Bs

Figure 14. Carbon adsorption system for Cr+® removal.

Smith et al. (1971} conducted laboratory experiments to study mercury removal from a 17 percent caustic solution produced by mercury electrolysis cells." In this study, the caustic solution was percolated through a column of activated carbon while another aqueous solution containing mercuric chloride was passed through the second column. Analysis of the treated solution indicated removal of 80 and 100 percent of the mercury from the first and second solution, respectively. Filtration of the feed solution also significantly removed very fine metallic mercury particles from the waste streams.

Huang (1984} evaluated removal of heavy metals such as mercury (EI), lead (II), and cadmium (II) from groundwater by use of activated carbon. Two types of carbon [Nuchar S.A. (L. type) and Filtrasorb 400 (H. type)] were used at different pH values (2.5 to 10.5) during the process. Results of the study indicated that, at low pH values, Nuchar S.A. achieved higher removal efficiencies for the metals than did Filtrasorb 400; however, at high pH values, the performance of both carbons was similar.

Results obtained from various studies conducted on mercury-contaminated wastes indicated the following:5

* Adsorption capacity of activated carbon increases with chelation of mercury in solution.

' Reduction of Hg*1 to the elemental state may continue even after its adsorption onto activated carbon.

* Sulfurizing agents (i.e., carbon disulfide) increase mercury removal from the waste streams,

* Because low pH values increase adsorption of mercury onto the activated carbon, carbon regeneration may be facilitated at high pH values.

Availability-

Activated carbon has been widely used to remove organic materials from aqueous wastes; however, its application for removal of metals from wastewaters has been limited. Areas in which activated carbon is applied to remove metals include material technology, in which valuable inorganics such as gold and silver are extracted from solution by activated carbon; analytical chemistry, In which activated carbon is used to enrich specific metal ions for quantitative analysis; and in water and wastewater treatment. In which carbon is used to remove metallic compounds.61

A few bench- and pilot-scale studies have been conducted on the treatment of waste streams by activated carbon adsorption. Also, a few full-scale carbon-adsorption systems are conwercially available for treatment of wastewaters containing chromium and mercury. Because of confidential agreement between carbon manufacturers and their customers and lack of sufficient characterization data, however, performance information related to these full-scale operations is currently Inadequate. Limited available information on full-scale applications of activated carbon, however, Indicates that wastewaters containing 0007 and DQ09 are potentially amenable for treatment by carbon adsorption. Additional studies are recommended to investigate the applicability of activated carbon for removal of metals from wastewaters.

Environmental Evaluation-Environmental impacts of activated carbon adsorption are primarily associated with regeneration or disposal of the activated carbon. The carbon material is regenerated by using a strong acid such as sulfuric acid or base to remove adsorbed metals from the pores of the activated carbon.

Costs-Direct capital costs for carbon adsorption systems applicable to metals treatment include the purchase of equipment such as storage tanks, pre-filters, carbon columns, waste feed pumps, piping materials, and automatic controls. Table 41 presents equations used to calculate the direct capital costs of the process. Indirect capital costs include primarily engineering and construction costs, contractor's fee, interest during construction, startup expenses, spare parts inventory, contingency, and working capital. Table 42 lists indirect cost factors for carbon adsorption systems. Direct and indirect capital costs are incurred prior to actual startup of the process.

Operation and maintenance costs consist of direct costs involved in the operation of the process, including operating labor, electricity, and activated carbon consumption. Costs such as insurance, taxes, general administation, and system overhead must also be included in indirect costs Table 43 lists operation and maintenance costs for carbon adsorption systems with flow rates in the range of 100 to 2500 gal/h.

TABLE 41. DIRECT COSTS OF CARBON ADSORPTION*'6

Carbon consumption Direct capital rate, lb/day costs, S

where C • Carbon consumption rate in pounds per day s - Storage volume in gallons

* Reference 52.

h Cost estimates were developed for three mode! treatment systems (three small scale and three large scale systems). The cost estimates for these systems were then used to develop a cost equation in the form of a proper curve.

TABLE 42. INDIRECT COST FACTORS OF CARBON ADSORPTION*

Item

Percent of direct capital costs

Percent of the sum of direct and indirect capital costs

Percent of total annual costb

Indirect Canital Costs

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