cited (1) Crist, Ray H.; Oberholser, Karl; Shank, Norman; Nguyen, Ming Environ. Sci. Technol. 1981, 15, 1212. (2) Davis, James A.; Leckie, James 0. J. Colloid Interface Sci. 1978, 67, 90.
Ray H. Crlst," Karl Oberholser Norman Shank, Mlng Nguyen
Department of Natural Science Messiah College Grantham, Pennsylvania 17027
0 1982 American Chemical Society
Environ. Sci. Technol., Vol. 16, No. 7, 1982
441
