In a 2-quart saucepan bring salted water to a boil and add rice and boil, uncovered, stirring occasionally, until cooked, about 25 minutes. Drain rice in a colander and rinse under cold running water until cool. Drain rice well.
In a saucepan of boiling salted water, blanch beans and carrots for 1 or 2 minutes, drain in a sieve and shock in ice bath. Drain vegetables well.
In a large bowl whisk together lemon juice, mustard, salt, and pepper. Add oil in a stream and whisk until emulsified. Add rice, blanched vegetables, celery, bell pepper, scallions, and chickpeas, toss well and adjust seasoning. Add dill and parsley. Serve on bed of arugula.