The aviation market continues to recover, with North American commercial passenger activity now back to 80% of pre-pandemic levels, while international activity still has a longer way to a full recovery.
At most of our 80 operated locations outside of the U.S., activity was substantially ahead of where we were a year ago.
Adjusted second quarter net income and earnings per share were $25 million and $0.39 per share, respectively.
Adjusted EBITDA for the second quarter was $60 million.
Volume improved significantly, as markets continue to recover, with second quarter consolidated volume up 9% sequentially and 33% year-over-year.
And lastly, generating $37 million of cash flow from operations during the second quarter and $500 million over the past 12 months, increasing our net cash position to more than $200 million, further strengthening our balance sheet.
Consolidated revenue for the second quarter was $7.1 billion, an increase of $1.1 billion or 19% sequentially and an increase of $3.9 billion or 126%, compared to the second quarter of last year.
The year-over-year increase is driven by the significant increase in volume across all of our operating segments, as well as our 130% increase in average fuel prices compared to the second quarter of 2020.
Our aviation segment volume was 1.8 billion [Phonetic] gallons in the second quarter, an increase of 230 million gallons or 20% sequentially, and double the volume generated in the second quarter of last year.
Although we've continued to experience increased activity with overall segment volume at more than 60% of pre-pandemic levels, at this time, we remain optimistic about the second half of the year and beyond.
Volume in our marine segment for the second quarter was 4.6 million metric tons, an increase of 360,000 metric tons or 8% sequentially and an increase of nearly 600,000 metric tons or 15% year-over-year.
Our land segment volume was 1.3 billion gallons or gallon equivalents during the second quarter, flat sequentially, but an increase of 120 million gallons or gallon equivalents of 10% year-over-year.
Consolidated volume for the second quarter was 3.9 billion gallons, an increase of 310 million gallons or 9% sequentially, and an increase of 960 million gallons or 43% compared to the second quarter of 2020.
Consolidated gross profit for the second quarter was $185 million, that's down 4% sequentially and 6% year-over-year.
Our aviation segment contributed $88 million of gross profit in the second quarter, an increase of 15% [Phonetic] sequentially or a decline of 3% year-over-year.
The land segment generated second quarter gross profit of $23 million, that's a decline of 11% sequentially and 39% year-over-year.
Our land segment delivered gross profit of $74 million in the second quarter, a decline of 18% sequentially, but an increase of 8% year-over-year when excluding the profitability related to the multi-service business from last year's results.
Core operating expenses, which exclude bad debt expense, were $147 million in the second quarter, which was in line with our guidance for the quarter, as we continue to manage our controllable costs well.
Looking ahead to the third quarter, operating expenses, excluding bad debt expense, should remain in the range of $146 million to $150 million.
As previously discussed, our team has continued to do an excellent job, managing our receivables portfolio throughout the pandemic, with more than 90% of our portfolio now current.
Our team's efforts have been paying off, with no leasing losses of any significance, compounded by successfully collecting certain high-risk receivables, which had been previously reserved for, this resulted in a credit to bad debt expense this quarter of approximately $1 million.
Adjusted income from operations for the second quarter was $39 million, that's down 7% sequentially, but up 17% year-over-year, related to the segment activity that I mentioned earlier.
Adjusted EBITDA for the second quarter was $60 million, down 3% sequentially, but up 11% compared to last year.
Second quarter interest expense was $10 million, that's flat year-over-year, as total interest expense continues to benefit from low average borrowings, as well as low rates, and we get into the quarter with no borrowings on our revolving credit facility, and in a net cash position, again, in excess of $200 million.
We expect interest expense for the third quarter to be approximately $9 million to $11 million.
Our adjusted effective tax rate for the quarter was 10.3%, which is significantly lower than our tax rate in the second quarter of 2020, and the rate we had previously forecast for this year's second quarter.
In a nutshell, our second quarter tax rate was much lower than forecast for the reasons just explained, but the forecast of changes in income mix will also contribute to a lower tax rate, compared to where we started the year, with our tax rate for the second half of the year now expected to be in the range of 29% to 33%.
Our total accounts receivable balance increased to approximately $1.8 billion at quarter end, principally related to the increase in volume in our aviation and marine segments, as well as the sequential rise in average fuel prices.
We remain focused on managing our working capital requirements, which resulted in operating cash flow generation of $37 million during the second quarter, again, despite a 14% sequential increase in prices and a 9% increase in volume.
And despite rising prices and increasing volumes, we again generated healthy operating cash flow, which now aggregates to nearly $750 million over the past six quarters.

Summary:
Adjusted second quarter net income and earnings per share were $25 million and $0.39 per share, respectively.
Adjusted EBITDA for the second quarter was $60 million, down 3% sequentially, but up 11% compared to last year.