Hi All! I am working through sample questions in the NCEES book, and hoping someone can help me with something. I am on page 31 - Morning Sample Questions - #131. I guessed at this during my "trial-run" and got it right, but when I look at the solution on pg. 79, it makes an actual reference to the LeGrega Haz Waste Mgmt book (1994 version; pg. 139). Here is my question - I don't have the same version of the Haz Waste mgmt book - I have the 2001 version and the answer is referencing the 1994 version. I tried to go read up on the explanation and can't find the same solution reference in my book - understanding page numbers and topics changed from the 1994 version to the 2001 version. I was hoping my version would still cover the material but I don't see it. Does anyone (1) either have the 1994 version and can provide me a brief synopsis of what pg. 139 says about oxidized lake sediment effects on chromium, or (2) have the 2001 version and can find the reference about oxidized lake sediment effects on chromium that I am obviously missing? Any help is greatly appreciated. Thanks Everyone - Hope your Monday is going well!
Help on Question from NCEES Env Engineering Sample Questions and Solutions
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FLBuff PE
Multi-disciplinary engineer
Looks like it is an error in the answer of the NCEES exam. I have both books (NCEES and LaGrega '94). Try page 193. When the microzone is oxidated, the chromium is reuced from CR+6 to CR+3. The CR+3 is "precipitated as relatively insoluble" (Table 4-15, pg. 193). Hope this helps!
EnvEngineer
Well-known member
I hate to be contrary but the chrome is oxidized not reduced and goes from the non soluable chrome 3 to soluable chrome 6. This is a major issue in chemical oxidation with fentons and permanganate. Answere A is correct.
FLBuff PE
Multi-disciplinary engineer
Actually, on the page I am referencing in LaGrega, the chemical equation is as follows:
2H2CrO4 + 3SO2 --> CR2(SO4)3 + 2H2O
The Cr is in +6 in the products, and is then reduced to Cr +3, thus precipitating out. That is why Answer A, "Promotes Release" is correct. If the chrome went from Cr 3 to Cr 6, then the chrome would not precipitate.
RIP - VTEnviro
His Memory Eternal
I took haz wastes in 2000 so I probably have the old edition. I'll look when I get home tonight. I'm always up for a spirited chromium debate.
That LaGrega book is a great reference. There was one problem on my exam I had never seen before. I found one of the key terms from the question in the index of that book and it led me to an example that was nearly identical to the test question.
That LaGrega book is a great reference. There was one problem on my exam I had never seen before. I found one of the key terms from the question in the index of that book and it led me to an example that was nearly identical to the test question.
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FLBuff PE
Multi-disciplinary engineer
I guess the question is: is the problem saying the chromium is in the sediment, or in the water? If it is in the sediment, then EnvEngineer is correct; the chromium would be oxidized from Cr +3 (insoluble) to Cr +6 (soluble), thus promoting release from the sediment to the water. I took the problem to mean that the chromium was present in the water, and as the sediment was oxidized, something had to be reduced; i.e. the chromium. Then, the chromium would be reduced from Cr +6 (soluble) to Cr +3 (insoluble), thus precipitating out of the lake water.
Bacon, looks like I have made this more confusing; I apologize. Regardless, the reference in the solution to the NCEES problem should have said pg. 193 in LaGrega.
Bacon, looks like I have made this more confusing; I apologize. Regardless, the reference in the solution to the NCEES problem should have said pg. 193 in LaGrega.
